- #1

Karagoz

- 52

- 5

## Homework Statement

From a physics multiple choice test: "A car is raised to a height of 10 meters. The car is at rest both before and after it has been lifted. The mass of the car is 900 kg. The total work that has been done on the car during the move is"

(The right answer is: zero).

Wnet =ΔKE (Net work = Change in Kinetic energy). The car's kinetic energy remains the same (it's still on rest), so in this equation no work is done on the car.

But how is it that the work done on the car is zero?

Doesn't it contradict with the equation: W = F*s?

A force upwards is applied to the car to raise it up, it requires force against the gravity force to raise the car. And the car is displaced 10 meters upwards with that force.

The force exerted on the car to lift it up is: 900KG*9.81 = 8,829 N.

So according to this equation: W = F*s the work is done on the car, because it's been displaced by a force.

The car is still on rest (no kinetic energy), but the car's potential energy is increased by ca 88 Kj (mgh = 900Kg*9.81*10m). Doesn't it mean that it's been done some work to increase the car's potential energy?

## Homework Equations

Wnet =ΔKE (Net work = Change in Kinetic energy). The car's kinetic energy is the remains the same (it's still on rest), so in this equation no work is done on the car.

But according to this equation: W = F*s the work is done on the car, because it's been displaced by a force.

## The Attempt at a Solution

I read some answers in Stack Exchange and PhysicsForums, but that just confuses even more:

https://www.physicsforums.com/threads/work-done-to-lift-an-object.828424/

https://www.physicsforums.com/threads/work-done-in-lifting-an-object-against-gravity.624909/

https://www.physicsforums.com/threads/work-done-lifting-an-object-1m-simple-question.477475/