Special Relativity Flashcards: Reference Frames, Time, and Length

Cards in this deck

1. Card 1

   Question

   What two kinds of coordinates identify an event in an inertial frame?

   Answer

   A position and a time. A camera shutter closing is one event, and the frame gives the coordinates of that closure.

2. Card 2

   Question

   An instrument is bolted to a pod that coasts past a lab. Which is the instrument’s rest frame?

   Answer

   The pod frame. The instrument stays at a fixed position in that frame.

3. Card 3

   Question

   A pod recedes from a lamp in vacuum. What speed does the pod measure for light from the lamp?

   Answer

   $c$, the same speed the lab measures for the light.

4. Card 4

   Question

   A beacon flashes and its light reaches a distant camera. Which event does the camera’s detection timestamp give directly?

   Answer

   The light’s arrival at the camera. The flash is a separate event at the beacon.

5. Card 5

   Question

   Two events occur beside the same clock at rest in a frame. What interval does that clock record between them?

   Answer

   The proper time, $\Delta\tau$, between the two events.

6. Card 6

   Question

   In one inertial frame, how can two separated events be timed locally and compared?

   Answer

   Place a clock at each event location. Keep both clocks at rest and synchronized in the chosen frame, then subtract the event readings.

7. Card 7

   Question

   In which frame is the proper length of a rigid calibration bar measured?

   Answer

   The bar’s rest frame, where both ends stay at fixed positions.

8. Card 8

   Question

   A probe coasts in a straight line at constant speed relative to an inertial lab. Is the frame attached to the coasting probe inertial?

   Answer

   Yes. Constant velocity relative to an inertial frame gives another inertial frame.

9. Card 9

   Question

   A detector at rest midway between two stationary lamps receives their vacuum flashes together. When were the flashes emitted in the lamps’ frame?

   Answer

   At the same time. The paths to the detector have equal length and light travels at the same speed along both.

10. Card 10

    Question

    How do you calculate the Lorentz factor $\gamma$ from the relative speed $v$?

    Answer

    $\gamma=1/\sqrt{1-(v/c)^2}$, where $c$ is the speed of light in vacuum.

11. Card 11

    Question

    A rod moves along the lab’s track. Which two lab events give the rod’s length in that frame?

    Answer

    Record the front and rear endpoint positions at the same lab time. Their separation at that time is the lab length.

12. Card 12

    Question

    Two events happen at different lab stations but both happen beside the same clock on a coasting probe. Which clock gives the proper time?

    Answer

    The probe clock. The events occur at its one fixed probe location.

13. Card 13

    Question

    A detector at rest in a lab receives a flash at lab time $6\,\mu\mathrm{s}$. The light traveled for $2\,\mu\mathrm{s}$ in the lab frame. When was the flash emitted in that frame?

    Answer

    At $4\,\mu\mathrm{s}$. Subtract travel time from arrival time in the same frame.

14. Card 14

    Question

    What is the Lorentz factor at relative speed $0.60c$?

    Answer

    $1.25$. The square root in the denominator is $\sqrt{1-0.60^2}=0.80$.

15. Card 15

    Question

    A lab timed two flashes when they were emitted. Which events should a passing pod use to compare the time interval in its own frame?

    Answer

    The same two emissions. The arrival of either flash at a detector is a separate event.

16. Card 16

    Question

    A moving rod’s front position is recorded first and its rear position one lab second later. Does the difference between the recorded positions give the lab length?

    Answer

    No. Use endpoint positions at one lab time. The rear has moved during that second.

17. Card 17

    Question

    A clock coasts past an inertial lab and records proper time $\Delta\tau$ between two ticks. What interval $\Delta t$ does the lab assign to those same ticks?

    Answer

    $\Delta t=\gamma\Delta\tau$. The lab uses synchronized clocks to time the ticks at different positions.

18. Card 18

    Question

    Two flashes at different positions along a test range are simultaneous in the range frame. A pod coasts along the line joining them. Are the emissions simultaneous in the pod frame?

    Answer

    No, for a nonzero relative speed along that line. The two frames assign different time differences to the same emissions.

19. Card 19

    Question

    If the relative velocity reverses direction without changing speed, what happens to the Lorentz factor?

    Answer

    It stays the same. The factor depends on $(v/c)^2$.

20. Card 20

    Question

    A bar coasts past an inertial lab with its length perpendicular to its velocity. How does the length measured in the lab compare with the bar’s rest length?

    Answer

    They are equal. The lab records both ends at the same lab time, and their separation is entirely transverse to the motion.

21. Card 21

    Question

    A pod coasting at $0.80c$ relative to the lab records $9\,\mathrm{s}$ between two ticks of its own clock. How much lab time separates those ticks?

    Answer

    $15\,\mathrm{s}$. The lab interval is $(5/3)\times9\,\mathrm{s}$.

22. Card 22

    Question

    How do you find the measured length $L$ of an object moving along its length at constant speed, given its proper length $L_0$?

    Answer

    $L=L_0/\gamma$. Measure both ends at one time in the frame where the object moves.

23. Card 23

    Question

    Two lamps and a detector are at rest in the same frame. Their flashes travel directly through vacuum to the detector and arrive together. If one lamp is farther away, which lamp emitted first in that frame?

    Answer

    The farther lamp. Both flashes traveled at the same speed, so the longer path took more time.

24. Card 24

    Question

    A boom’s endpoints are measured at the same lab time while it moves lengthwise. Does the boom frame assign the same time to those two measurements?

    Answer

    No. Simultaneity is frame-dependent for these two events because they occur at different points along the direction of relative motion.

25. Card 25

    Question

    Two ticks of a clock coasting at $0.60c$ relative to the lab are $15\,\mathrm{s}$ apart in lab time. How much time elapsed on the moving clock?

    Answer

    $12\,\mathrm{s}$. Its proper time is $15\,\mathrm{s}/1.25$.

26. Card 26

    Question

    A $75\,\mathrm{m}$ bar in its rest frame moves lengthwise at $0.80c$ in the lab. What length does the lab measure?

    Answer

    $45\,\mathrm{m}$. Divide $75\,\mathrm{m}$ by $5/3$.

27. Card 27

    Question

    Two inertial crews coast past one another and each uses clocks synchronized in its own frame. How can each assign a slower rate to the other crew’s clock?

    Answer

    They compare different pairs of ticks. Each times two ticks of the other crew’s clock with separated clocks synchronized in its own frame.

28. Card 28

    Question

    The nose of a coasting probe passes two checkpoints at rest in an inertial lab. Which frame gives the proper time between those crossings?

    Answer

    The probe frame. Both crossings happen at its nose, where one clock on the probe can time them.

29. Card 29

    Question

    Two light pulses reach the same detector at the same instant. Do all inertial frames agree that the pulses arrive together?

    Answer

    Yes. They arrive at one place and time, so their arrivals coincide in a single event. Each frame still assigns its own coordinates to that event.

30. Card 30

    Question

    A bar moves lengthwise at $0.60c$ relative to the lab, which measures its length as $52\,\mathrm{m}$. What is the bar’s proper length?

    Answer

    $65\,\mathrm{m}$. Multiply the lab length by $1.25$.

30 cards

Special Relativity Flashcards: Reference Frames, Time, and Length

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