Suppose we have Bob the astronaut sitting without a space suit in a spaceship full of air on a mission to Mars. Bob is very fond of balloons and is holding on to a nice, big, red helium balloon via a piece of string. Bob is sitting facing the front of the spaceship. Mission Control decides to slowly accelerate the spaceship. In which direction will the balloon move relative to Bob? Why?
When the spaceship starts to accelerate, the balloon will move in the opposite direction to Bob, relative to his perspective. This occurs due to an effect called inertia.
Inertia is the tendency of an object to resist changes in its velocity or motion. When the spaceship accelerates, Bob's body tries to resist the change in velocity and tends to stay in its current state of rest. However, the spaceship pushes Bob forward, causing him to move in the same direction as the acceleration.
The balloon, on the other hand, is not directly connected to the spaceship. It is only connected to Bob via the string. As the spaceship accelerates forward, the balloon will continue to move at its original velocity due to its own inertia. Since the balloon is not affected by the spaceship's acceleration, it appears to move backward relative to Bob, who is moving forward with the spaceship.
So, from Bob's perspective, the balloon will move in the opposite direction (backward) relative to him when the spaceship accelerates.