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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, Fnet = 0

The car is moving to the right.





*** At Constant Velocity:


 Fapplied = Friction Force


a = 0

Fnet = 0





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



... the weight force.

Fn = weight force


= w = mg






On a horizontal surface:


fs= Fnus = FgUs = mgUs


fk= FnUk = FgUk = mgUk






Both blocks are being
pulled at constant
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

fs= Fnus = Fgus = mgus


Kinetic friction equation

fk= FnUk = FgUk = mgUk



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


a) Uk (the roughness number) were halved?



fk= FnUk



f would be halved




b) tripled the weight force (F)?

fk= wUk

f would be tripled




c) turned the block on its' side?

fk= FnUk = wUk = mgUk



f would be unchanged




Friction Questions




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