do floating objects move with a moving object.?

Question

if you were travelling in a car down a road. if theorhetically you had a ball floating in the car, would that ball move with the car, or hit the rear window.

this maybe stupid, but i honestly don't know.

Answer 1

It depends on what's making the ball move and whether it was moving with the car before it started floating.

The principle that applies here is that an object moving relative to an inertial frame of reference will continue to do so if no external force (in the plane of motion, in this case) is applied that has magnitude relative to the inertial frame.

A simple experiment will verify this. Imagine that you have a helium balloon and do one of the following:

(i) You start at your house in the car. The car is not moving and you let the balloon go (inside the car). Then start the car moving. The balloon will "move" to the back of the car until it hits the back window (or something else). This is because the car is moving forward but the balloon is "staying still" relative to the road.

(ii) You start at your house in the car. You hold onto the balloon tight and then accelerate the car to a constant speed along the road. Then, you let the balloon go (in the car, again). Now, the balloon will simply float straight up relative to yourself because it's moving forward relative to the road at the same speed as the car.

I hope this helps.

Edit: BTW, not a stupid question. While there *are* stupid questions in physics, most "I wonder if..." and "what if..." questions lead to better understanding. The story goes that Einstein generated his theory of special relativity from one such question: "What if one rode a beam of light?"

Answer 2

We need more information about how the ball "floats," and how the car is moving.

Examples:
1. The car is moving along at a steady 40 mph with the ball in somebody's lap, and THEN the ball "levitates". In this case, the ball will travel along with the car. That's because the ball still has its initial forward momentum from when it was sitting in the person's lap.

2. The ball levitates while the car is sitting still, and THEN the car starts moving. In that case, the ball hits the rear window. That's because the ball has no forward momentum, so it doesn't move forward when the car does.

A couple of real-world examples that don't involve levitation:

3. If you are in a moving car and you toss a ball straight up, it will land in your lap instead of hitting the rear window. That's because it keeps its forward momentum when you toss it up.

4. If you have a helium balloon on a string in a stationary car, and then the car starts moving, the balloon will actually tilt _forward_ as the car takes off. This is a neat experiment. It happens because the air (which is supporting the balloon) "sloshes" toward the back of the car as it takes off, and the resulting air pressure forces the balloon forward.

Answer 3

Simbha_07's helium balloon example is incorrect. If you have a floating helium balloon in a car and accelerate the car, the balloon will move *forward* in the car because the effective force of gravity is directed slightly backwards, and the air density decreases opposite the direction of apparent gravity.

The answer to your question is that as long as the car is moving with uniform speed and direction, the ball will not crash into the rear window, regardless of why it is floating. No experiment can ever possibly return a result that depends on the uniform motion of the laboratory. The result of ANY experiment in a uniformly moving laboratory MUST be the same as the same experiment done in a stationary laboratory. That is the Principle of Relativity.

Answer 4

okay if the ball is floating and is not connected to the car by any means, then why would it move ?!
yet when the car moves the floating -assumed steady- ball will hit the rear window.