DP Physics (last assessment 2024)

Question 18N.3.SL.TZ0.3b.i

Date	November 2018	Marks available	[Maximum mark: 2]	Reference code	18N.3.SL.TZ0.3b.i
Level	SL	Paper	3	Time zone	TZ0
Command term	Explain	Question number	b.i	Adapted from	N/A

b.i.

[Maximum mark: 2]

18N.3.SL.TZ0.3b.i

The diagram shows the axes for two inertial reference frames. Frame S represents the ground and frame S′ is a box that moves to the right relative to S with speed v.

When the origins of the two frames coincide all clocks show zero. At that instant a beam of light of speed c is emitted from the left wall of the box towards the right wall. The box has proper length L. Consider the event E = light arrives at the right wall of the box.

Using Galilean relativity,

(b.i)

explain why the time coordinate of E in frame S is $t = \frac{L}{c}$ .

[2]

Markscheme

ALTERNATIVE 1:

the time in frame S′ is $t' = \frac{L}{c}$ ✔

but time is absolute in Galilean relativity so is the same in S ✔

ALTERNATIVE 2:

In frame S, light rays travel at c + v ✔

so $t = \frac{L}{{\left( {c + v} \right) - v}} = \frac{L}{c}$ ✔