Gravity/physics - a question

[Electric Skeptic]

I have a question about which my wife and I have been arguing, and I'd appreciate it if anyone of the many on these forums who know more about physics than I could answer it conclusively, along with an explanation as to why:

Imagine you are standing on a perfectly flat plain. In one hand, you hold a loaded, high-powered handgun. In the other hand you hold a bullet of the same type with which the handgun is loaded. Both hands are at the same height above the ground. You point the handgun exactly parallel to the ground and, at precisely the same time, pull the trigger and drop the bullet in your other hand.

The question: which bullet hits the ground first, the one you dropped or the one you fired?

 

[Lucretius]

Both hit the ground at the same time. The effect of gravity is independent of horizontal motion. Both fall at 9.8m/s^2.

I'm assuming you also didn't fire the bullet fast enough to just start orbiting the Earth.

 

[Koz]

in a vacuum, they both hit at the same time

in air i dunno

 

[David Gould]

Air is the problem, I think. I am wondering if the fact that the bullet is moving forward creates any lift for it in the turbulence around the bullet. (Or it could create downward force, I guess - difficult to say.)

 

[David Gould]

But, assuming no differences, they would hit at the same time.

 

[Kahalachan]

Gravity would pull down on them equally and they'd hit the ground at the same time.

 

[Adriac]

As others have said: Discounting the effects of air and the curvature of the earth, both will land at exactly the same time. This is because perpendicular vectors have no effect on each other.

If you want to explain it intuitively, look at it this way: if you fire the gun up, the falling bullet will land first. If you fire the gun down, the shot bullet will hit first. In between, it's the same time.

Aside, it doesn't matter what you drop. A VW Bug dropped from the same height will land at the same time.

 

[Electric Skeptic]

Thanks all - that's what I've been trying to tell her. But she just says that it's common sense that the shot bullet will take longer to land. I don't have sufficient science to show her why that's wrong

And yes, I was ignoring such things as the curvature of the earth, air resistance, and so forth.

 

[Kahalachan]

Thanks all - that's what I've been trying to tell her. But she just says that it's common sense that the shot bullet will take longer to land. I don't have sufficient science to show her why that's wrong

And yes, I was ignoring such things as the curvature of the earth, air resistance, and so forth.

I think she is confusing this with personal experiences. If you see a rock skipping on a pool or lake, it does seem like it's taking longer to hit the ground. If you just throw the rock and let another rock drop, they'll both hit at the same time.

Do the actual experiment maybe? Sounds like it'd be a fun way to do an "I told you so"

 

[Electric Skeptic]

I think she is confusing this with personal experiences. If you see a rock skipping on a pool or lake, it does seem like it's taking longer to hit the ground. If you just throw the rock and let another rock drop, they'll both hit at the same time.

Do the actual experiment maybe? Sounds like it'd be a fun way to do an "I told you so"

lol Here in australia, it'd be a good way to get arrested

 

[Physics_guy]

Common sense is notoriously unreliable.

Explain it to her like this: before shooting the gun or dropping the bullet, both bullets are in equilibrium with respect to forces acting upon them. The force of gravity on the bullets is exactly matched by the normal force of the compression of the molecules in the barrel of the gun or in the fingers of your hand. However, once the bullet leaves the barrel and you release the other bullet, then the forces are no longer in balance but they are still equal. Outside of the barrel the initial explosion is no longer applying a force to the bullet in the gun, so the only force acting on it is gravity, just as the only force acting on the bullet you drop is gravity. Now the moving bullet might have a lot of forward momentum, but that is irrelevant because it is at 90 degrees to the angle of the force of gravity - that forward momentun does not have to be overcome before the bullet hits the ground.

Therefore, since the forces affecting them are identical as are their masses, a = f/m says that their accelerations will be identical - both towards the ground at 9.8 m/s^2.

 

[azmurath]

I have a question about which my wife and I have been arguing, and I'd appreciate it if anyone of the many on these forums who know more about physics than I could answer it conclusively, along with an explanation as to why:

Imagine you are standing on a perfectly flat plain. In one hand, you hold a loaded, high-powered handgun. In the other hand you hold a bullet of the same type with which the handgun is loaded. Both hands are at the same height above the ground. You point the handgun exactly parallel to the ground and, at precisely the same time, pull the trigger and drop the bullet in your other hand.

The question: which bullet hits the ground first, the one you dropped or the one you fired?

They both hit at the same time. Gravity pulls on objects the same , and unles you had the gun slightly tipped upward or downward, they would hit at the same time.

 

[Elduran]

Thanks all - that's what I've been trying to tell her. But she just says that it's common sense that the shot bullet will take longer to land. I don't have sufficient science to show her why that's wrong

And yes, I was ignoring such things as the curvature of the earth, air resistance, and so forth.

See if you can find the video clip of the astronaut dropping a feather and a golf ball (or whetever heavy object it was) on the moon, and deomstrate to her that way that common sense doesn't always apply!

 

[arunma]

Thanks all - that's what I've been trying to tell her. But she just says that it's common sense that the shot bullet will take longer to land. I don't have sufficient science to show her why that's wrong

And yes, I was ignoring such things as the curvature of the earth, air resistance, and so forth.

Actually, if you were firing something that goes a bit faster than a bullet (say...a ballistic missile), then the curvature of the earth would matter quite a bit. It's possible that you could send your projectile into orbit, which would cause it to never hit the ground. Even in the case of a bullet, the curvature of the earth would technically cause it to hit the ground slightly after the bullet that is dropped from rest.

Someone else mentioned aerodynamic effects. If the bullet does not come out of the gun precisely parallel to the ground, the angle of attack could potentially cause a small lift effect, which would cause the moving bullet to fall either faster or slower than the bullet that is dropped from rest.

As you can tell from the responses people have given, there are all kinds of fun ways to make this a difficult problem. Even simple things like air resistence makes this a complicated issue, even for those of us who have taken an advanced course in classical mechanics. But neglecting all of those peripheral issues, if you'd like to explain to your wife why the two bullets hit the ground at the same time, you can tell her that horizontal and vertical motion are completely decoupled. In other words, moving something horizontally doesn't affect its vertical motion, which is why both bullets accelerate at the same rate.

Well, I'm sorry for rambling. I do that a lot when it comes to physics. But I hope that helps.

 

[BVZ]

Imagine a train that moves as fast as a bullet. When riding on this train, dropping a bullet will take x amount of time, weather the train is moving or not.

 

[jwu]

http://www.glenbrook.k12.il.us/gbssci/phys/Class/vectors/u3l2b.html
This site is pretty good...i*m sure there are also videos somewhere on the net which show this with two falling balls as well, one of them moving horizontally and triggering the second one in the moment when it begins to drop of the track.

 

[Adriac]

Well, actually, it's worth noting that technically, if you pull the trigger and drop the object at the exact same time, the bullet will begin falling only after it leaves the barrel, slightly after the one you drop.


For some reason I though that was worth mentioning

 

[h2whoa]

Hang on. Let me get this question straight in my mind.

You are pointing a gun straight ahead of you on a totally straight surface. You fire the gun and at exactly the same moment drop a bullet from the same height to see which one will hit the floor first? Am I right about this?

If I am, then it seems to me that it is the bullet that you drop.

Although the effect of gravity is the same on both there is a fundamental difference between them. The bullet you've dropped has only one velocity: a downward one. The bullet however technically has two: a downward one but also a tremendous forward velocity.

It's like if you drop a piece of paper or throw a paper airplane. The piece of paper hits the floor first before the airplane (so long as you haven't fudged the plane design). Similarly if you have two identical (decent) paper airplanes and you let one just drop from shoulder height and throw the other from the shoulder then the one you dropped will hit the floor first.

 

[z3ro]

Although the effect of gravity is the same on both there is a fundamental difference between them. The bullet you've dropped has only one velocity: a downward one. The bullet however technically has two: a downward one but also a tremendous forward velocity.

Which is why the problem is so counter-intuitive. Horizontal and vertical motion are independent. In a vacuum(which avoids most of the stickies already mentioned) the only force pulling the bullets down is gravity, which is why they hit at the same time(the horizontal velocity is not pulling the bullet down, nor is it holding it up. Which is why wind resistance is an issue).

It's like if you drop a piece of paper or throw a paper airplane. The piece of paper hits the floor first before the airplane (so long as you haven't fudged the plane design). Similarly if you have two identical (decent) paper airplanes and you let one just drop from shoulder height and throw the other from the shoulder then the one you dropped will hit the floor first.

In an atmosphere. Do it in a vacuum, and both will hit at the same time.

The video is actually of an astronaut on the moon, dropping a hammer and feather at the same time, and they both land at the same time.

 

[RealityCheck]

Well, actually, it's worth noting that technically, if you pull the trigger and drop the object at the exact same time, the bullet will begin falling only after it leaves the barrel, slightly after the one you drop.


For some reason I though that was worth mentioning

Damn, you beat me to the punch.