# What's relative motion got to do with it?

Asked by Poser (7800) May 28th, 2010

Hypothetical situation: You are the lead car in a car chase. You are traveling 100mph. The cop behind you is also traveling 100mph. You pop out of the sunroof and point your weapon backwards to take out the cop. Your round is faulty, and the slug only leaves your gun going 100mph. Two questions:

1. To a stationary observer, does the slug simply drop to the ground, directly below the point where the barrel was when the gun was fired (assuming it doesn’t hit anything else)? If not, why? If so, why?

2. If the slug hits the police officer, could you argue in court that you didn’t shoot him, but that his high rate of speed caused him to run into the bullet?

Observing members: 0 Composing members: 0

1. Yes, it does. Relative to the outside observer, the bullet before it is fired is actually moving at 100mph, together with the car. Firing the bullet and accelerating it to 100 mph relative to the reference frame of the shooter cancels out the 100mph velocity that the bullet has relative to the outside observer. Thus, to the observer, the bullet will drop straight to the ground.
This was experimentally verified by Mythbusters. (The only thing the Mythbusters can not break are the laws of physics, thank Zeus.)
2. I doubt it. The police officer can be considered part of the shooters reference frame, since s/he is moving at the same velocity. Furthermore, it would have been your intention to hit the police officer with the bullet. That you exploited a certain aspect of the laws of physics for that is irrelevant.

ragingloli (44206)

In terms of the bullet’s speed, the speed of the cars is irrelevant, since they were going the same speed. The bullet still hits the officer at about 100 mph. As a matter of fact, the bullet would hit at a higher rate of speed than normal due to decreased wind resistance.

CaptainHarley (22414)

@ragingloli Is correct. The observer will see the bullet fall to the ground. If the trailing car hits the bullet before it hits the ground he will hit it at 100 mph

Rarebear (25154)

2. “You pop out of the sunroof and point your weapon backwards to take out the cop.” If he gets shot and dies, you’re charged with manslaughter at least.

lilikoi (10079)

@ragingloli has it correct.

@ragingloli Has it. Just to add a bit:

If the stationary observer didn’t see the lead car and only saw the bullet and cop, than it would look like the cop drove into a falling bullet.

roundsquare (5512)

@roundsquare Exactly. That’s what I was trying to say above, but you said it better.

Rarebear (25154)

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