Physics H2/2019/I/5

This is a very well-written question that illustrates what Cambridge / SEAB is looking for: In-depth learning.

This question tests you on how well you understand the Principle of Conservation of Momentum, which says:

“The linear momentum of a system is conserved provided no net external force is exerted on the system”

In particular, this question wants to know if you understood 2 small details:

1. What does “system” refer to?

2. What does “external” net force mean?

According to the encyclopaedia Brittanica:  

“A system is a portion of the universe that has been chosen for studying the changes that take place within it in response to varying conditions.”

In the context of this question, the system comprises of the blocks X and Y, as well as the container. According to the Principle of Conservation of Momentum, the total momentum of these three objects which make up the system must be conserved, i.e. constant.

Thus we can make a few true statements:

1. the initial linear momentum of the “system” is zero.

2. there was no “external” net force on the system. (The force of the explosion was exerted presumably between X and Y, which are both included in the system, hence are classified as “internal forces”. )

Therefore the PCOM tells us that the momentum of the system should remain at ZERO, hence allowing us to conclude that the answer is A: the centre of gravity will not move.

Many students mistakenly think that physics is about memorising equations and applying them at the right time. That strategy might have served you well in Lower Secondary and maybe even upper secondary physics, but it does not work very well at the A levels. This is a great example of what we teachers mean by “in-depth understanding”, and I hope it gives you a better idea of what to look out for when preparing for your physics exams.