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Physics 101Thursday 9/15/11Class 7

Chapter 5.3 – 5.5 Newtonʼs Second law of motion Newtonʼs Third law of motion Forces in 2 dimensions

In the SI (metric) system, the unit of force is the .. A) Galileo B) joule C) newton D) Eiffel E) none of these

Reading Quiz

Newton's 1st Law

You kick a smooth flat stone out on a frozen pond. The stone slides, slows down, and eventually stops. You conclude that: A) the force pushing the stone forward finally stopped pushing it B) no net force acted on the stone C) a net force acted on it all along D) the stone simply “ran out of steam” E) The stone has a natural tendency to be at rest

Newton's 2nd Law

Newton's 2nd Law: Solving problems with this: 1) Sketch the Forces 2) Isolate the particle of interest 3) Choose a convenient coordinate system 4) Resolve Forces into components with this coordinate system 5) Apply the 2nd law to each component

!F = m

!a

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Newton's 2nd Law 1-D

Situation I: A constant force is exerted for a short time interval on a frictionless cart that is initially at rest. The cart acquires a final velocity vf. (1D Motion) Situation II: The same constant force is exerted for the same short time interval on a frictionless cart that is initially moving at velocity v1. The cart has a final velocity of v2 . The change in the speed of the cart Δv = v2 - v1 , compared to the final speed in situation I, is.. A: the same, Δv = vf. B: greater, Δv > vf. C: less, Δv < vf. D: answer depends on the sign of v1 and v2.

Newton's 2nd Law 1-D

From rest you push on the accelerator of your car, providing a force F for 4 secs. If the force was only ½ F, how long would the force need to be supplied to reach the same final speed? A) 16 s B) 8 s C) 4 s D) 2 s E) 1 s

Newton's 2nd Law 1-D

A force F acts on mass M for a time interval T, giving it a final speed v. If the same force acts for the same time on a different mass 2M, what would be the final speed of the bigger mass? A) 4v B) 2v C) v D) 1/2 v E) 1/4 v

Newton's 2nd Law 1-D

A constant rightward force (F) is exerted for a short time interval (t) on a cart that is initially at rest on an air track. This gives the cart a certain velocity, v. Suppose we begin with the cart moving with the velocity v above, and then we apply a leftward force (-F) for the same short time interval (t). What will the cart be doing after time t? A: Moves forward with 2*v. B: Moves forward with v. C: Final speed is 0. D: Cannot be determined from the info provided.

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Free-body diagram Free-body diagram

A glider on a level air track is coasting along at constant

velocity. Which of the following free-body diagrams correctly indicates all the forces on the glider? Assume

there is air resistance.

v = constant

A B C D

E) None of these

Free-body diagram Force components

The vector !A is shown. Axes and

positive direction have been chosen as

shown. What is Ax?

A) Ax = A cos!

B) Ax = A sin!

C) Neither of these.

x

y

!

!A

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Net Force

An astronaut picks up a rock. The rock has a mass of 5.0 kg, and on this planet it’s weight is 40 N. If the astronaut exerts an upward force of 45 N on the rock, what is it’s acceleration? A) 9 m/s2

B) 1 m/s2

C) 8 m/s2

D) 17 m/s2

Newton's 2nd Law 1-D

Next Week Force Components

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Force Components

Three people are pulling on a ring in a "2-D" tug of war.

Shown is a "top view".

No one is winning - the

ring is sitting still.

The pulls are configured

as shown (teams 1 and 2

are each pulling with a

force of 100 N.

Team 3 pulls with

unknown force T3)

Which expression gives the sum of the forces in the y

direction? (calling up "+") Fnet, y =

A) +200 N B) +200 N * sin(45) (= +141 N)

C) 0 N D) 200 N * sin(45) - T3

E) More than one of the above is correct.

100 N 100 N

45045

0

T3 ?

Force Components

Newton's 3rd Law

A moving van collides with a sports car in a high-speed head-on collision. Crash! During the impact, the truck exerts a force Truck on the car and the car exerts a force Far on the truck. Which of the following statements about these forces is true) A) Ftruck = Fcar’

B) Ftruck > Fcar C) Ftruck < Fcar

 When you climb up a rope, the first thing 

you do is pull down on the rope.  How do 

you manage to go up the rope by doing 

that?  a) this slows your initial velocity, which is already upward

b) you don’t go up, you’re too heavy

c) you’re not really pulling down—it just seems that way

d) the rope actually pulls you up

e) you are pulling the ceiling down

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Newton's 3rd Law

A giant, extremely massive moon is headed straight for an inhabited planet. The Enterprise has decided to "tractor" the moon out of the planet's path. The Capt. orders the tractor beam on at maximum strength, (but forgets to ALSO order their impulse engines to be turned on) What happens? A: The moon is tractored out of the planet's path, slowly but surely, and the Enterprise saves the day once again. B: The Enterprise accelerates rapidly towards the moon and crashes into it, killing all aboard and ending the series.

Newton's 3rd Law

Consider a car at rest. We can conclude that the downward gravitational pull of Earth on the car and the upward contact force of Earth on it are equal and opposite because A: the two forces form an "action reaction" pair. B: the net force on the car is zero. C: neither of the above

Newton's 3rd Law

A locomotive engine pulls a series of train cars. Which is the correct analysis of the situation? A: The train moves forward because the locomotive pulls forward slightly harder on the cars than the cars pull backward on the locomotive. B: The locomotive's force on the cars equals the force of the cars on the locomotive, but the frictional force on the locomotive is forward and large, while the backward frictional force on the cars is smaller. © From Mazur, "Peer Instruction"

Newton's 3rd Law

Contact forces

!a

!F

m1

m2

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 If you push with force F on either the heavy box (m1) or the light box (m2), in which of the two cases below is the contact force between the two boxes larger? 

a) case A

b) case B

c) same in both cases

F m2

m1

A

F m2

m1

B 2m m

F

 Two blocks of masses 2m  and 

m  are in contact on a horizontal 

fricEonless surface.  If a force F 

is applied to mass 2m, what is 

the force on mass m ? 

a) 2F

b) F

c) ½F

d) 1/3F

e) ¼F

Next Week

A force F acts on mass m1 giving acceleration a1. The same force acts on a different mass m2 giving acceleration a2 = 2 a1. If m1 and m2 are glued together and the same force F acts on this combination, what is the resulting acceleration?

A) ¾ a1 B) 3/2 a1

C) ½ a1 D) 4/3 a1

E) 2/3 a1

F a1

a2 F

F a

m1

m2

m1 m2

Next Week

Reading Assignments: Tuesday 9/20 sections 5.6 – 6.1 Thursday 9/22 sections 6.2 – 6.5