Two blocks X and Y are falling through a vacuum in a uniform gravitational field, as shown.

Question 19

Two blocks X and Y are falling through a vacuum in a uniform gravitational field, as shown.

Block X has weight 2w.

Block Y has weight w.

The blocks do not move apart.

Which value best represents the force exerted by block X on block Y?

A 0                   B w                  C 1.5w                         D 2w

Reference: Past Exam Paper – November 2015 Paper 11 Q14

Solution:

Answer: A.

The blocks are falling due to the gravitational force acting on them.

Since this occurs through a vacuum, they both accelerate downwards with the same acceleration (of free fall). On Earth, this would have been 9.81 m s^-2.

As the blocks are falling freely with the same acceleration, they are moving equal distances at all time. So, they do not exert any force on each other.

If, say, block X was falling faster it would have exerted its weight 2w on block Y.

If block Y was falling faster, the distance between the two blocks would have increased.

 

It is important to understand that the acceleration of free fall is the same for both, even if their masses differ. This difference in mass results in a difference in weight, not in acceleration.