# What happens if you hit a bullet from the side?

1. Jul 26, 2015

### TheShredder

My brother actually said this and felt that it would not effect the person's hand. He said, theoretically, if a person were to move their hand from the side of the bullet (so as to slap the bullet) , at the same speed as the bullet, the bullet would would change its direction and the hand would be left unharmed. I feel that the bullet would graze the person's hand, while it is in contact with the bullet, also the speed of the hand does not need to be at the same speed as the bullet to cause it to change direction, it would require a certain amount but not exactly the same as the bullet.
Which one is right and why?
also here is a picture I made in case I could not explain the problem properly: http://imgur.com/ceATo5d
Thank you!

2. Jul 26, 2015

### Bystander

You have a collision between two objects. Is it elastic, or inelastic?

3. Jul 26, 2015

### rootone

I guess this collision would be elastic since a hand is a soft object.
Either way though there will be some exchange of momentum.
The moving hand may very slightly alter the trajectory of the bullet,
however the much faster moving bullet could apply a very considerable force to a part of the hand which it touches, and very possibly this could result in grazing or worse damage depending on factors such as the exact angle of contact

4. Jul 26, 2015

### TheShredder

If the hit is made at a 90o angle

5. Jul 26, 2015

### rootone

Probably would result in a nasty graze along with some tissue damage due to heat caused by friction, but no permanent injury.
The trajectory of the bullet would probably be altered by a very small amount.

6. Jul 26, 2015

### Staff: Mentor

That is my expectation as well. A frictionless cold bullet would not harm the hand (assuming it is completely flat to make that collision exactly as described).

7. Jul 26, 2015

### stedwards

Bullet grazing wounds to the palm and wrist.

8. Jul 26, 2015

### DaveC426913

I realize you're trying to make an ideal scenario here, to isolate the mechanisms, but I'm not sure that discounting friction illuminates the issue here. Of course a frictionless bullet would do no harm.

9. Jul 26, 2015

### Alasmic

If you were to hit the bullet and cause the exact force of the bullet it would change direction with no effect. But if the force of the object hitting it was less then the force measured with the speed of the bullet it would cause damage. Also this all depends on the angle of hitting the bullet and the direction it was hit. To the side up or down gravity does play effect in this equation

10. Jul 26, 2015

### DaveC426913

I refute both these claims.

We're talking about a human hand here. Damage can and probably will occur regardless of whether the oblique force is equal, less than or greater than the momentum of the bullet.

11. Jul 26, 2015

### Alasmic

If your talking about heat yes but for the amount of time touching the bullet there would not be enough time to transfer the heat needed to cause damage

12. Jul 26, 2015

### Alasmic

Regardless of skin blood bone or metal if the force is exact then their shouldn't be variants

13. Jul 26, 2015

Staff Emeritus
A bullet moves at 1000 mph. People don't move that fast.

14. Jul 26, 2015

### Alasmic

His question was theoretical

15. Jul 26, 2015

### stedwards

This won't work. You have a 1700 mph bullet and 1700 mph hand. They have a collision velocity of 1700*sqrt(2) = 2400 miles per hour. The bullet puts a hole in the hand no matter how cold or frictionless it is.

16. Jul 26, 2015

### rootone

I can say that I have 'experimental evidence' that a very brief contact of human flesh with a hot object (such as a soldering iron) will cause tissue damage.

17. Jul 26, 2015

### Bystander

That was my take on the OP as written. The other possible "twist" is that "The Bulletproof Monk" can parallel the trajectory for a couple milliseconds and "tap/slap/nudge" it sideways by some measureable amount.

18. Jul 26, 2015

### Alasmic

At a 90 degree angle it would knock the bullet of its course but at and degree above that I believe yes it would go through the skin probably at twice the speed of a stationary hand

19. Jul 26, 2015

### stedwards

No. In the center of velocity frame you just get hit by a faster bullet. Bystander understands what trajectory is required of the magical 1700+ mph hand.

20. Jul 26, 2015

### Alasmic

Do you propose constructing a lab with the object of impact having the same elasticity and angle!?