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Friction

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D) Friction - force between 2 surfaces that opposes motion

Depends only on:

1) texture of each surface

What do plows have on their wheels to increase  friction?

Chains

2) force holding objects together

What do race cars have mounted on their rear hood to increase  their friction?

Spoilers increase the force between tire and road

Why do some people put barbells in their car trunk on a snowy day?

Weights in a car truck
increase the tires force on the road

Two Kinds of Friction

(f_s) 1) Static Friction - force of friction that must be overcome to move an object at rest.

(f_k) 2) Kinetic Friction - force of friction that opposes a moving object.

Kinetic friction is weaker than static friction

f_k<f_s

Friction Equations

Static friction: f_s= u_sF_n    

Kinetic friction: f_k = u_kF_n

f_s = force of static friction

f_k = force of kinetic friction

F_n = normal force (supporting force)

u_s = coefficient of static friction - "the roughness number"

u_k = coefficient of kinetic friction - "the roughness number"

1. Relates to texture of surfaces in contact

2. Calculated in lab

ice and skate - low uk

What do skiers do to their skis to lower the coefficient of friction between their skis and  the snow?

Skiers put wax on the bottom
of their skis to lower the coefficient of friction

The FRICTION force vector
always drawn opposite to motion

This object is moving to the right. Describe its velocity.

Object's velocity is increasing

This object is moving to the right. Describe its velocity.

Constant Velocity, Fnet = 0

*** At Constant Velocity:

 F_applied = Friction Force

a = 0

F_net = 0

F_n = weight force

= w = mg

On a horizontal surface:

f_s= F_nu_s = wU_s = mgU_s

f_k= F_nU_k = wU_k = mgU_k

Both blocks are being pulled at constant velocity. What is the reading on the force scale on the  right?

Hint: fk = mgUk

f = 10 N, m, g, Uk don't change when blocks are stacked!

Ex 1) If an object is moving on a horizontal surface and you doubled the weight force on the object, how would that change the kinetic friction force?

(Hint: weight force = normal force on horizontal surface.)

kinetic Friction
 Would Double 

On a horizontal surface

Static Friction equation

f_s= F_nu_s = wu_s = mgu_s

Kinetic friction equation

f_k= F_nU_k = wU_k = mgU_k

1) What would happen to the kinetic friction (f_k) if:

a) U_k (the roughness number) were halved?

f_k= F_nU_k

f_k  would be halved

b) tripled the weight force (F)?

f_k= wU_k

f_k  would be tripled

c) turned the block on its' side?

f_k= F_nU_k = wU_k = mgU_k

f_k  would be unchanged

Friction Questions