# Sample Problem Force And Motion

Sample Problem Force And Motion

A constant horizontal force r; of magnitude 20 N is applied to block A of mass m” = 4.0 kg, which pushes against block B of mass = 6.0 kg. The blocks slide over a friction less surface, along an x axis.

SOLUTION: We shall first examine a solution with a serious error. then a dead end solution. and then a successful solution.

Successful Solution: The key idea here is that, because of the direction in which force is applied. the two form a rigidly connected system. We can relate the net force on the system to the acceleration of the system with Newton’s second law. Here, once again for the x axis, we can write that law as (a) A constant horizontal force F is applied to block A. which pushes against block B,Twohorizontal forces act on block A: applied force F,p and force Ffrom block B. (e) Only one horizontal force acts on block B:force r; from block

SOLUTION: The Key Idea here is that we can relate the net force on block B to the block’s acceleration with Newton’s second law, Here we can write that law. still for components along the  axis. Thus, force  is in the positive direction of the .r axis and has a magnitude .