# Coefficient of Kinetic Friction

"A hockey puck is hit on a frozen lake and starts moving with a speed of 12 m/s. Exactly 5.0 s later, its speed is 6.0 m/s. What's the puck's average acceleration? What is the coefficient of kinetic friction between the puck and the ice?"

I've calculated the acceleration, but I'm stuck when it comes to finding the coefficient of kinetic friction since I don't know the mass of the object.

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You don't need the mass of the object, just like when calculating how fast something accelerates towards the Earth you don't need to know its mass. :)

How would you calculate that without the mass, though?

Well, think about it. The force on the puck is mgµ, and its acceleration is F/m. Right?

Solve the equations algebraically and you will see that at some point the mass cancels out of the equation providing a solution independent of the mass.

Write Newton's second law for the puck. What forces acts on the puck?