# Puck on Ice

Psyguy22
A hockey puck on a frozen pond with an initial speed of 12.3 m/s stops after sliding a distance of 198.9 m. Calculate the average value of the coefficient of kinetic friction between the puck and the ice.

So I started with finding the acceleration. I divided 12.3m/s by 198.9m then took the inverse (so units would go back seconds) and came up with 16.17s. So now I have Δv and Δt so I divided 12.3m/s by 16.17s to get an acceleration of .76m/s^2

I also know that friction equals μN and that F=ma. Since N=mg and friction is the only force acting on the object, i divided the mass out. So I had μg=a, or μ=a/g. (using 9.81m/s^2 as gravity) So μ=.0774 which seemed reasonable for ice. But I dont know how to find the actual force without knowing the mass. Am I even heading in the right direction?

## Answers and Replies

Mentor
So I started with finding the acceleration. I divided 12.3m/s by 198.9m then took the inverse (so units would go back seconds) and came up with 16.17s. So now I have Δv and Δt so I divided 12.3m/s by 16.17s to get an acceleration of .76m/s^2
That does not work.
To travel 198.9m in 16.17s, it would have to travel at 12.3m/s the whole time, which is clearly not the case. You need a different approach here.

I also know that friction equals μN and that F=ma. Since N=mg and friction is the only force acting on the object, i divided the mass out. So I had μg=a, or μ=a/g. (using 9.81m/s^2 as gravity) So μ=.0774 which seemed reasonable for ice. But I dont know how to find the actual force without knowing the mass. Am I even heading in the right direction?
That part is good, once you fix your acceleration value. You cannot, and do not have to, calculate the force.

Psyguy22
Then how do I find acceleration? I don't have time..

MalachiK
But you do know lots of other things. For example, you know the initial and the final speeds. I think it's okay to assume uniform acceleration here. You probably have a list of equations for dealing with this sort of motion that look something like, for example, v = u + at.

What you need to do is find an equation that includes the quantities that you know along with the acceleration. Then there's some algebra to find a.