# Kinetic Friction Problem

1. Mar 1, 2009

### DoctorB2B

1. The problem statement, all variables and given/known data

A force of 85N is applied to a block on an incline that makes an angle of 35 degrees with the horizontal. The coefficient of kinetic friction between the block and the incline is $$\mu$$k=0.45. The block is moving and accelerating up the incline at 1.5 m/s2.

2. Relevant equations

(a) Find the mass of the block
(b) If the block is initially moving down at 2.6 m/s, what is its speed 3.6s after the 85N force is applied?

3. The attempt at a solution

(a) F=ma --> m= 85N/1.5m/s = 56.7kg
(b) I have no idea

2. Mar 1, 2009

### LowlyPion

Unfortunately the acceleration is after taking away for frictional slowing. So the net acceleration doesn't reflect the acceleration of the mass or the downward acceleration from gravity down the incline for that matter.

3. Mar 1, 2009

### AEM

You should actually write Newton's second law in this instance as

$$\Sigma F = ma$$

You have three force to consider: (1) The applied force of 85 Newtons; (2) The downward force of gravity; The force of friction. You have to write out the resulting equation in full and then you can compute the mass.

For the second part, you will have to use the same equation to find the acceleration. Then you can use one of your kinematic equations to compute the velocity.