Problem Animation
Click to view movie (961k)

Crazy Al has just acquired a 1967 Ford Mustang and wants to do a thorough checkup on it before he gives it a price. Since the hydraulic jack is broken, he has decided that in order to get a good look at the underside of the car, he must drive it up a jury-rigged ramp that rests against a large wooden crate.

What is known:

  • The tires create a 1,000 N force perpendicular to the ramp. Assume that the box cannot move.
  • The length of the ramp is 3 m. The distance from the start of the ramp, point A, to the front wheels is 2 m.
  • The ramp is elevated at a 25° angle.
  • The coefficient of static friction at point B is 0.2.

Known Information


What is the minimum value for the coefficient of static friction at point A that will prevent the ramp from moving?


  • Find the normal force at point B.
  • Use the normal force to find the friction force at B.
  • Use the equilibrium equations to solve for the reaction forces at point A, and then find the coefficient of friction.