The trolley experiment
One simple setup settles what kinetic energy depends on, and the answer turns out to be two things rather than one.
Figure to be added
Method
- Stand a light trolley B on a horizontal surface.
- Push an object A along the surface so that it strikes the trolley, and note how far the trolley is pushed forward.
- Repeat, sending the same object in at a greater velocity.
- Repeat again with objects of different mass, each sent in at the same velocity.
Observation
The greater the velocity given to the object, the further the trolley travels.
The greater the mass of the object, the further the trolley travels.
What the trolley is measuring
When A strikes B, part of the energy A held because of its motion is transferred to the trolley, and the trolley begins to move. The distance the trolley runs is therefore a measure of how much energy it received — so anything that makes the trolley travel further means the object arrived with more energy.
Two factors, not one
Kinetic energy depends on the object's mass and on its velocity. Any formula for it must therefore contain both, which is what the next page confirms.