Question

Engine of a car of mass 1500kg, keeps car moving with constant velocity 5m/s. If frictional force is 1000 N, power of engine is?

Answer 1

The power is the work done per unit of time.

The mass of the car is irrelevant. It only really matters if you try and accelerate the car, or take it uphill. To convince yourself of that, think about how little power you would need to keep the car going if it were sliding on perfectly smooth ice - in fact it would be hard to slow the car down.

If you are pushing against a 1000N force over the distance of 5m, you are doing 5000Nm of work, which is 5000J.

Since it takes one second to cover that 5m, you are doing 5000J of work in a second, which is 5000J/s=5000W.

So the power is 5000Watts.