Get your head round this please!

[mr. piper]

If, and this is hypothetical, you were travelling in a car at 100mph, and you fired a projectile in the same direction at 100mph, it would, relative to an onlooker be travelling at 200mph, ok? Then if the projectile was fired at 100mph in the opposite direction to the car, the on looker would see the object actually standing still (disregarding air resistance and gravity) as would a person that walks the wrong way up an escalator to an onlooker appears to go nowhere. Now with the effect of gravity and air to slow the object down it should start to drop and go backwards at the rate that it would decelerate and start to fall. I admit that there are a lot of variables in this, but in a perfect world, am i correct in my thinking?

[Loosehead]

do you mean backwards towards or back wards further away from the car than it would have been in a vaccum? the latter is correct

Generally this is a though experiment and they are usually done ignoring air resistance etc. Any way as you are introducing the air then it depends on the object, it will move slightly away ie away from where the stationary point is in your description above that is further away from the car. This is because when you fire the object you get the 100 + the bit of wind resistance pushing it back. The amount depends on the projectile, heavy iron ball hardly any movement, drop straight to the ground. Feather lots of movement.

The though experiments that are famous all disregard air resistance as it's mainly on Earth that these matter for the most part the univers is free from this tedium!

[Space]

in your experiment, admitting the wind speed is 0,
the bullet would be still once it is launched, and it wouldn't be pushed by the wind because the wind has the same speed: 0.
So it would just fall down.

However, if there is a car behind just behind, at 100mph, they will see things in a different way...

[jake-the-peg]

Whilst you're talking about 100mph you're right.

If you now extent this up towards the speed of light things start to get a little different.

This sounds rather like the classic introduction to special relativity

[bernardo]

Yes.

[fo3nix]

- if the onlooker is stationary on the ground that the car is travelling at 100mph on

"yes"

[Peter Pedant]

give or take a drink or two,

it falls like a stone