I was having a chat with my friend about classical physics and I got this situation on my mind: imagine that a person are in a car at 200 Km/h and he shoots a gun that throws a bullet with the same speed as the car, the bullet will be flying through the window of the car or will stop where the person are sitting? I know it has something with momentum (I guess).

The car, shooter and gun are all .co-moving at the same speed , so the bullet speed from the shooters' measurement is 200Km/H. Note: In the frame of the shooter he is travelling at 0Km/h i.e.stationary so you just add the speed of the bullet to the 0Km/H.

 

To an observer standing on the road, watching the driver shoot, you add the speed of car plus the bullet speed, which is 400Km/H. I'm ignoring air- friction forces on the bullet.

 

The basic idea to measuring speed is that whichever position you take a measurent from should be considered stationary. i,e. 0 speed. The measurement of speed of a position is always relative to the speed of another position.

Edited March 10, 201510 yr by StringJunky

The car, shooter and gun are all .co-moving at the same speed , so the bullet speed from the shooters' measurement is 200Km/H. Note: In the frame of the shooter he is travelling at 0Km/h i.e.stationary so you just add the speed of the bullet to the 0Km/H.

 

To an observer standing on the road, watching the driver shoot, you add the speed of car plus the bullet speed, which is 400Km/H. I'm ignoring air- friction forces on the bullet.

 

Yes, for who is watching outside the car (0 km/h) the bullet speed will be 400 km/h. So in this case the bullet will go out the window, right?

Yes, the bullet will exit for both of them but the observer on the ground will measure 400Km/H for the bullet. The driver will measure 200Km/H

Edited March 10, 201510 yr by StringJunky

What about if the bullet is fired from the front of the car out the back window, do you then subtract the speeds? (same speed but with opposite velocity) as soon as it goes through the window will it just drop to the ground? I think that is the case and the previous case was right too where the bullet was fired forward. (Added when they have the same velocity as the car.)

What about if the bullet is fired from the front of the car out the back window, do you then subtract the speeds? (same speed but with opposite velocity) as soon as it goes through the window will it just drop to the ground? I think that is the case and the previous case was right too where the bullet was fired forward. (Added when they have the same velocity as the car.)

 

There's a short video further down that's pretty good; the ball drops. In the drivers' frame-of-reference, the bullet would still be travelling at 200Km/H because he's moving away from the stationary bullet at that speed. You would see this better in a gravity-free environment because the bulletl wouldn't drop.

Edited March 10, 201510 yr by StringJunky

What about if the bullet is fired from the front of the car out the back window, do you then subtract the speeds? (same speed but with opposite velocity) as soon as it goes through the window will it just drop to the ground? I think that is the case and the previous case was right too where the bullet was fired forward. (Added when they have the same velocity as the car.)

Mythbusters did this firing a basketball out of an air cannon off the back of a truck (I believe that was the set up). It took them a lot of tries to perfectly match the speed of the truck to the speed of the ball coming out of the cannon, but eventually they did manage to get the ball to just drop and bounce straight up and down.

Mythbusters did this firing a basketball out of an air cannon off the back of a truck (I believe that was the set up). It took them a lot of tries to perfectly match the speed of the truck to the speed of the ball coming out of the cannon, but eventually they did manage to get the ball to just drop and bounce straight up and down.

The vid I linked to is that one.

The vid I linked to is that one.

This is what I get for not clicking links.

This is what I get for not clicking links.

Easily done.

one other point,it would take the bullet the same time to fall to the ground,if you fire it out of the front or back of the car.