"You pull your 20 kg little brother horizontally across a beack on a .7 kg innertube at 65 N to get the tube moving. What is the coefficient of static friction between the sand and the tube? You keep pulling at 60 N with the innertube at a constant velocity. What is the coefficient of kinetic friction?
I'm totally stuck. Please please please help me! "
OK, first step is to go back to the definition: The static friction is the force that must be applied before the object will move. You have the force in the problem already, and you have the mass that you are trying to move (20.7 kg). Solving for the coefficient of static friction a straightforward plugging in the numbers.
Now for the second piece, you know the force necessary to keep the tube moving at constant velocity (60N). The key is constant velocity, this means the force to pull is equal to the force of friction (no acceleration). Again, plug in your numbers from the problem and you should get your result.