# Homework Help: Finding the kinetic energy

1. Feb 14, 2010

### lbella026

1. The problem statement, all variables and given/known data
A high diver, mass 50kg sits atop the diving platform with 10,000 Joules of stored gravitational potential energy (PE). The platform has a height of 20m. The diver makes his leap. When the diver has dropped 3/4 of the height what is diver total energy (KE + PE).

2. Relevant equations
KE=(1/2)mass x speed squared

3. The attempt at a solution
I don't know how to find the Kinetic energy using the equation with out knowing the speed.

2. Feb 14, 2010

### pgardn

Have you "guys" done any conservation of Energy problems?

3. Feb 14, 2010

### lbella026

We havent yet. I'm unsure of what equation to use which would apply. That equation and the W=force x distance are the only ones I can find in the book. Is there another equation I could use? thanks!

4. Feb 14, 2010

### pgardn

So you have not talked about Potential Energy, or gravitational potential energy in class? Along with F x d = Work and Kinetic energy?

5. Feb 14, 2010

### lbella026

ohhh would the potential energy be the same as the kinetic energy?? KE =10,000 J. would the potenial energy be lost and kinetic energy gainned?

6. Feb 14, 2010

### pgardn

The diver has the same amount of Mechanical energy (PE + KE) or (U + K) all the way through his dive. His mechanical energy is conserved.

So if the diver is not moving he has all his energy in the form of potential energy. If the diver is moving and he is above the lowest point in his path he has potential energy and kinetic energy (yes some potential energy is now kinetic energy). And at the lowest point in the path he is moving his fastest, all his potential energy is now kinetic energy. But at every position the total energy is the same.

7. Feb 14, 2010

### lbella026

ok now I understand. Thank you so much!