Target: 10 Questions in 10 minutes

An IB Physics data booklet is helpful

1. What of these is the best description for an 'inertial frame of reference'?

  • A. A frame of reference that is stationary.
  • B. A frame of reference that has constant velocity.
  • C. A frame of reference that is accelerating.
  • D. A frame of reference that has momentum.

2. Which of Newton's laws of motion are the same for 'Galilean relativity' between different inertial reference frames?

  • A. Laws I and II
  • B. Laws I and III.
  • C. Laws II and III
  • D. Laws I, II and III.

3. In Einstein's Postulates for special relativity, only two statements are made. Which row correctly states these postulates?

Postulate I Postulate II
A
 The laws of physics are the same regardless of motion The speed of light measured in an inertial reference frame is independent to the motion of the observer or source
B
 The laws of physics are the same regardless of motion The speed of light increases or decreases dependent on the motion of the observer or source
C
 The laws of physics are the same in all inertial reference frames The speed of light measured in an inertial reference frame is independent to the motion of the observer or source
D
 The laws of physics are the same in all inertial reference frames The speed of light increases or decreases dependent on the motion of the observer or source

Q4-6: These questions are about an event being observed on the ground or by an observer on a moving train.

4. A train passes a point Q on the ground at a speed of 4m s-1, such that the centre of the train P passes the point at a time t = 0 seconds.

event in train , Galilean relativity question

A ball is dropped on the ground, 5 m to the right of Q as shown in the diagam, hitting the ground at a time t = 8 seconds.
Using Galilean relativity, how far will this event occur from the point P in the reference frame of an observer on the train?

  
  • A. 37 m to the right of P
  • B. 27 m to the right of P
  • C. 37 m to the left of P
  • D. 27 m to the left of P

5. What is the Lorentz factor γ if the train in the diagram above could move to the right at 4/5 c (80% of the speed of light)?

  • A. 0.60
  • B. 1.67
  • C. 2.78
  • D. 5.00

6. The ball takes a time t to fall a distance h to the ground once dropped, as observed from the ground. The time for this event to occur is t', as recorded by an observer on the train. Which of these statments is true, assuming the train is still moving at v = 4/5 c?

  • A. t' = t
  • B. t' > t
  • C. t' < t
  • D. t - t' = v/h

7-8. A'light clock' consists of a very short duration pulse of light travelling up and down, reflecting off 2 mirrors as shown below. The time t it takes to travel up and down a distance = 2d is one 'tick' of the clock.

The clock is placed on a moving train. For an observer on the train, the light pulse still travels up and down in time t. As seen from the ground, the light ray now takes a diagonal path as the train moves to the right, as shown below:

light clock pulse, stationary and moving observers

7. Which of these statments is correct for an observer on the train and an observer on the ground?

I. The time taken for the pulse to travel up and down is the same for both observers.
II. The speed of the pulse is the same for both observers.
III. The distance travelled by the light pulse is the same for both observers.

Michael Schmid (edited)
C.C licence 3.0
  • A. I and II
  • B. I only
  • C. II only
  • D. I and III

8. How does the distance travelled and the time taken for the pulse to travel up and down change if the train slows down?

distance travelled time taken
A
 decreases decreases
B
 increases increases
C
 decreases stays the same
D
 stays the same decreases

Q9 +10: To test the theory of special relativity, particles called muons are observed. These are created by cosmic rays in the upper atmosphere. They travel an average distance d to the ground , at a very high speed, v.

muons created in upper atmosphere

Muons have a very short half life, T. As a result, the percentage of muons reaching the ground should be P, according to Galilean relativity, and this number is very small. However, the percentage detected is much larger than P.

9. An observer on the ground measures the half life of the fast moving muons as they decay.
Choose the correct word to complete the sentence below:

  
Due to special relativity, the measured half life is T.

10. In the reference frame of the muons, how can the observations be explained?
  • A. The half life is greater than T
  • B. The distance travelled decreases
  • C. The velocity increases
  • D. The value of P decreases