Friction on an Inclined Plane

Two blocks of masses M1 and M2 are joined by a string over a pulley attached to an inclined plane.  To simplify the problem, we will set the mass M2 to zero and only be concerned with the motion of the mass M1 as it slides down the ramp against friction.  Forget all about M2 – pretend it isn’t there!


  1. Make a prediction:

Imagine you have three blocks, all exactly alike except that their masses are 5, 10, and 20 kg.  The coefficient of friction between each block and the ramp is the same.   The angle of the ramp with the horizontal is 30°.  If each block was released from rest near the top of the ramp, predict which one you think would have the largest acceleration.  Explain briefly why you chose that block. (Note that your mark is based on your explanation, not whether it is correct!)

  1. Now load the physlet:

Be sure to wait until the applet has finished loading before you begin.

Set up the physlet so that the ramp angle is 30°, the coefficient of kinetic friction m = 0.24, and mass M1 = 5 kg.  (Don’t forget to set M2 = 0!)  Run the physlet and record the acceleration of the 5 kg block.  Then repeat for M1 = 10 kg and then 20 kg, recording the acceleration each time.  How does your prediction compare to what you observe for the three blocks?

  1. Use a free-body diagram and Newton’s second law to derive an expression (i.e. no number values) for the acceleration of the block in terms of M, g, m and the angle q.  Do you notice anything in the expression you derived that accounts for the observed acceleration of the three blocks?

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