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Date May 2019 Marks available 1 Reference code 19M.2.SL.TZ1.5
Level Standard level Paper Paper 2 Time zone 1
Command term Calculate Question number 5 Adapted from N/A

Question

A small metal pendulum bob of mass 75 g is suspended at rest from a fixed point with a length of thread of negligible mass. Air resistance is negligible. The bob is then displaced to the left.

At time t = 0 the bob is moving horizontally to the right at 0.8 m s–1. It collides with a small stationary object also of mass 75 g. Both objects then move together with motion that is simple harmonic.

Calculate the speed of the combined masses immediately after the collision.

[1]
a.

Show that the collision is inelastic.

[3]
b.

Describe the changes in gravitational potential energy of the oscillating system from t = 0 as it oscillates through one cycle of its motion.

[1]
c.

Markscheme

0.40 «m s−1» ✔

a.

initial energy 24 mJ and final energy 12 mJ ✔

energy is lost/unequal /change in energy is 12 mJ ✔

inelastic collisions occur when energy is lost ✔

b.

maximum GPE at extremes, minimum in centre ✔

c.

Examiners report

[N/A]
a.

Candidates fell into some broad categories on this question. This was a “show that” question, so there was an expectation of a mathematical argument. Many were able to successfully show that the initial and final kinetic energies were different and connect this to the concept of inelastic collisions. Some candidates tried to connect conservation of momentum unsuccessfully, and some simply wrote an extended response about the nature of inelastic collisions and noted that the bobs stuck together without any calculations. This approach was awarded zero marks.

b.

This straightforward question had surprisingly poorly answers. Candidate answers tended to be overly vague, such as “as the bob went higher the GPE increased and as it fell the GPE decreased.” Candidates needed to specify when GPE would be at maximum and minimum values. Some candidates mistakenly assumed that at t=0 the pendulum bob was at maximum height despite being told otherwise in the question stem.

c.

Syllabus sections

Core » Topic 2: Mechanics » 2.4 – Momentum and impulse
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