A bullet and force


by x86
Tags: bullet, force
x86
x86 is offline
#1
Nov4-13, 09:53 PM
P: 86
I was thinking about force lately, after reading some physics 100 questions on the internet to see if I have what it takes to become a physicist.

For example, when you shoot a bullet, the chemical reaction that takes places exerts a force on the bullet for a short period of time (due to pressure). After that, the only force acting on the bullet is air resistance (slowing it down).

However, isn't it also true that the bullet exerts a force on anything it may encounter? Such as say air molecules? (The bullet exerts a force on any air molecules it comes in contact with, losing energy as thermal and slowing down)

What is the correct answer to a question like this one? How does this whole thing work?

"A horizontally moving bullet slows
down. Is anything exerting a force on it? How do you
know? Is it exerting a force on anything? How do you know?"

This isn't a homework question, its just something on the internet I'd like to know the answer to.

How would one answer this question? Am I thinking too deeply about physics? Or is deep thought what is required?

I would personally think this is the fact:
It exerts a force on air molecules, or anything that it may encounter because air around the bullet moves.
and
Air exerts a force on the bullet, because the bullet slows down.

EDIT: Also, any advice for someone looking to study physics?
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Drakkith
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Nov4-13, 10:24 PM
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Yes, the bullet exerts a force on anything it touches. The force exerted on the air is equal to the force the air exerts on it.

Newton's Third Law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body.
Simon Bridge
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#3
Nov4-13, 11:08 PM
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Quote Quote by x86 View Post
What is the correct answer to a question like this one? How does this whole thing work?

"A horizontally moving bullet slows
down. Is anything exerting a force on it?
Yes.

How do you know?
Newton's 1st law.
An object remains at rest or moves at a constant velocity unless it is acted on by an unbalanced force.
The bullet is slowing down - therefore, there is an unbalanced force on the bullet.

Is it exerting a force on anything?
Yes.

How do you know?"
Newton's third law.
See Drakith.
The bullet has a force on it, therefore it is exerting a force on something else.

Notice how nothing in the question says what the bullet is moving through - only that it is "horizontal" and it is "slowing down". Sometimes you have to make assumptions of things that are not mentioned in the question, but you should try not to.

Am I thinking too deeply about physics?
Nope. About average.

Or is deep thought what is required?
You are doing fine - it usually requires quite shallow thought.
The question does not ask you to provide details about the force, where it comes from etc.
Having established the existence of a force or forces on the bullet and exerted by the bullet, you are then able to investigate the situation to figure out the details.

In this example, the bullet has to push the air out of the way - which means that there is moving air in the wake of the bullet.
This has knock-on effects like heat and sound... which are usually more easily handled in terms of energy. Something you seemed to naturally start to do when you wrote "...losing energy as thermal and slowing down"... which suggests you may have some talent there ;)


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