# Velocity on a hill Kinematics Problem with Dynamics

1. Feb 23, 2014

### alingy1

In the sport of luge, a competitor slides on a sled down a 500-m long straight ice track inclined at 20o to the horizontal. Her initial speed is 2.0 m s-1. The coefficient of dynamic friction between the sled and the ice is 0.050. Neglect air resistance.

What is her acceleration? What is her speed at the bottom?

Okay, this is weird.

I spent three hours on this. I feel the mass of the competitor is missing.

I separated the two dimensions. Both have non-zero acceleration (because there is a net force that is diagonal to the ice).

I get these equations:
-171=-0.68+1/2(-1.15+0.017/m)t^2
469=1.88t+1/2(3.15-0.047/m)t^2
a=3.35-0.05/m
I can't solve because I get 0=0 on both sides if I try to add the equations.

What am I supposed to do? Please, I have an exam soon.

2. Feb 24, 2014

### jackarms

It'd help if you could explain what all those values mean, or write down the equations symbolically first, and also explain what your strategy is with them. I'm having trouble understanding what you're trying to do.

I think your process is more complicated than necessary, though, since your equations involve time, which isn't involved with the acceleration, so those won't be any good. To find the acceleration, it won't involve kinematics since you aren't given enough information in the problem. Try finding it by using Newton's second. What forces act on the sled that add up to the net force?