Question

Suppose you are in a rocket with no windows, traveling in deep space far from other objects. Without looking outside the rocket or making any contact with the outside world, explain how you could determine whether the rocket is

(a) moving forward at a constant 80% of the speed of light and

(b) accelerating in the forward direction.

Short answer

(a) When the rocket is moving at a constant speed the passengers don't feel any force.

(b) When the rocket is accelerating in the forward direction the passengers feel a backward force towards their seats.

Step-by-step solution

Given data:

A rocket having no windows is moving with passengers in outer space.

Inertia of motion:

A body in motion tends to be in motion unless an external force acts on it.

Determining the effect of rocket motion:

1. When the rocket is moving at a constant speed all objects inside the rocket along with the passengers are at rest with respect to each other.

Thus, the seats don't exert any force on the passengers and vice versa. So, the passengers won't feel any force.

b. When the rocket is accelerating the seats are accelerating instantaneously along with it. However, the passengers tend to remain at constant speeds due to the inertia of motion. The seats are thus accelerating with respect to the passengers and apply a force on the passengers.

Hence, the passengers feel a continuous backward force towards their seats. Such forces are also experienced in accelerating cars, trains, and airplanes.