Question

I have a problem on mastering physics that I need help on.

A person pulls horizontally on block B in the figure, causing both blocks to move together as a unit.
While this system is moving, make a carefully labeled free-body diagram of block if there is friction between block B and the table and the pull is equal to thefriction force on block B due to the table.
Draw all relevant force vectors with their tails at the dot. The orientation of your vectors will be graded. The exact length of your vectors will not be graded butthe relative length of one to the other will be graded.
(In the figure there is a picture of two blocks one on top of the other. Block A is on top and Block B is on the bottom and they are sitting on a horizontaltable.)

Originally I thought that normal and weight would be longer than the friction vector, but that was incorrect. so now I'm thinking that the friction vector is the onlyrelevent force and and should be moving to the left. Can anyone help me solve this problem? Thanks!!

Answer 1

Ok. On block A (the top one), there is the force of gravity on the block (which is proportional to the mass of A) directed into the table, and anequivalent normal force exerted by B on A in the opposite direction. (There is also the force of static friction on block A, but at the relevant point intime, that force = 0)

On block B, there is a gravitational force proportional of the mass of A and B combined directed downward, and an equivalent normal force exerted by thetable in the opposite direction. There are two equal and opposite horizontal forces - *pull* (this is what i think you were missing) and friction. both areproportional to the mass of A+B, and, as you suspected, they should both be shorter then the gravitational / normal vectors. Since neither block isaccelerating, every vector should have an equally long vector in the opposite direction.