Question

When is a body said to be at rest?
A.) A body is said to be at rest when its position doesn’t change with time.
B.) A body is said to be at rest when its acceleration doesn’t change with time.
C.) A body is said to be at rest when its acceleration changes with time.
D.) A body is said to be at rest when its position changes with time.

Answer 1

Hint: A body is considered to be stationary in a state of rest w.r.t. if its location does not change as shown by the stationary body over time. Similarly, it is in the state of motion w.r.t. a stationary body if its position changes with time as seen by the stationary body.

Complete step-by-step answer:

In this question we will use the first law of motion by Newton which is called the law of inertia. Inertia is a property of a body to rest or stay in motion at a constant velocity. Some objects have greater inertia than others, because an object's inertia is equal to its mass. Which is why it's easier to adjust a boulder 's direction than a baseball.

Therefore, it is said that a body is in a state of rest when it does not move or it does not change its position with time.

Note: A body is in a moving state w.r.t. if its position changes as seen by the moving body with time, otherwise it is the moving body in a state of rest w.r.t. This occurs only if, at all times, the velocity of the two bodies is equal. For example, two people standing on the Earth's surface see each other as being at rest.