Question

a) We raise a hammer a certain fixed distance with a constant speed. Next, we raise the same distance, but with a constant acceleration upwards. Compare the work done in both cases. b) A horse is hooked to a car. Since the car pulls behind the horse as strong as this car pull, why is the car not in balance, no matter how hard the horse pulls the car?

Answer 1

A) For raising a hammer through a distance we only increase its potential energy but to accelerate the hammer through that distance will also increase its kinetic energy because accelerating will cause increase in speed resulting in increase in kinetic energy.

So the work done in second case i.e the accelerating case will be more.

B) The force between the horse and car is equal on each other, other wise the cord connecting horse and car would either break or loosen. But the car will not be in balance because one force acting on it is the pull by the horse and the other is the friction which are not equal in magnitude. So that is why the car moves. If in case the car doesnot move it implies that the pulling force by the horse is not enough to overcome the frictional force on the car.