Teaching Physics with the Physics Suite Edward F. Redish

Friction is doing what?

 You are pushing on a large block that is resting on the table. On top of that block rest another, smaller block. You press on the block to start it moving. After about 0.25 s, it is moving at a constant speed and the block on the top is not slipping. Draw a labeled free-body diagram for the two boxes during the time when the boxes are accelerating, specifying all the forces acting on the block. (Be sure to specify the type of force and the object causing each force.) Wherever you can, compare the magnitudes of forces. Draw a labeled free-body diagram for the two boxes during the time when the boxes are moving at a constant speed, specifying all the forces acting on the block. (Be sure to specify the type of force and the object causing each force.) Wherever you can, compare the magnitudes of forces. Suppose the bottom block has a mass of 0.4 kg and the coefficient of friction between the block and the table is 0.3. The top block has a mass of 0.1 kg and the coefficient of friction between the two blocks is 0.2. What force do you need to exert to keep the blocks moving at a constant speed of 10 cm/s? (You may use g = 10 N/kg and you may treat kinetic and static friction as the same.)

Note to the instructor: Students often have a lot of trouble believing that friction can speed up an object as well as slow it down. This problem, combined with an extended and interactive classroom discussion, can help somewhat.

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