# Homework Help: Just a quick physics question

1. Apr 8, 2005

$$K = 1/2(1300kg)(10km/h)^2 + 1/2(1600kg)(6.6km/h)^2-1/2(2900)(8.12)^2 = 320J$$

how is that 320J? when i plug that into my calculator, i get 4243.12(not sure what units this is in). can someone tell me why that K is 320 j?

2. Apr 8, 2005

### cepheid

Staff Emeritus
You need to convert EVERYTHING into SI (MKS) units if you want your energy to be nicely expressed in joules at the end, you cannot just mix in km and hours.

Edit: and what do you mean you don't know what units your answer is in? Obviously it is in kg * km^2 / h^2, which is not a joule.