# Calculating kinetic energy

I have the question:

"Calculate the kinetic energy, $E_k$, when a car of mass $1500$ kg travels at a speed of $108$ km/h."

I converted the $108$ km/h to $108000$ m/h by multiplying by $1000$. I then divided $108000$ by $60$ to get minutes and then again by $60$ to get $30$ m/s I then put these values into the equation for kinetic energy and got $675000$ Joules.

So the final answer I get for this is $675000$ Joules [J] is this correct and if not could you please explain where I went wrong ?

• You got it exactly! The downvote is hard to understand. – Peter Oct 20 '16 at 15:25
• Bravo,, here is a +1 – Ahmad Bazzi Oct 20 '16 at 15:26

## 2 Answers

Your conversion from km/h to m/s is correct and as long as you used $p^2/2m$ where p is the momentum and m is the mass or $1/2mv^2$ the more familiar one you are ok.

• Can I also leave the answer as 6.75 x 10^5 [J] ? – Dan Oct 20 '16 at 16:03
• yes, that is ok – Amara Oct 20 '16 at 16:14

Kinetic energy is simply ${\frac{mv^2}{2}}$, where $v$ is velocity and $m$ is mass. Since the velocity given to you is in $Km/hr$ and the standard unit of velocity is $metres/sec$ so on converting the velocity to $m/s$ you get $v=\frac{108\times5}{18}=30 m/s$.

The mass given to you is $1500 kg$ so the Kinetic energy becomes ${\frac{mv^2}{2}}=\frac{1500\times 30\times30}{2}= 675000$ Joules.