How to Determine the Velocity of a 100kg Crate Pulled by a Motor?

[adoado]

Homework Statement

Determine the velocity of the 100kg crate being pulled horizontally by a rope connected to a motor in 15s if the motor operates with an efficiency of e = 0.8. The power input of the motor is 2Kw. The coefficient of kinetic friction between the crate and the plane is 0.2.

The Attempt at a Solution

The friction force I worked out to be 20g N.

Hence, the equation of motion given by Newton's second law is:

Friction - MotorForce = ma.
20g - MF = 100a.

From here, I need the velocity. So I know the motor's power, P. So P/v = MF.

20g - P/v = ma.

This is where I get stuck. I would usually assume the question means no acceleration, but the time is stated (15sec). Similarly, the assumption does not hold in accordance with the answer...

Any advice would be greatly appreciated ;)
Adrian

 

[diazona]

So P/v = MF.

That equation is normally only useful when the velocity is constant. In this case, you can't assume the velocity is constant.

Have you tried doing it with energy?