Friction

Ms Mikaela Fudolig

Physics 71

Friction

Friction is a force exerted by the surface on an objectPARALLEL to the surfacePERPENDICULAR to the normal forceDISSIPATING force

STATIC and KINETIC friction

Static Friction,

Balances the force you apply on an objectobject REMAINS AT REST (v=0)magnitude:

sf

,max0 s sf f

,maxs sf Nnormal force

coefficient of static friction

Exercise 1a

rough surfaceno

applied force

?sf

v=0

Exercise 1b

rough surface

?sf

F

v=0

sf

ˆ( ) 0net sF F f i

Exercise 1c

rough surface

?sf

F

v=0, box just about to slide

sf

ˆ( ) 0net sF F f i

Exercise 1c

rough surface

?sf

F

v=0, box just about to slide

sf

s sf N

Kinetic Friction,

acts OPPOSITE the direction of MOTION of an objectv0

kf

k kf N

normal force

coefficient of kinetic friction

k s

Kinetic Friction, kf

k s

Kinetic Friction

Draw free-body diagram.Identify the direction of motion.Identify the direction of fk.

Apply Newton’s 2nd Law.

Exercise 2a

You want to move a 1-N object across a flat surface with s=0.5 and k=0.2. What force should you exert on the object for the object to start moving?

Exercise 2b

The 1-N object is now moving, and you want to keep it moving at constant velocity. What is the magnitude of the force that you must exert on the object? s=0.5 and k=0.2.

Exercise 2c

An object weighing 1N is at rest on a flat surface with s=0.6 and k=0.4. If you exert on the object a horizontal force of 0.4N, will the object move?

Exercise 2d

An object is at rest on a flat surface. You exert on the object a force in the +x direction. Can friction be strong enough to move the object in the –x direction?

Example 1

A box of mass m is initially at rest on a ramp inclined at an angle above the horizontal. The angle is gradually increased until it has reached a critical value C. At this angle, the box is about to slide. What is s?

Exercise 2e

An object is on a flat surface, moving to the left. In what direction does friction act? Is the friction static or kinetic?

An object is on a flat surface, moving to the right. In what direction does friction act? Is the friction static or kinetic?

Exercise 3

A box is going down a ramp inclined at 30 above the horizontal. Its speed increases by 10m/s after 10s. What is the coefficient of kinetic friction?

Exercise 4: A girl pulling a box

Exercise 4: A girl pulling a box

The box’s mass is 20kg. If k=0.4 and =30, what should the tension in the string be if the box is to move at constant velocity?

Conceptual Exercise 5

In what direction should the girl pull the box to decrease the magnitude of the frictional force (hence, possibly increase the box’s acceleration)?

Example 2

Given: s, k

Find the mass m2 for which mass m1 moves up the plane with constant velocity.

Exercise 6

If the masses are moving at constant velocity, what is the tension in the string?

Exercise 7

Given: s, k

Find the mass m2 for which mass m1 moves down the plane with constant velocity.

Exercise 8

Given: s, k

Find the mass m2 for which mass m1 is just about to move.

Fluid Resistance

aka “drag force”“friction” in gases and liquidscaused by motion of fluid

Fluid Resistance: Direction

OPPOSITE the direction of the velocity of the moving body (relative to the fluid)

Conceptual Exercise 8

In what direction is the force exerted by the fluid on the ball if:

vdragF

v

dragF

v

dragF

Fluid Resistance: Magnitude

Speed of object

Fluid resistance

f kv 2f DvLow speed High speed

Exercise 9

Draw the free-body diagram of a rock (dropped from a height h) falling in air. Include air resistance.

mg

f

Exercise 9

Draw the free-body diagram of a rock (dropped from a height h) falling in air. Include air resistance.

At t=0, what is the acceleration of the rock? What happens to the acceleration at t=5?

mg

f

Acceleration in the presence of Fluid Resistance

NOT constant!

Terminal Speed

Eventually, the moving body will stop accelerating and reach its terminal speed.

Set a=0:

t

mgv

k

Terminal speed for objects

moving at low speeds

Terminal Speed

Eventually, the moving body will stop accelerating and reach its terminal speed.

Set a=0:

t

mgv

D

Terminal speed for objects

moving at high speeds

Conceptual Exercise 10

A bowling ball and a styrofoam ball of the same size are dropped from the same height. Which reaches the ground faster (in the presence of air resistance)?

Conceptual Exercise 11

What happens to the range of the projectile when there is air resistance?

What happens to the maximum height of a projectile if there is air resistance?

Summary

At rest/constant velocity a=0, F=0accelerating a0, F 0|a| and |v| are equal for connected

bodiestension in a massless and taut string is

the same everywhere on the string

Summary

Friction opposite the direction of motion

Static friction at rest/about to moveKinetic friction moving (constant

velocity/accelerating)

Summary

Fluid resistance:direction: OPPOSITE the direction of the

velocity of objectmagnitude: increases with object’s speedterminal velocity

mass, mdrag coefficient (D, k)