St. Mary's H.S. Physics - Intro to Friction Notes

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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

to the right.

Describe its velocity.

Constant Velocity, Fnet = 0

*** 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 = 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: 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 = 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