# Kinetic energy physics exercise

1. Nov 4, 2014

### Fabian901

I'm going through a physics exercise at high school level. A ball is dropped through a slide and when it reaches the bottom of the slide it will travel horizontally. Assuming there is no energy loss due to friction or air resistance draw a velocity time graph.
I tried working it out, the answer on the book says that when the ball reaches the bottom of the slide it will start moving horizontally with a positive constant velocity. I'm a bit confused on the constant velocity part. When the ball is moving horizontally, it obviously has KE. And the fact that it has energy doesn't it mean that there is a force applied through a distance, and therefore it should accelerate?

2. Nov 4, 2014

### Staff: Mentor

The ball's total energy is constant through the entire experiment. A force is only required to gain energy. All this experiment does is convert potential energy to kinetic energy.

3. Nov 4, 2014

### Fabian901

Many thanks Russ! I don't think I understand the principle of conservation of enery that well then. Imagine that before someone put the ball on top of the slide the ball was standing on the floor with no motion, what was the energy of the ball standing still on the floor? Is it 0 joules? Was energy being created to place the ball on top of the slide? If so, wouldn't this contradict the principle of conservation energy which states that energy cannot be created or destroyed?

4. Nov 5, 2014

### Staff: Mentor

Measured with respect to the floor, the mechanical energy of the ball would be zero. (0 gravitational PE + 0 kinetic energy)

Energy was used to lift the ball and place it on the top of the slide, but that energy wasn't 'created'. For example, if a person lifted it, that person had to do work on the ball, essentially converting some biochemical energy to give the ball increased potential energy.

Not to worry. Conservation of energy is safe and sound!

5. Nov 6, 2014

### Fabian901

Thanks a lot for the help guys!