# Kinetic Energy calculation

1. Dec 28, 2011

### mailhiot

If a metal sphere that weights 1.006 kg is dropped from 2.5 meters, what is it's kinetic energy when it hits the ground?

Is this kinetic energy the same value as mechanical energy?

If this kinetic energy was converted in electrical energy by a 1000 watt electric generator that was 95% efficient, how much mechanical energy (in joules) is required? In other words, does a 1000 watt generator require 1000 joules of mechanical energy?

2. Dec 28, 2011

### Thundagere

In this case, what you want to do is apply conservation of energy—in other words, total mechanical energy will always be conserved. Use that principal, and potential & kinetic energy to answer your question.

3. Dec 28, 2011

### mailhiot

Thank you for the answer, but I am not a physics person so don't really understand the answer. So in my question, with a metal sphere that is 1.006 kg dropped from 2.5 meters, what is the potential energy, kinetic energy, and mechanical energy? I need to know this value because I need to know how many spheres need to be dropped per second to power a 1000 watt electric generator. THANKS!

4. Dec 29, 2011

### Thundagere

OK, some background info then.
The mechanical energy is the total energy the object posseses from position and from motion. This will always be conserved.

Potential energy is energy from position. GRAVITATIONAL potential energy is the potential energy that gravity causes, and is given by PE= mgh, where m is the mass, g is the acceleration due to garvity (9.8 m/s^2), and h is the height. You have all these values.

Kinetic energy is the energy due to motion, and is given by KE= 0.5 m v^2, where m is mass and v is velocity.

Now ask yourself, before the sphere is dropped, what is it's kinetic energy? Since it's velocity is 0, logically it has no kinetic energy. However, it has gravitational potential energy, which you can calculate.
Since mechanical energy is potential energy and kinetic energy. then the GPE in this scenario makes up all the mechanical energy.

Remember that all the mechanical energy will be conserved. At 0 m, all of it will be kinetic energy.
Hopefully, I've given you something to think about!