# Dynamics Block

## Homework Statement

A 2.5 kg block is initially at rest on a horizontal surface. A horizontal force F of magnitude 6.0 N and a vertical force P are then applied to the block (the vertical force is upwards). The coefficients of friction for the block and surface are Ms = 0.40 and Mk = 0.25. Determine th emagnitude of the frictional force acting on the block if the magnitude of P is a) 8.0 N, b) 10 N, and c) 12 N

fs = Mk * Fn
Fnet = ma
Fg - Fn - P = 0

## The Attempt at a Solution

I tried using the third equation to get Fn and plug this value into the first equation to get force of friction. I didn't get the right answer... for P = 8.0 N the answer should be 6.0 N. How?

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Doc Al
Mentor
Show exactly what you did. Hint: First you must determine whether the relevant friction is static or kinetic.

Show exactly what you did. Hint: First you must determine whether the relevant friction is static or kinetic.
Fg - Fn - P = 0
24.5 = Fn + 8
Fn = 16.5

fs = 16.5 * 0.25 = 4.1 N

Doc Al
Mentor
Fg - Fn - P = 0
24.5 = Fn + 8
Fn = 16.5
Good.

fs = 16.5 * 0.25 = 4.1 N
This assumes that the block moves. Does it?

Good.

This assumes that the block moves. Does it?
well Yea if the force applied is 6.0 N and the friction is 4.1 then yes it should move.

well Yea if the force applied is 6.0 N and the friction is 4.1 then yes it should move.
Oh wait no it doesn't because Fn * Ms = 8.25 which is greater than 6 N so it doesn't move. Now what?

Doc Al
Mentor
Oh wait no it doesn't because Fn * Ms = 8.25 which is greater than 6 N so it doesn't move. Now what?
How does static friction work? Note that for static friction, μN is the maximum possible static friction that the surfaces can generate before being forced to move. But static friction will be only what it needs to be to prevent slipping (up to the maximum). How much does static friction need to be in this case to prevent slipping?

How does static friction work? Note that for static friction, μN is the maximum possible static friction that the surfaces can generate before being forced to move. But static friction will be only what it needs to be to prevent slipping (up to the maximum). How much does static friction need to be in this case to prevent slipping?
6 to prevent slipping?

Doc Al
Mentor
6 to prevent slipping?
Right!