Dynamics

Friction Relationships

Friction

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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.

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Object's velocity is increasing.

This object is moving. Describe its velocity. Which way is it moving?

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Constant Velocity, Fnet = 0

The car is moving to the right.

At Constant Velocity

Fapplied = Friction Force

a = 0

Fnet = 0

When an object is moving on a horizontal surface, the normal force equals…

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…the weight force.

Fn = weight force

= w = mg

On a Horizontal Surface

fs = Fn·μs = Fg·μs = mg·μs

fk = Fn·μk = Fg·μk = mg·μk

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

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Hint: fk = mgμk

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

Try it

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.)

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Kinetic Friction Would Double

On a Horizontal Surface

Static Friction equation

fs = Fn·μs = Fg·μs = mg·μs

Kinetic friction equation

fk = Fn·μk = Fg·μk = mg·μk

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

a) μk (the roughness number) were halved?

fk = Fn·μk

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fk would be halved

b) tripled the weight force (F)?

fk = w·μk

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fk would be tripled

c) turned the block on its side?

fk = Fn·μk = w·μk = mg·μk

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fk would be unchanged

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