Solving Hockey Puck Friction Problem

[PvtDonutDJ88]

Homework Statement

A hockey puck is hit on a frozen lake and starts moving with a speed of 12.0m/s. Exactly 5.0sec later, its speed is 6.0m/s. What is the puck's average acceleration? What is the coefficient of kinetic friction between the puck and ice?

Homework Equations

V_F=V_I+at
F_N=mg
F_F=UF_N
F_F=ma
EF_Y=F_N+F_gcos(0)
(cos(0)=1)

The Attempt at a Solution

I have the acceleration it's -1.2m/s2 (that was easy been doing that for a while)The problem is I am having trouble on finding the coefficient of Friction. I'm pretty sure the last four equations are correct, do i need to find Fn next? Because i don't have the mass and it's not moving at a constant velocity, so i do need mass right?if and multiply that by gravity to get Fn? All I know i have the info just a little confused on how to use itNever mind i got it no worry's

 

[luijsu]

If you solve the equation for F_F=ma, you'll find that the m's cancel out.

 

[PvtDonutDJ88]

uh? what do u mean m's cancel out? The mass cancels out? and you that's one formula forgot (i edited it in)

 

[luijsu]

Yes. You'll find that writing the frictional force in terms of m,g, and u and setting it equal to ma will result in the masses canceling out.

 

[bpaterni]

edit: yep ^^^^^^^^

I believe he/she means that you can replace m*a for the force of friction, so:

-1.2m = k*-9.81m

By solving this, you see that the mass cancels and you're left with the coefficient of friction.