Show Me The Physics - Intro to Friction Notes

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Two bricks slide across the same table. One brick is 2 kg and the other has a mass of 4 kg. Which experiences a greater amount of friction?

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Kinetic Friction or Static?

Name two characteristics of this friction.

FRICTION force vector
always drawn opposite to motion

This object below is

moving to the right.

Describe its velocity.

Object's velocity is increasing.

This object is moving.

Describe its velocity.

Which way is it moving?

Constant Velocity, F_net = 0

The car is moving to the right.

*** At Constant Velocity:

F_applied = Friction Force

a = 0

F_net = 0

*** When object is moving
on a horizontal surface,
the normal force equals .....

... the weight force.

F_n= weight force

= w = mg

On a horizontal surface:

f_s= F_nu_s = F_gU_s = mgU_s

f_k= F_nU_k = F_gU_k = mgU_k

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

Hint: f_k = mgU_k

f = 10 N,

m, g, U_kdon'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 = F_gu_s = mgu_s

Kinetic friction equation

f_k= F_nU_k = F_gU_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