LECTURE 8 Ch5 F16 covered

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Lecture 8

Chapter 6

Friction forces

Physics I

Course website:http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsI


Today we are going to discuss:

Chapter 6:

Some leftover (Ch.5) Kinetic/Static Friction: Section 6.4 Gravity: Section 6.3 (read if you want. We will get to

this in Ch.13)


Exam I

Results

Max 30 20 20 20Average 20 11.6 8.0 12.7

Average 52.3 points out of 90 pointsAverage 58.1%

MCQ Pr.1 Pr.2 Pr.3

I added three problems to your current homework!


Some leftover Ch5.


Newton’s laws

In 1687 Newton published his three laws in his Principia Mathematica.

These intuitive laws are remarkable intellectual achievements and work spectacular for everyday physics


Newton’s 1st Law (Law of Inertia)

In the absence of force, objects continue in theirstate of rest or of uniform velocity in a straight line

i.e. objects want to keep on doing what they are already doing

It helps to find inertial reference frames


Inertial reference frame

An inertial reference frame is one in which Newton’s first law is valid.

Inertial reference frame– A reference frame at rest – Or one that moves with a constant velocity

This excludes rotating and accelerating frames (non-inertial reference frames), where Newton’s first law does not hold.

How can we tell if we are in an inertial reference frame? - By checking if Newton’s first law holds!


A physics student cruises at a constant

velocity in an airplane.A ball placed on the

floor stays at rest relative to the airplane.

Inertial Reference Frame (Example)

There are no horizontal forces on the ball, so when .

Newton’s first law is satisfied, so this airplane is an inertial reference frame.


Newton’s first law is violated, therefore this airplane is not an inertial reference frame.

Non-Inertial Reference Frames (Example)A physics student is

standing up in an airplane during takeoff.

A ball placed on the floor rolls toward the back of the plane.

There are no horizontal forces on the ball, and yet the ball accelerates in the plane’s reference frame.

In general, accelerating reference frames are not inertial reference frames.


Newton’s Second Law of MotionNewton’s second law is the relation between acceleration and force. Acceleration is proportional to force and inversely proportional to mass.

amF

2smkgN

New unit of force Newton

• It takes a force to change either the direction or the speed of an object.

• More force means more acceleration;

• the same force exerted on a more massive object will yield less acceleration.


Mass

Mass is the measure of inertia of an object, sometimes understood as the quantity of matter in the object. In the SI system, mass is measured in kilograms.

Mass is not weight.

Mass is a property of an object. Weight is the force exerted on that object by gravity.

If you go to the Moon (gravitational acceleration is about 1/6 g),you will weigh much less. Your mass, however, will be the same.

amF Proportionality defines mass of object.


ExampleWhat average net force is required to bring a 1000-kg car to rest from a speed of 20 m/s within a distance of 100 m? Assume acceleration is constant

Force to stop a car.

When the fly hit the truck, it exerted a force on the truck (only for a fraction of a second). So, in this time period, the truck accelerated (backward) up to some speed. After the fly was squashed, it no longer exerted a force, and the truck simply continued moving at constant speed.

ConcepTest Truck on Frozen LakeA very large truck sits on a frozen lake. Assume there is no friction between the tires and the ice. A fly suddenly smashes against the front window. What will happen to the truck?

A) it is too heavy, so it just sits thereB) it moves backward at constant

speedC) it accelerates backwardD) it moves forward at constant speedE) it accelerates forward


FrictionStatic Kinetic

Kinetic/Static Friction: Section 6.4


A new contact force…friction

Friction is always present when two solid surfaces slide along each other.

The microscopic details are not yet fully understood.


Friction. Static-vs-Kinetic

m

mg

m

NFA

mg

N

mg

m

NFA

fsfs

no motionstatic

no motionstatic

Static. Limit.Motion is about to start

Fx max0 sA fF

As Ff

0

ASs FNf

frf

AF

NS

45°

m

NFA

fkSliding.

Friction (kinetic) is constantNf Kk

As Ff

Kinetic friction

NKk Ff Static friction

frictionstaticoftcoefficienS frictionkinetic

oftcoefficienK

NS

Nf Ss

If a horizontal force is applied on an object , the object does not move.It means a second force, Static Friction, must be opposing the applied force.

An object remains at rest as long as fs < fs max.

The object just begins to slip when fs = fs max.


Static friction

Static friction applies when two surfaces are at rest with respect to each other (such as a book sitting on a table).

The static frictional force is as big as it needs to be to prevent slipping, up to a maximum value.

Nf Ss

Static friction points opposite the direction in which the object would move if there were no static friction.

N


Kinetic friction

Sliding friction is called kinetic friction.

Approximation of the kinetic frictional force:

N is the normal force

μk is the coefficient of kinetic friction, which is differentfor each pair of surfaces.

Nf Kk

Usually it is easier to keep an object sliding than it is to get it started.

Movie “New in Town” (6.09)

N

The kinetic friction force is directedopposite to the velocity of the object relative to the surface


Coefficients of FrictionNote that, in general, μs > μk.


Summary


ExampleA 10-kg box moves on a horizontal surface. The system has coefficients of friction μK=0.5. [Use g=10 m/s2]A horizontal force of 60N is applied. What is the acceleration?

Fy may

x

y

Fg=mg

mFfr

N

=μKNmgN

NF Kfr mgK

maFFF frAx

FA

mamgF KA m

mgFa KA 2

2 110

10105.060s

msm

kgkgN

The static friction force has a maximum of sN = 50 N. The tension in the rope is only 30 N. So the pulling force is not big enough to overcome friction.

ConcepTest Will It move?A) moves to the left

B) moves to the right

C) the box does not move

D) moves down

E) moves up

A box of weight 100 N is at rest on a floor where s = 0.5.A rope is attached to the box and pulled horizontally with tension T = 30 N. Which way does the box move?

T=30Nm

Follow-up: What happens if the tension is 55 N?

NSfr FF NmgS 501005.0


Demonstration

Two Interleaved BooksSimply lay the pages of two phone books on top of each other one by one before

attempting to pull them apart.

FA Fperp

Fpull

It increases friction

It pulls books apart

Ffr

NxFfr

http://www.youtube.com/watch?v=AHq82D78Igg


mg

N

How to measure μS?

Fx max

Fy mayGiven: μs, with frictionFind: θ

A block lies on an inclined plane with μk. The angle of the incline is slowly increased. At what angle does the block start to move?

Ffr0cos mgNFy

cosmgN NF Sfr cosmgS

Fx mgsin Ffr 0mgsin smgcos 0

mgsin smgcossincos

tan s

0

About to start moving

By measuring angle, when an object gets lose, we can find static friction coefficient


Thank youSee you on Monday

The block is moving at constant speed, so

it must have no net force on it. The forces

on it are N (up) and mg (down), so N = mg,

just like the block at rest on a table.

ConcepTest Going UpA block of mass m rests on the floor of an elevator that is moving upward at constant speed. What is the relationship between the force due to gravity and the normal force on the block?

A) N > mg

B) N = mg

C) N < mg (but not zero)

D) N = 0

E) depends on the size of the elevator

m

v

v=consta


A 65-kg woman descends in an elevator that briefly accelerates at 0.20g downward. She stands on a scale that reads in kg. What does the scale read?

y

m

Example: Apparent weight loss.

Y direction

) )

Although her weight is still mg, the scale would read less, 0.8mg

Woman is a described

object

An object in free fall has no weight!

ConcepTest Going Up I A 50-kg student (mg = 490 N) gets in

an elevator at rest and stands on a metric

bathroom scale. Sadly, the elevator cable

breaks. What is the student’s weight during

the few second it takes the student to

plunge to his doom?

A) > 490 N

B) 490 N

C) < 490 N (but not zero)

D) N = 0

E) depends on the size of the elevator

m

0If an object is accelerating downward with ay = g

ym

ay = g

He is weightless like an astronaut.

LECTURE 8 Ch5 F16 covered

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