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Sample problems involving forces and friction

  1. A box of mass M sits on a frictionless floor. A smaller box of mass m lies on top of the big box. The coefficient of static friction between the boxes is mu. How hard can you pull horizontally on a rope attached to the big box in order before the small box starts to slide across the top of the big box?

  2. Now you pull at an angle theta above the horizontal. Does the critical force required to cause the little box to slide change? If so, does it become larger or smaller than before?
  3. A long rod has two strings attached to its ends. The top string is 1.1 m long and oriented at a 30 degree angle; the shorter bottom string is tilted at a 40 degree angle. Both strings meet at a ball of mass 0.2 kg. The rod is spun so that the ball makes one full rotation around the rod every 0.8 s.

    For starters, ignore the force of gravity on the ball ....

    1. What is the length of the bottom string?
    2. What is the acceleration of the ball?
    3. What is the net force applied to the ball by the strings?
    4. What is the tension in the top string?
    5. What is the tension in the bottom string?

    What happens if you include the force of gravity, too?

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.