# Finding speed on kinetic energy question

1. Jan 12, 2012

### Tygra

Hi all.

I have come across a question in the kenetic energy section on a physics classroom website. The question asks you to find the speed, and have given the energy output and mass. The energy is 12,000 joules and the mass 40kg.

The formula for calculating the kinertic energy is KE = 0.5 * m * v2.

The answer given states the speed is 24.5 m/s. Below is how they explain the question:

KE = 0.5*m*v2
12 000 J = (0.5) * (40 kg) * v2

300 J = (0.5) * v2

600 J = v2

v = 24.5 m/s

What I don't understand is where does the 600 come from. I see you sqaure root this figure to get the speed, but I cannot see how this figure comes from 12,000 J and 40kg. Obviously the 300 J comes from the 12,000 being divied by 40, I just need to see where the 600 comes from.

Thanks.

2. Jan 12, 2012

### Skrambles

300 / 0.5 = 600

3. Jan 12, 2012

### Tygra

Thanks, mate.

When I first glanced at that I was thinking 150. After some shock I finally understood it.

4. Jan 12, 2012

### GAURAV DUBEY

Hello friend! explanation for your question is fine. Take care of physical units as well.