Random thought about force and motion

  • Thread starter tadpole
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  • #1
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It has been awhile since I took Physics. The other day my daughter asked some questions about force. She told her teacher explained if the object is moving horizontal the push force is 0. I said that can't be right. There has to be a force to overcome friction. This lead me thinking about other scenarios. Some of these questions are kind of silly. I hope some of you can enlighten me. Thanks for taking the time to answer my random thought.

If you shoot a gun in space, after the bullet left the gun:

a) does it accelerate ?

b) if yes, what cause it to accelerate since there is no propulsion or any kind of power to change velocity ? Does it continue to accelerate or at some point it will reach a constant velocity ?

c) if it reach a constant velocity, than does that mean the bullet no longer have force since
F = M X A. If the force is 0, than when the bullet hit me, it would not hurt, right ? :))

d) does it decelerate ? what cause it to decelerate ?
 

Answers and Replies

  • #2
Doc Al
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a) does it accelerate ?
No.

c) if it reach a constant velocity, than does that mean the bullet no longer have force since
F = M X A. If the force is 0, than when the bullet hit me, it would not hurt, right ? :))
A moving bullet doesn't 'have force'--it has momentum and energy. A force is an interaction between objects. When the bullet hits you, you and the bullet will exert forces on each other.

If the net force on an object is zero, then it will keep moving at constant speed in a straight line. Presumably there's no force acting on the bullet after it leaves the gun.
 
  • #3
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So even though the bullet does not have any force, it has momentum and energy and when it hits another object, this energy somehow converts pack to force at impact. Thanks, I am glad you told me, I was going to jump in front of the bullet to prove an experiment.
 
  • #4
Doc Al
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So even though the bullet does not have any force, it has momentum and energy and when it hits another object, this energy somehow converts pack to force at impact.
Force and energy are different things. One doesn't convert into the other.
 
  • #5
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Ok, so what cause the damage if not some energy conversion, is it the momentum ? I know I heard on TV, they always say something hit something with a force of blah. I never heard they said impact with momentum of blah or energy of blah. Don't give me up on yet.
 
  • #6
Doc Al
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I know I heard on TV, they always say something hit something with a force of blah.
There's nothing wrong with saying something hits something else with some force. That doesn't mean that the something was 'carrying' the force around with it. Forces between objects are generated upon contact.
 
  • #7
Yes, so -question to tadpole - what forces are in play while the gun is being fired? (assuming you can somehow get gunpowder to ignite in outer space) How long (how much time) does the force act on the bullet? And also, how do you know in general when you have a force? Could you think of another scenario where you need to identify the forces acting? You mentioned friction...
 
  • #8
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I don't think "space" is specific enough, since most space missions are close to the earth where everything is constantly being accelerated by gravity.

Hell, even 10 universes away there's still gravity, so the bullet will always be accelerated. That's probably not the intent of the problem, but imo it should at least be specified to disregard gravity, just like we usually specify to disregard friction.

I don't think gunpowder needs air to burn.
 

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