# Can someone explain a simple physics question for me?

1. Nov 5, 2009

### pooker

Okay this isn't really a problem, but I want it explained too me instead of the books way. We are doing a section on mechanical energy, and for the entire course with vectors, force, etc, we have always used rcostheta and rsintheto to find an x and y force.

Now tell me why we do this? Is this to elimate the forces from a vector component form to a basic x y form, where y is a 90 degree value, and x is a 0 degree value?

Also another question. We used the equation for change in mechanical energy where energy potential, and kinetic energy where the same regardless of its position along a path when looking at conservative forces.

I used rcostheta and rsintheta to plug in values for velocity when they where at angles, but I noticed that this data does not accurately give the TOTAL potential energy correctly.

To use an example say that

a ball is fired at 600m/s at 30 degrees with a mass of 24kg.

To plug this into the isloated system model for velocity I had to use velocity x sin30

but when it asked for its total mechanical energy at the balls maximum height I used 1/2mvf^2 and just plugged in 600.

2. Nov 5, 2009

### DaveC426913

Please use the question template provided.