Dynamics Applying Newton's Laws Circular...

DynamicsApplying Newton’s Laws

Circular Motion

Lana Sheridan

De Anza College

Oct 17, 2017

Last time

• friction

• problem solving with forces

From last time: Pulley with an Incline

Let’s change up our Atwood machine apparatus so that one of themasses is on a slanted surface with no friction:

5.7 Analysis Models Using Newton’s Second Law 129

Example 5.10 Acceleration of Two Objects Connected by a Cord

A ball of mass m1 and a block of mass m2 are attached by a lightweight cord that passes over a frictionless pulley of negligible mass as in Figure 5.15a. The block lies on a frictionless incline of angle u. Find the magnitude of the acceleration of the two objects and the tension in the cord.

Conceptualize Imagine the objects in Figure 5.15 in motion. If m2 moves down the incline, then m1 moves upward. Because the objects are con-nected by a cord (which we assume does not stretch), their accelerations have the same magnitude. Notice the normal coordinate axes in Figure 5.15b for the ball and the “tilted” axes for the block in Figure 5.15c.

Categorize We can identify forces on each of the two objects and we are looking for an acceleration, so we categorize the objects as particles under a net force. For the block, this model is only valid for the x9 direction. In the y9 direction, we apply the particle in equilibrium model because the block does not accelerate in that direction.

Analyze Consider the free-body diagrams shown in Figures 5.15b and 5.15c.

AM

S O L U T I O N

x

y

m1

2g cosx!

y!

m2

m2g sin u

uum

m1u

m2

nS

TS

TS

gS

gS

aS

aS

a

c

b

Figure 5.15 (Example 5.10) (a) Two objects connected by a lightweight cord strung over a frictionless pulley. (b) The free-body diagram for the ball. (c) The free-body diagram for the block. (The incline is frictionless.)

Apply Newton’s second law in the y direction to the ball, choosing the upward direction as positive:

(1) o Fy 5 T 2 m1g 5 m1ay 5 m1a

For the ball to accelerate upward, it is necessary that T . m1g. In Equation (1), we replaced ay with a because the accel-eration has only a y component. For the block, we have chosen the x9 axis along the incline as in Figure 5.15c. For consistency with our choice for the ball, we choose the positive x9 direction to be down the incline.

Apply the particle under a net force model to the block in the x9 direction and the particle in equilibrium model in the y9 direction:

(2) o Fx9 5 m2g sin u 2 T 5 m2ax9 5 m2a

(3) o Fy9 5 n 2 m2g cos u 5 0

Solve Equation (1) for T : (4) T 5 m1(g 1 a)

Substitute this expression for T into Equation (2): m2g sin u 2 m1(g 1 a) 5 m2a

Solve for a: (5) a 5 am2 sin u 2 m1

m1 1 m2bg

Substitute this expression for a into Equation (4) to find T :

(6) T 5 am1m2 1sin u 1 1 2m1 1 m2

bg

Finalize The block accelerates down the incline only if m2 sin u . m1. If m1 . m2 sin u, the acceleration is up the incline for the block and downward for the ball. Also notice that the result for the acceleration, Equation (5), can be interpreted as the magnitude of the net external force acting on the ball–block system divided by the total mass of the system; this result is consistent with Newton’s second law.

What happens in this situation if u 5 90°?WHAT IF ?continued

In Equation (2), we replaced ax9 with a because the two objects have accelerations of equal magnitude a.

Acceleration? Tension?

Pulley with an Incline

m1a = T −m1g (1)

m2a = m2g sin θ− T (2)

Add eq (1) and (2):

(m1 +m2)a = m2g sin θ−m1g

a =(m2 sin θ−m1)g

m1 +m2

Putting a into (1):

m1(m2 sin θ−m1)g

m1 +m2= T −m1g

T =m1m2(sin θ+ 1)g

m1 +m2

Does this agree with what we had for the Atwood machine whenθ = 90◦?

Overview

• Circular motion and force

• Centripetal force

• Examples

• Non-uniform circular motion

Circular Motion - Now with ForceIf an object moves in a uniform circle, its velocity must always bechanging. ⇒ It is accelerating.

F = ma ⇒ F 6= 0

Any object moving in a circular (or curved) path must beexperiencing a force.

We call this the centripetal force.

152 Chapter 6 Circular Motion and Other Applications of Newton’s Laws

Example 6.1 The Conical Pendulum

A small ball of mass m is suspended from a string of length L. The ball revolves with constant speed v in a horizontal circle of radius r as shown in Figure 6.3. (Because the string sweeps out the surface of a cone, the system is known as a conical pendulum.) Find an expression for v in terms of the geometry in Figure 6.3.

Conceptualize Imagine the motion of the ball in Figure 6.3a and convince your-self that the string sweeps out a cone and that the ball moves in a horizontal circle.

Categorize The ball in Figure 6.3 does not accelerate vertically. Therefore, we model it as a particle in equilibrium in the vertical direction. It experiences a cen-tripetal acceleration in the horizontal direction, so it is modeled as a particle in uniform circular motion in this direction.

Analyze Let u represent the angle between the string and the vertical. In the dia-gram of forces acting on the ball in Figure 6.3b, the force T

S exerted by the string on the ball is resolved into a vertical

component T cos u and a horizontal component T sin u acting toward the center of the circular path.

AM

S O L U T I O N

Apply the particle in equilibrium model in the vertical direction:

o Fy 5 T cos u 2 mg 5 0

(1) T cos u 5 mg

Use Equation 6.1 from the particle in uniform circular motion model in the horizontal direction:

(2) a Fx 5 T sin u 5 mac 5mv2

r

Divide Equation (2) by Equation (1) and use sin u/cos u 5 tan u:

tan u 5v2

rg

Solve for v: v 5 "rg tan u

Incorporate r 5 L sin u from the geometry in Figure 6.3a: v 5 "Lg sin u tan u

Finalize Notice that the speed is independent of the mass of the ball. Consider what happens when u goes to 908 so that the string is horizontal. Because the tangent of 908 is infinite, the speed v is infinite, which tells us the string can-not possibly be horizontal. If it were, there would be no vertical component of the force T

S to balance the gravitational

force on the ball. That is why we mentioned in regard to Figure 6.1 that the puck’s weight in the figure is supported by a frictionless table.

Imagine a moving object that can be mod-eled as a particle. If it moves in a circular path of radius r at a constant speed v, it experiences a centripetal acceleration. Because the particle is accelerating, there must be a net force acting on the particle. That force is directed toward the center of the circular path and is given by

a F 5 mac 5 m v2

r (6.1)

Analysis Model Particle in Uniform Circular Motion (Extension)

Examples

acting on a rock twirled in a circle

traveling around the Sun in a perfectly circular orbit (Chapter 13)

particle moving in a uniform magnetic field (Chapter 29)

nucleus in the Bohr model of the hydrogen atom (Chapter 42)

r

! vS

acS

FS

r

L

m

u

u

T sin u

T cos uTS

gS mgS

a b

Figure 6.3 (Example 6.1) (a) A conical pendulum. The path of the ball is a horizontal circle. (b) The forces acting on the ball.

1Figures from Serway & Jewett.

Uniform Circular Motion

For an object moving in a uniform circle, a = ac = v2

r .

This gives the expression for centripetal force:

F = ma

so,

Fc =mv2

r

As a vector:

Fc = −mv2

rr

Centripetal Force

Something must provide this force: 6.1 Extending the Particle in Uniform Circular Motion Model 151

The acceleration is called centripetal acceleration because aSc is directed toward the center of the circle. Furthermore, aSc is always perpendicular to vS. (If there were a component of acceleration parallel to vS, the particle’s speed would be changing.) Let us now extend the particle in uniform circular motion model from Section 4.4 by incorporating the concept of force. Consider a puck of mass m that is tied to a string of length r and moves at constant speed in a horizontal, circular path as illustrated in Figure 6.1. Its weight is supported by a frictionless table, and the string is anchored to a peg at the center of the circular path of the puck. Why does the puck move in a circle? According to Newton’s first law, the puck would move in a straight line if there were no force on it; the string, however, prevents motion along a straight line by exerting on the puck a radial force F

Sr that makes it follow

the circular path. This force is directed along the string toward the center of the circle as shown in Figure 6.1. If Newton’s second law is applied along the radial direction, the net force caus-ing the centripetal acceleration can be related to the acceleration as follows:

a F 5 mac 5 m v2

r (6.1)

A force causing a centripetal acceleration acts toward the center of the circular path and causes a change in the direction of the velocity vector. If that force should vanish, the object would no longer move in its circular path; instead, it would move along a straight-line path tangent to the circle. This idea is illustrated in Figure 6.2 for the puck moving in a circular path at the end of a string in a horizontal plane. If the string breaks at some instant, the puck moves along the straight-line path that is tangent to the circle at the position of the puck at this instant.

Q uick Quiz 6.1 You are riding on a Ferris wheel that is rotating with constant speed. The car in which you are riding always maintains its correct upward ori-entation; it does not invert. (i) What is the direction of the normal force on you from the seat when you are at the top of the wheel? (a) upward (b) downward (c) impossible to determine (ii) From the same choices, what is the direction of the net force on you when you are at the top of the wheel?

�W Force causing centripetal acceleration

m

r

r

r

FS

F S

A force F , directed toward the center of the circle, keeps the puck moving in its circular path.

Sr

Figure 6.1 An overhead view of a puck moving in a circular path in a horizontal plane.

Figure 6.2 The string holding the puck in its circular path breaks.

r

When the string breaks, the puckmoves in thedirection tangentto the circle.

vS

Pitfall Prevention 6.1Direction of Travel When the String Is Cut Study Figure 6.2 very carefully. Many students (wrongly) think that the puck will move radially away from the center of the circle when the string is cut. The velocity of the puck is tangent to the circle. By Newton’s first law, the puck continues to move in the same direction in which it is moving just as the force from the string disappears.

It could be tension in a rope.

Centripetal Force

Something must provide this force:

Example 6.2 How Fast Can It Spin?

A puck of mass 0.500 kg is attached to the end of a cord 1.50 m long. The puck moves in a horizontal circle as shown in Figure 6.1. If the cord can withstand a maximum tension of 50.0 N, what is the maximum speed at which the puck can move before the cord breaks? Assume the string remains horizontal during the motion.

Conceptualize It makes sense that the stronger the cord, the faster the puck can move before the cord breaks. Also, we expect a more massive puck to break the cord at a lower speed. (Imagine whirling a bowling ball on the cord!)

Categorize Because the puck moves in a circular path, we model it as a particle in uniform circular motion.

AM

S O L U T I O N

Analyze Incorporate the tension and the centripetal acceler-ation into Newton’s second law as described by Equation 6.1:

T 5 m v2

r

continued

Solve for v: (1) v 5 ÅTrm

Example 6.3 What Is the Maximum Speed of the Car?

A 1 500-kg car moving on a flat, horizontal road negotiates a curve as shown in Figure 6.4a. If the radius of the curve is 35.0 m and the coefficient of static friction between the tires and dry pavement is 0.523, find the maximum speed the car can have and still make the turn successfully.

Conceptualize Imagine that the curved roadway is part of a large circle so that the car is moving in a circular path.

Categorize Based on the Conceptualize step of the problem, we model the car as a particle in uniform circular motion in the horizontal direction. The car is not accelerating vertically, so it is modeled as a particle in equilibrium in the vertical direction.

Analyze Figure 6.4b shows the forces on the car. The force that enables the car to remain in its circular path is the force of static friction. (It is static because no slipping occurs at the point of contact between road and tires. If this force of static friction were zero—for example, if the car were on an icy road—the car would continue in a straight line and slide off the curved road.) The maximum speed vmax the car can have around the curve is the speed at which it is on the verge of skidding outward. At this point, the friction force has its maximum value fs,max 5 msn.

AM

S O L U T I O N

Find the maximum speed the puck can have, which corre-sponds to the maximum tension the string can withstand:

vmax 5 ÅTmaxrm

5 Å 150.0 N 2 11.50 m 20.500 kg

5 12.2 m/s

Finalize Equation (1) shows that v increases with T and decreases with larger m, as we expected from our conceptual-ization of the problem.

Suppose the puck moves in a circle of larger radius at the same speed v. Is the cord more likely or less likely to break?

Answer The larger radius means that the change in the direction of the velocity vector will be smaller in a given time interval. Therefore, the acceleration is smaller and the required tension in the string is smaller. As a result, the string is less likely to break when the puck travels in a circle of larger radius.

WHAT IF ?

nS

fsS

fsS

mgS

a

b

Figure 6.4 (Example 6.3) (a) The force of static friction directed toward the center of the curve keeps the car moving in a cir-cular path. (b) The forces acting on the car.

6.1 Extending the Particle in Uniform Circular Motion Model 153

It could be friction.

Centripetal Force

Consider the example of a string constraining the motion of a puck: 6.1 Extending the Particle in Uniform Circular Motion Model 151




a F 5 mac 5 m v2

r (6.1)




m

r

r

r

FS

F S


Sr



r


vS


Centripetal Force

Question. What will the puck do if the string breaks?

(A) Fly radially outward.

(B) Continue along the circle.

(C) Move tangentially to the circle.





a F 5 mac 5 m v2

r (6.1)




m

r

r

r

FS

F S


Sr



r


vS


Centripetal Force

Question. What will the puck do if the string breaks?

(A) Fly radially outward.

(B) Continue along the circle.

(C) Move tangentially to the circle. ← 6.1 Extending the Particle in Uniform Circular Motion Model 151




a F 5 mac 5 m v2

r (6.1)




m

r

r

r

FS

F S


Sr



r


vS


Finding Centripetal Force Example

Page 169, # 4

Problems 169

10. A pail of water can be whirled in a vertical path such that no water is spilled. Why does the water stay in the pail, even when the pail is above your head?

11. “If the current position and velocity of every par-ticle in the Universe were known, together with the laws describing the forces that particles exert on one another, the whole future of the Universe could be cal-culated. The future is determinate and preordained. Free will is an illusion.” Do you agree with this thesis? Argue for or against it.

Section 6.1 Extending the Particle in Uniform Circular Motion Model 1. A light string can

support a station-ary hanging load of 25.0 kg before breaking. An object of mass m 5 3.00 kg attached to the string rotates on a friction-less, horizontal table in a circle of radius r 5 0.800 m, and the other end of the string is held fixed as in Figure P6.1. What range of speeds can the object have before the string breaks?

2. Whenever two Apollo astronauts were on the surface of the Moon, a third astronaut orbited the Moon. Assume the orbit to be circular and 100 km above the surface of the Moon, where the acceleration due to gravity is 1.52 m/s2. The radius of the Moon is 1.70 3 106 m. Determine (a) the astronaut’s orbital speed and (b) the period of the orbit.

3. In the Bohr model of the hydrogen atom, an electron moves in a circular path around a proton. The speed of the electron is approximately 2.20 3 106 m/s. Find (a) the force acting on the electron as it revolves in a circular orbit of radius 0.529 3 10210 m and (b) the centripetal acceleration of the electron.

4. A curve in a road forms part of a horizontal circle. As a car goes around it at constant speed 14.0 m/s, the total horizontal force on the driver has magnitude 130 N.

AMTM

What is the total horizontal force on the driver if the speed on the same curve is 18.0 m/s instead?

5. In a cyclotron (one type of particle accelerator), a deuteron (of mass 2.00 u) reaches a final speed of 10.0% of the speed of light while moving in a circular path of radius 0.480 m. What magnitude of magnetic force is required to maintain the deuteron in a circu-lar path?

6. A car initially traveling eastward turns north by traveling in a circular path at uniform speed as shown in Figure P6.6. The length of the arc ABC is 235 m, and the car completes the turn in 36.0 s. (a) What is the acceleration when the car is at B located at an angle of 35.08? Express your answer in terms of the unit vectors i and j. Deter-mine (b) the car’s average speed and (c) its average acceleration during the 36.0-s interval.

7. A space station, in the form of a wheel 120 m in diameter, rotates to provide an “artificial gravity” of 3.00 m/s2 for persons who walk around on the inner wall of the outer rim. Find the rate of the wheel’s rotation in revolutions per minute that will produce this effect.

8. Consider a conical pendulum (Fig. P6.8) with a bob of mass m 5 80.0 kg on a string of length L 5 10.0 m that makes an angle of u 5 5.008 with the vertical. Deter-mine (a) the horizontal and vertical components of the

y

A

O

B

Cx

35.0!

Figure P6.6

W

W

space and used as colonies. The purpose of the rota-tion is to simulate gravity for the inhabitants. Explain this concept for producing an effective imitation of gravity.

8. Consider a small raindrop and a large raindrop fall-ing through the atmosphere. (a) Compare their termi-nal speeds. (b) What are their accelerations when they reach terminal speed?

9. Why does a pilot tend to black out when pulling out of a steep dive?

Problems

The problems found in this

chapter may be assigned online in Enhanced WebAssign

1. straightforward; 2. intermediate; 3. challenging

1. full solution available in the Student Solutions Manual/Study Guide

AMT Analysis Model tutorial available in Enhanced WebAssign

GP Guided Problem

M Master It tutorial available in Enhanced WebAssign

W Watch It video solution available in Enhanced WebAssign

BIO

Q/C

S

rm

Figure P6.1

Problems 169

10. A pail of water can be whirled in a vertical path such that no water is spilled. Why does the water stay in the pail, even when the pail is above your head?

11. “If the current position and velocity of every par-ticle in the Universe were known, together with the laws describing the forces that particles exert on one another, the whole future of the Universe could be cal-culated. The future is determinate and preordained. Free will is an illusion.” Do you agree with this thesis? Argue for or against it.

Section 6.1 Extending the Particle in Uniform Circular Motion Model 1. A light string can

support a station-ary hanging load of 25.0 kg before breaking. An object of mass m 5 3.00 kg attached to the string rotates on a friction-less, horizontal table in a circle of radius r 5 0.800 m, and the other end of the string is held fixed as in Figure P6.1. What range of speeds can the object have before the string breaks?

2. Whenever two Apollo astronauts were on the surface of the Moon, a third astronaut orbited the Moon. Assume the orbit to be circular and 100 km above the surface of the Moon, where the acceleration due to gravity is 1.52 m/s2. The radius of the Moon is 1.70 3 106 m. Determine (a) the astronaut’s orbital speed and (b) the period of the orbit.

3. In the Bohr model of the hydrogen atom, an electron moves in a circular path around a proton. The speed of the electron is approximately 2.20 3 106 m/s. Find (a) the force acting on the electron as it revolves in a circular orbit of radius 0.529 3 10210 m and (b) the centripetal acceleration of the electron.

4. A curve in a road forms part of a horizontal circle. As a car goes around it at constant speed 14.0 m/s, the total horizontal force on the driver has magnitude 130 N.

AMTM

What is the total horizontal force on the driver if the speed on the same curve is 18.0 m/s instead?

5. In a cyclotron (one type of particle accelerator), a deuteron (of mass 2.00 u) reaches a final speed of 10.0% of the speed of light while moving in a circular path of radius 0.480 m. What magnitude of magnetic force is required to maintain the deuteron in a circu-lar path?

6. A car initially traveling eastward turns north by traveling in a circular path at uniform speed as shown in Figure P6.6. The length of the arc ABC is 235 m, and the car completes the turn in 36.0 s. (a) What is the acceleration when the car is at B located at an angle of 35.08? Express your answer in terms of the unit vectors i and j. Deter-mine (b) the car’s average speed and (c) its average acceleration during the 36.0-s interval.

7. A space station, in the form of a wheel 120 m in diameter, rotates to provide an “artificial gravity” of 3.00 m/s2 for persons who walk around on the inner wall of the outer rim. Find the rate of the wheel’s rotation in revolutions per minute that will produce this effect.

8. Consider a conical pendulum (Fig. P6.8) with a bob of mass m 5 80.0 kg on a string of length L 5 10.0 m that makes an angle of u 5 5.008 with the vertical. Deter-mine (a) the horizontal and vertical components of the

y

A

O

B

Cx

35.0!

Figure P6.6

W

W

space and used as colonies. The purpose of the rota-tion is to simulate gravity for the inhabitants. Explain this concept for producing an effective imitation of gravity.

8. Consider a small raindrop and a large raindrop fall-ing through the atmosphere. (a) Compare their termi-nal speeds. (b) What are their accelerations when they reach terminal speed?

9. Why does a pilot tend to black out when pulling out of a steep dive?

Problems

The problems found in this

chapter may be assigned online in Enhanced WebAssign

1. straightforward; 2. intermediate; 3. challenging

1. full solution available in the Student Solutions Manual/Study Guide

AMT Analysis Model tutorial available in Enhanced WebAssign

GP Guided Problem

M Master It tutorial available in Enhanced WebAssign

W Watch It video solution available in Enhanced WebAssign

BIO

Q/C

S

rm

Figure P6.1


9, # 4

Fc =mv2

r

m

r=

Fcv2

=130 N

(14.0m/s)2

= 0.6633 kg/m

F ′c =m

rv2

= (0.6633 kg/m)(18.0 m/s)2

= 215 N


Page 169, # 4

Fc =mv2

r

m

r=

Fcv2

=130 N

(14.0m/s)2

= 0.6633 kg/m

F ′c =m

rv2

= (0.6633 kg/m)(18.0 m/s)2

= 215 N

Ferris Wheel Forces

A Ferris wheel is a ride you tend to see at fairs and theme parks.

a b c

nbotS

mgS

ntopS

vS

R

Bottom

TopvS

mgS

Figure 6.6 (Example 6.5) (a) A child rides on a Ferris wheel. (b) The forces acting on the child at the bottom of the path. (c) The forces acting on the child at the top of the path.


continued

Write Newton’s second law for the car in the radial direc-tion, which is the x direction:

(1) a Fr 5 n sin u 5mv 2

r

Apply the particle in equilibrium model to the car in the vertical direction:

o Fy 5 n cos u 2 mg 5 0

(2) n cos u 5 mg

Divide Equation (1) by Equation (2): (3) tan u 5v 2

rg

Solve for the angle u: u 5 tan21 c 113.4 m/s 22135.0 m 2 19.80 m/s2 2 d 5 27.68

Finalize Equation (3) shows that the banking angle is independent of the mass of the vehicle negotiating the curve. If a car rounds the curve at a speed less than 13.4 m/s, the centripetal acceleration decreases. Therefore, the normal force, which is unchanged, is sufficient to cause two accelerations: the lower centripetal acceleration and an acceleration of the car down the inclined roadway. Consequently, an additional friction force parallel to the roadway and upward is needed to keep the car from sliding down the bank (to the left in Fig. 6.5). Similarly, a driver attempting to negotiate the curve at a speed greater than 13.4 m/s has to depend on friction to keep from sliding up the bank (to the right in Fig. 6.5).

Imagine that this same roadway were built on Mars in the future to connect different colony centers. Could it be traveled at the same speed?

Answer The reduced gravitational force on Mars would mean that the car is not pressed as tightly to the roadway. The reduced normal force results in a smaller component of the normal force toward the center of the circle. This smaller component would not be sufficient to provide the centripetal acceleration associated with the original speed. The cen-tripetal acceleration must be reduced, which can be done by reducing the speed v. Mathematically, notice that Equation (3) shows that the speed v is proportional to the square root of g for a roadway of fixed radius r banked at a fixed angle u. Therefore, if g is smaller, as it is on Mars, the speed v with which the roadway can be safely traveled is also smaller.

WHAT IF ?

normal force nS has a horizontal component toward the center of the curve. Because the road is to be designed so that the force of static friction is zero, the component nx 5 n sin u is the only force that causes the centripetal acceleration.

▸ 6.4 c o n t i n u e d

Example 6.5 Riding the Ferris Wheel

A child of mass m rides on a Ferris wheel as shown in Figure 6.6a. The child moves in a vertical circle of radius 10.0 m at a constant speed of 3.00 m/s.

(A) Determine the force exerted by the seat on the child at the bottom of the ride. Express your answer in terms of the weight of the child, mg.

Conceptualize Look carefully at Figure 6.6a. Based on experiences you may have had on a Ferris wheel or driving over small hills on a roadway, you would expect to feel lighter at the top of the path. Similarly, you would expect to feel heavier at the bottom of the path. At both the bottom of the path and the top, the nor-mal and gravitational forces on the child act in opposite directions. The vector sum of these two forces gives a force of constant magnitude that keeps the child moving in a circular path at a constant speed. To yield net force vec-tors with the same magnitude, the normal force at the bottom must be greater than that at the top.

AM

S O L U T I O N

During the ride the speed, v , is constant.

Ferris Wheel Forces

Quick Quiz 6.11 You are riding on a Ferris wheel that is rotatingwith constant speed. The car in which you are riding alwaysmaintains its correct upward orientation; it does not invert.

(i) What is the direction of the normal force on you from the seatwhen you are at the top of the wheel?

(A) upward

(B) downward

(C) impossible to determine

1Page 153, Serway & Jewett

Ferris Wheel Forces


(i) What is the direction of the normal force on you from the seatwhen you are at the top of the wheel?

(A) upward ←(B) downward



Ferris Wheel Forces


(ii) From the same choices, what is the direction of the net forceon you when you are at the top of the wheel?

(A) upward

(B) downward



Ferris Wheel Forces


(ii) From the same choices, what is the direction of the net forceon you when you are at the top of the wheel?

(A) upward

(B) downward ←(C) impossible to determine


Ferris WheelAssume the speed, v , is constant.

a b c

nbotS

mgS

ntopS

vS

R

Bottom

TopvS

mgS



continued



r



(2) n cos u 5 mg


rg





WHAT IF ?


▸ 6.4 c o n t i n u e d





AM

S O L U T I O N

ntop < mg : Fnet points down

a b c

nbotS

mgS

ntopS

vS

R

Bottom

TopvS

mgS



continued



r



(2) n cos u 5 mg


rg





WHAT IF ?


▸ 6.4 c o n t i n u e d





AM

S O L U T I O N

nbot > mg : Fnet points up

a b c

nbotS

mgS

ntopS

vS

R

Bottom

TopvS

mgS



continued



r



(2) n cos u 5 mg


rg





WHAT IF ?


▸ 6.4 c o n t i n u e d





AM

S O L U T I O N

A Banked Turn

Sharp turns in roads are often banked inwards to assist cars inmaking the turn: the centripetal force comes from the normalforce, not friction.


Finalize This speed is equivalent to 30.0 mi/h. Therefore, if the speed limit on this roadway is higher than 30 mi/h, this roadway could benefit greatly from some banking, as in the next example! Notice that the maximum speed does not depend on the mass of the car, which is why curved highways do not need multiple speed limits to cover the various masses of vehicles using the road.

Suppose a car travels this curve on a wet day and begins to skid on the curve when its speed reaches only 8.00 m/s. What can we say about the coefficient of static friction in this case?

Answer The coefficient of static friction between the tires and a wet road should be smaller than that between the tires and a dry road. This expectation is consistent with experience with driving because a skid is more likely on a wet road than a dry road. To check our suspicion, we can solve Equation (2) for the coefficient of static friction:

ms 5v2

max

gr

Substituting the numerical values gives

ms 5v2

max

gr5

18.00 m/s 2219.80 m/s2 2 135.0 m 2 5 0.187

which is indeed smaller than the coefficient of 0.523 for the dry road.

WHAT IF ?

Example 6.4 The Banked Roadway

A civil engineer wishes to redesign the curved roadway in Example 6.3 in such a way that a car will not have to rely on friction to round the curve without skidding. In other words, a car moving at the designated speed can negotiate the curve even when the road is covered with ice. Such a road is usually banked, which means that the road-way is tilted toward the inside of the curve as seen in the opening photograph for this chapter. Suppose the designated speed for the road is to be 13.4 m/s (30.0 mi/h) and the radius of the curve is 35.0 m. At what angle should the curve be banked?

Conceptualize The difference between this example and Example 6.3 is that the car is no longer moving on a flat roadway. Figure 6.5 shows the banked roadway, with the center of the circular path of the car far to the left of the figure. Notice that the horizontal component of the normal force participates in causing the car’s centripetal acceleration.

Categorize As in Example 6.3, the car is modeled as a particle in equilibrium in the vertical direction and a particle in uniform circular motion in the horizontal direction.

Analyze On a level (unbanked) road, the force that causes the centripetal accelera-tion is the force of static friction between tires and the road as we saw in the pre-ceding example. If the road is banked at an angle u as in Figure 6.5, however, the

AM

S O L U T I O N

Figure 6.5 (Example 6.4) A car moves into the page and is round-ing a curve on a road banked at an angle u to the horizontal. When friction is neglected, the force that causes the centripetal accelera-tion and keeps the car moving in its circular path is the horizontal component of the normal force.

nx

ny

u

u

FgS

nS

▸ 6.3 c o n t i n u e d

Apply Equation 6.1 from the particle in uniform circular motion model in the radial direction for the maximum speed condition:

(1) fs,max 5 msn 5 m v 2

max

r

Apply the particle in equilibrium model to the car in the verti-cal direction:

o Fy 5 0 S n 2 mg 5 0 S n 5 mg

Solve Equation (1) for the maximum speed and substitute for n: (2) vmax 5 Åmsnrm

5 Åmsmgrm

5 "ms gr

Substitute numerical values: vmax 5 "10.523 2 19.80 m/s2 2 135.0 m 2 5 13.4 m/s

←

A Banked Turn

A turn has a radius r . What should the angle θ be so that a cartraveling at speed v can turn the corner without relying on friction?





ms 5v2

max

gr


ms 5v2

max

gr5

18.00 m/s 2219.80 m/s2 2 135.0 m 2 5 0.187


WHAT IF ?






AM

S O L U T I O N


nx

ny

u

u

FgS

nS

▸ 6.3 c o n t i n u e d



max

r


o Fy 5 0 S n 2 mg 5 0 S n 5 mg


5 Åmsmgrm

5 "ms gr


Hint: consider what the net force vector must be in this case.

A Banked Turn





ms 5v2

max

gr


ms 5v2

max

gr5

18.00 m/s 2219.80 m/s2 2 135.0 m 2 5 0.187


WHAT IF ?






AM

S O L U T I O N


nx

ny

u

u

FgS

nS

▸ 6.3 c o n t i n u e d



max

r


o Fy 5 0 S n 2 mg 5 0 S n 5 mg


5 Åmsmgrm

5 "ms gr


y -direction (vertical):

Fy ,net = 0

ny −mg = 0

n cos θ = mg

n =mg

cos θ

A Banked Turn





ms 5v2

max

gr


ms 5v2

max

gr5

18.00 m/s 2219.80 m/s2 2 135.0 m 2 5 0.187


WHAT IF ?






AM

S O L U T I O N


nx

ny

u

u

FgS

nS

▸ 6.3 c o n t i n u e d



max

r


o Fy 5 0 S n 2 mg 5 0 S n 5 mg


5 Åmsmgrm

5 "ms gr


x-direction (horizontal):

Fx ,net = Fc

nx =mv2

r

n sin θ =mv2

r

mg

cos θsin θ =

mv2

r

tan θ =v2

rg⇒ θ = tan−1

(v2

rg

)

Non-Uniform Circular MotionIf the speed is changing the net force will not be pointed into thecenter of the circle.

6.2 Nonuniform Circular Motion 157

must also have a tangential and a radial component. Because the total accelera-tion is aS 5 aSr 1 aSt, the total force exerted on the particle is g F

S5 g F

Sr 1 g F

St

as shown in Figure 6.7. (We express the radial and tangential forces as net forces with the summation notation because each force could consist of multiple forces that combine.) The vector g F

Sr is directed toward the center of the circle and is

responsible for the centripetal acceleration. The vector g FS

t tangent to the circle is responsible for the tangential acceleration, which represents a change in the par-ticle’s speed with time.

Q uick Quiz 6.2 A bead slides at constant speed along a curved wire lying on a horizontal surface as shown in Figure 6.8. (a) Draw the vectors representing the force exerted by the wire on the bead at points !, ", and #. (b) Suppose the bead in Figure 6.8 speeds up with constant tangential acceleration as it moves toward the right. Draw the vectors representing the force on the bead at points !, ", and #.

Figure 6.8 (Quick Quiz 6.2) A bead slides along a curved wire.

#

"

!

Example 6.6 Keep Your Eye on the Ball

A small sphere of mass m is attached to the end of a cord of length R and set into motion in a vertical circle about a fixed point O as illustrated in Figure 6.9. Determine the tangential acceleration of the sphere and the tension in the cord at any instant when the speed of the sphere is v and the cord makes an angle u with the vertical.

Conceptualize Compare the motion of the sphere in Figure 6.9 with that of the child in Figure 6.6a associated with Example 6.5. Both objects travel in a circular path. Unlike the child in Example 6.5, however, the speed of the sphere is not uniform in this example because, at most points along the path, a tangen-tial component of acceleration arises from the gravitational force exerted on the sphere.

Categorize We model the sphere as a particle under a net force and moving in a circular path, but it is not a particle in uniform circu-lar motion. We need to use the techniques discussed in this sec-tion on nonuniform circular motion.

Analyze From the force diagram in Figure 6.9, we see that the only forces acting on the sphere are the gravitational force

AM

S O L U T I O N

Figure 6.9 (Example 6.6) The forces acting on a sphere of mass m connected to a cord of length R and rotating in a vertical circle centered at O. Forces acting on the sphere are shown when the sphere is at the top and bottom of the circle and at an arbitrary location.

R

O

mg sin u

u

umg cos uvbotS

vtopS

TS

TbotS

TtopS

mgS

mgS

mgS

Figure 6.7 When the net force acting on a par-ticle moving in a circular path has a tangential component o Ft , the particle’s speed changes.

!

!

!

FS

FtS

FrS

The net force exerted on the particle is the vector sum of the radial force and the tangential force.

Non-Uniform Circular Motion ExampleConsider a ball of mass m swinging in a vertical circle, radius R.What is the tension T when the string makes an angle θ with thevertical if the speed at that instant is v?

6.2 Nonuniform Circular Motion 157

must also have a tangential and a radial component. Because the total accelera-tion is aS 5 aSr 1 aSt, the total force exerted on the particle is g F

S5 g F

Sr 1 g F

St

as shown in Figure 6.7. (We express the radial and tangential forces as net forces with the summation notation because each force could consist of multiple forces that combine.) The vector g F

Sr is directed toward the center of the circle and is

responsible for the centripetal acceleration. The vector g FS

t tangent to the circle is responsible for the tangential acceleration, which represents a change in the par-ticle’s speed with time.

Q uick Quiz 6.2 A bead slides at constant speed along a curved wire lying on a horizontal surface as shown in Figure 6.8. (a) Draw the vectors representing the force exerted by the wire on the bead at points !, ", and #. (b) Suppose the bead in Figure 6.8 speeds up with constant tangential acceleration as it moves toward the right. Draw the vectors representing the force on the bead at points !, ", and #.

Figure 6.8 (Quick Quiz 6.2) A bead slides along a curved wire.

#

"

!

Example 6.6 Keep Your Eye on the Ball

A small sphere of mass m is attached to the end of a cord of length R and set into motion in a vertical circle about a fixed point O as illustrated in Figure 6.9. Determine the tangential acceleration of the sphere and the tension in the cord at any instant when the speed of the sphere is v and the cord makes an angle u with the vertical.

Conceptualize Compare the motion of the sphere in Figure 6.9 with that of the child in Figure 6.6a associated with Example 6.5. Both objects travel in a circular path. Unlike the child in Example 6.5, however, the speed of the sphere is not uniform in this example because, at most points along the path, a tangen-tial component of acceleration arises from the gravitational force exerted on the sphere.

Categorize We model the sphere as a particle under a net force and moving in a circular path, but it is not a particle in uniform circu-lar motion. We need to use the techniques discussed in this sec-tion on nonuniform circular motion.

Analyze From the force diagram in Figure 6.9, we see that the only forces acting on the sphere are the gravitational force

AM

S O L U T I O N

Figure 6.9 (Example 6.6) The forces acting on a sphere of mass m connected to a cord of length R and rotating in a vertical circle centered at O. Forces acting on the sphere are shown when the sphere is at the top and bottom of the circle and at an arbitrary location.

R

O

mg sin u

u

umg cos uvbotS

vtopS

TS

TbotS

TtopS

mgS

mgS

mgS

Figure 6.7 When the net force acting on a par-ticle moving in a circular path has a tangential component o Ft , the particle’s speed changes.

!

!

!

FS

FtS

FrS

The net force exerted on the particle is the vector sum of the radial force and the tangential force.

Non-Uniform Circular Motion Example

In the radial direction:

Fr ,net =mv2

R

T −mg cos θ =mv2

R

T =mv2

R+mg cos θ

T = mg

(v2

Rg+ cos θ

)

Summary

• Uniform circular motion with forces

• Centripetal force

• Banked turns

• Non-uniform circular motion

(Uncollected) Homework Serway & Jewett,

• Ch 6, onward from page 169. Questions: Section Qs 1, 5, 7,9, 15, 17

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