Potential Energy = Kinetic Energy

1. Dec 23, 2003

Ohm

Even the average student can assume in cases where a tennis ball falls freely or is hit that Ke = Pe.
The question is why do we take it as if they are equal?

2. Dec 23, 2003

HallsofIvy

Average students probably assume a lot of things that aren't correct. It certainly isn't the case that "where a tennis ball falls freely or is hit that Ke= Pe". As a tennis ball falls its Kinetic energy is increasing and its Potential energy is decreasing. They can't always be the same. In addition, while kinetic energy depends upon speed, potential energy is always relative to some arbitrary reference point. Because of that you can always take potential energy to be equal to kinetic energy as some specific time but they won't stay equal.

3. Dec 23, 2003

Ohm

Thanks.

4. Dec 23, 2003

AHolico

What can be equal in those ideal cases is that the change in potential energy = - change in kinetic energy. This comes from the fact that you are dealing with conservatory forces and therefore the total energy is conserved (total energy = kinetic energy+potential energy). Therefore, since the total energy is a constant, if the potential energy goes down, then the kinetic energy goes up (case of a tennis ball falling freely).

5. Dec 23, 2003

Staff: Mentor

To say it in a different way, kinetic energy at impact equals potential energy at release for a dropped tennis ball. This is true if you make some assumptions such as no wind resistance.

6. Dec 23, 2003

Ohm

Yup, energy is conserved in isolated systems. Thanks for the insight. Would you believe that i finished Mechanics A-level at school(Average mark 87.5%) and i was not familiar with the reason of equating Loss in Pe with Gain in Ke ? Lately i discovered that i was not familiar with basic physics - I relied heavily on memory when i was doing O-level. In fact, i discovered that the majority of my friends who are doing A-level physics know everything by heart. I mean I can solve simple "critical" questions but when it comes to using basic theory to "extract" an idea, well that's where I get stuck.
Most Uncanny though, is the fact that i managed to get through Mechanics A-level incredibly successfuly. I'm currently searching on the internet to get my self obtained with basic information and get puzzled with questions. I'm gonna be using your forums since i do not consider internet to be a perfectly reliable source.