Moving Blocks and Forces

You are pushing on a block that is resting on the table.

(a) You press on the block but not hard enough to start it moving. What are 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. Give a brief explanation for how you know each of the results you state.

(b) You press a bit harder and the block begins to move. After a moment of starting up, you press so that the block is moving with a constant velocity. For the time while the block is moving with a constant velocity have any of the comparisons you stated in (a) changed? Which ones? Have any of the forces changed? Which ones?

(c) Suppose the block has a mass of 0.4 kg and the coefficient of friction between the block and the table is 0.3. What force will you have to use to keep it going at a constant velocity of 0.2 m/s? (You may take g = 10 N/kg.)

(From University of Maryland PERG Website on "Thinking problems in physics")