Question

In each of the following situations, Emma is in the exact center of a glass-sided freight car speeding to the right at a very high speed v relative to you. For each question, explain your reasoning.

1. Emma has a flashlight in each hand and directs them at the front and rear ends of the freight car. She then switches the flashlights on at the same time.

   1. In Emma

Answer 1

In each of the following situations, Emma is in the exact center of a glass-sided freight car speeding to the right at a very high speed v relative to you. For each question, explain your reasoning.

1. Emma has a flashlight in each hand and directs them at the front and rear ends of the freight car. She then switches the flashlights on at the same time.

   1. In Emma�s frame of reference, which beam of light travels at a greater speed, the one directed toward the front or the one directed toward the rear of the train, or do they travel at the same speed?
      Light always travels at the same speed.

   2. In Emma�s frame of reference, which end, front or rear, is struck by light first, or are they struck at the same time? Which end is struck first in your frame of reference?
      In Emma�s frame of reference, the front and rear of the car are equal distances away and so light strikes both at the same time.

2. Now, Emma directs her flashlights at the ceiling and floor of the freight car. The flashlights are located midway between the ceiling and the floor and Emma switches them on at the same time.

   1. In Emma�s frame of reference, which surface, ceiling or floor, is struck by light first, or are they struck at the same time?
      Both at the same time.

   2. Which surface is struck first in your frame of reference?

      Both at the same time as each other, but not at the same time that Emma

      observes.

   3. In which frame of reference (yours or Emma�s) does the light travel a longer distance from the flashlight to the ceiling of the car?
      The light travels a longer distance to the ceiling in your frame.

   4. In which frame of reference (yours or Emma�s) does the light spend more time traveling toward the ceiling?
      The light takes more time in your frame.

3. Emma is playing laser tag in the freight car. Two �assassins� sneak into the freight car with Emma. One is positioned against the front end and the other is against the rear end. They each fire a laser at Emma. The two lasers strike Emma at the same time.

   1. In Emma�s frame of reference, who fired first, the person against the rear end of the car or the person against the front end, or did they fire at the same time? Both at the same time.

   2. In your frame of reference, who fired first?

      If you observe both lasers strike Emma at the same time, then the assassin at the front of the car fired first, since her light must travel a longer distance to reach Emma.