A child on a playground swing is swinging back and forth (one complete oscillation) once every four seconds, as seen by her father standing next to the swing. At the same time, a spaceship hurtles by at a speed close to the speed of light. According to special relativity (and ignoring the Doppler effect for this question), a person on the spaceship finds that the time for one full swing is:

A - less than 4 seconds when the spaceship is approaching the swing and longer than 4 seconds when it is moving away.

B - less than 4 seconds.

C - more than 4 seconds.

D - equal to 4 seconds.

A

Explanation:

Solution:-

- According to the law of relativity the relative speed between two moving objects is inversely proportional to the the time taken.

- Ignoring Doppler Effect.

- So if the relative speeds of two objects in motion i. e (swing and spaceship) are positive then the time frame of reference for both object relative to other other decreases. So in other words if spaceship approaches the swing i. e relative velocity is positive then the time period of oscillation observed would be less than actual i. e less than 4 seconds.

- Similarly, if spaceship moves away from the swing i. e relative velocity is negative then the time period of oscillation observed would be more than actual i. e more than 4 seconds.