# Why doesn't any object that doesn't have direct contact in a fast moving vehicle feel G-force or slam behind or ahead when braked or accelerated?

Why doesn't any object that doesn't have direct contact in a fast moving vehicle feel G-force or slam behind or ahead when braked or accelerated?

For example when we are in a car (even with all windows open) and driving speed is of 100 kmph (for that matter, even plane, whose speed is over 500 kmph) and we throw up a ball (which is a non-living thing and has sufficient mass and it isn't flying on its own unlike a fly or bee etc..) in the car, and for that brief moment it is not in any direct contact with any car or any interiors, so by common physics logic it should slam into our face but it doesn't. Why?

Explanation

When the car accelerates, it accelerates any object bound to it at the same time*: everything in it gains kinetic energy and everything travels at the same speed. When you remove the constraints (by throwing a ball), it still has the speed it has gained from the car acceleration because of inertia. Only exterior forces change the velocity of a solid. So if you throw it in the car while it's going straight at a constant speed, from the car referential the behaviour (trajectory) of the ball will be the same as if you were standing still on the ground. If you had thrown it out the window though, the friction from the air would have slowed it down and it would have appeared as though it was moving backwards whereas it would have been moving slowlier than you (and decelerating).

Example 1

One of the first examples that come to my mind is in action movies when the hero is throwing himself out a car to avoid being smashed against a wall at great speed: he doesn't just get out and walk, but injures himself rolling in the direction the car was heading. He's in fact (trying to) losing(/lose) kinetic energy (hence speed) through friction with the ground, because he had acquired that kinetic energy from being in the car (otherwise the car would move but not him with it, meaning he was a ghost - plot twist).

Example 2

Another way to think about it: you're already travelling at more than 3km/s in a Earth centred inertial reference frame (axes pointing at fixed stars), and yet you're still falling vertically when you're jumping. That's because you've already acquired that kinetic energy, and you're still travelling at the same speed even in the air because the Earth is not changing its rotation speed.

*: What happens is, the car starts moving but everything in it is at rest - as it moves it eventually makes contact with people and objects if they weren't against a backrest, and starts applying a force to continue going forward, which will accelerate them (if they're too heavy, that force will have to be high and the car will accelerate slowlier).

• ok. Got your explanation for ball. But what about fly or a mosquito or a bee who are flying in a plane. They are not in any contact with the plane nor have I thrown them, and they themselves aren't capable of flying at 500 kmph. So by physics law, they should be slammed when the plane picks up speed, and stay slammed behind until the plane slows down. But they (bees, mosquitoes, flies) keep on flying around the plane when the plane is in mid-air flying at 500 kmph... – Ram Sagar Mar 5 '14 at 7:36

When you tossed the ball inside the car, its initial speed was 100mph, like of all other objects including you and your hand which was holding the ball. Since no force is applied to the ball, it'll keep moving at this speed. It's called inertia.

• ok. Got your explanation for ball. But what about fly or a mosquito or a bee who are flying in a plane. They are not in any contact with the plane nor have I thrown them, and they themselves aren't capable of flying at 500 kmph. So by physics law, they should be slammed when the plane picks up speed, and stay slammed behind until the plane slows down. But they (bees, mosquitoes, flies) keep on flying around the plane when the plane is in mid-air flying at 500 kmph... – Ram Sagar Mar 5 '14 at 7:36
• Did you observe an air born fly during take off our landing? – Aksakal Mar 5 '14 at 12:25
• both,take-off and landing – Ram Sagar Mar 6 '14 at 17:24
• think of this. what is the acceleration of a plane during take-off and landing? how does it compare to G? then think of this: G is pulling a fly or a bee when it's flying around, why then doesn't it fall down to the ground? – Aksakal Mar 6 '14 at 18:06

Assume the car is moving in the X direction, let's say to the right. You are inside the car (so you are also moving in the x direction as fast as the car is) and throw the ball in the Y direction, let's say upwards. The motion of the ball, as seen from somebody standing still outside the car, is that of a projectile shot upwards with whatever speed you throw it at, and to the right with the same speed as the car. The key concept to understand is that since you throw it from within the car, its trajectory follows the one of a projectile thrown up and to the right, not just up, this is why it keeps up with the speed the car is moving and doesn't fall behind.

• ok. Got your explanation for ball. But what about fly or a mosquito or a bee who are flying in a plane. They are not in any contact with the plane nor have I thrown them, and they themselves aren't capable of flying at 500 kmph. So by physics law, they should be slammed when the plane picks up speed, and stay slammed behind until the plane slows down. But they (bees, mosquitoes, flies) keep on flying around the plane when the plane is in mid-air flying at 500 kmph... – Ram Sagar Mar 5 '14 at 7:37

Because the air in the vehicle is moving at the velocity of the vehicle containing it. The fly is hovering in the air and the air accelerates the fly at the same rate as the vehicle. If the inside of the vehicle were a vacuum and the fly were somehow hovering (it can't) the fly would hit the rear windshield.