Question

You are pushing a set of blocks along a frictionless horizontal surface. The blocks are arranged in order by the 1kg block, then the 2kg block, and then the 3kg block; you push the 1kg block.

a. Calculate the force exerted by the 3kg block on the 2kg block.

b. Calculate the force exerted by the 2kg block on the 1kg block.

For both parts show if you are using Newton's 2nd or 3rd Law and where.

Answer 1

The figure for the given situation is shown below.

Assume 1kg, 2kg and 3kg be m1, m2 and m3 respectively. Let a force F1 be applied to the 1kg block. If the system moves with an acceleration a, then by Newton's second law of motion,

The 1kg block will exert a force F2 to the 2kg block and that force is given by,

Similarly the 2kg block will exert a force F3 to the 3kg block and is given by,

(a) By Newton's third law, for every action, there is an equal and opposite reaction. Therefore when the 2kg block applies a force F3 on the 3kg block, the 3kg block exerts a reactive force equal to F3 on the 2kg block. So the force exerted by the 3kg block on the 2kg block is,

(b) Similarly when the 1kg block applies a force F2 on 2kg block, reactive force F2 is applied by the 2kg force on 1kg block. So the force exerted by the 2kg block on the 1kg block is,