Forces misconception hammer and nail

  • Thread starter eulerddx4
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Can someone please explain how a hammer can drive a nail. A person swings a hammer in the air and exerts a force on a nail but newton's third says the nail exerts the same force back on the hammer. So if newton is right then how does the nail accelerate.

I just don't get it.
 

Answers and Replies

  • #2
A.T.
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Can someone please explain how a hammer can drive a nail. A person swings a hammer in the air and exerts a force on a nail but newton's third says the nail exerts the same force back on the hammer. So if newton is right then how does the nail accelerate.

I just don't get it.
What exactly don't you get about it?
 
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  • #3
Pengwuino
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Can someone please explain how a hammer can drive a nail. A person swings a hammer in the air and exerts a force on a nail but newton's third says the nail exerts the same force back on the hammer. So if newton is right then how does the nail accelerate.

I just don't get it.
The hammer does get accelerated backwards (as in up). What do you think stops the motion of the hammer as you slam down on the nail?
 
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  • #4
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I guess my question might be is the hammer able to exert a force on the nail because it has mass and acceleration?
Also if the nail exerts the same force back then how does the nail accelerate
 
  • #5
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If i have a coffee mug on a table why am i able to push it with my hand if whenever i push it the coffee mug pushes back.
 
  • #6
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Thats what im trying to get at and i feel extremely stupid right now but im just not getting it
 
  • #7
A.T.
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Also if the nail exerts the same force back then how does the nail accelerate
Why should the force on the hammer prevent the nail from accelerating?
 
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  • #8
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because the force on the hammer is equal to the force on the nail
 
  • #9
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so shouldnt the forces on the nail and hammer just go away if newtons law is true
 
  • #10
Pengwuino
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If i have a coffee mug on a table why am i able to push it with my hand if whenever i push it the coffee mug pushes back.
You exert the same force that the coffee mug exerts on you. However, since you have a far far greater mass, by F = ma, the coffee mug will provide a very small (and to you, unnoticeable) acceleration back on you compared to the acceleration you provide to the mug.
 
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  • #11
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oooooo
 
  • #12
A.T.
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because the force on the hammer is equal to the force on the nail
But it is acting on the hammer. Why should it affect the nail?
 
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  • #13
A.T.
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so shouldnt the forces on the nail and hammer just go away if newtons law is true
Why? They act on two different objects.
 
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  • #14
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well im thinking that theres a force acting on the hammer just as theres a force acting on the nail
 
  • #15
A.T.
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well im thinking that theres a force acting on the hammer just as theres a force acting on the nail
Yes, and therefore they both are accelerated in opposite directions.
 
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  • #16
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i just dont get why theres acceleration if the force of the hammer is the same and opposite the force on the nail. To me it seems like the force should just go away if thats the case
 
  • #17
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says to self: "why is this so hard for me to understand"
 
  • #18
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You exert the same force that the coffee mug exerts on you. However, since you have a far far greater mass, by F = ma, the coffee mug will provide a very small (and to you, unnoticeable) acceleration back on you compared to the acceleration you provide to the mug.
but why should i be providing acceleration if the force is canceled. Doesn't there have to be some outside force. When i push on a car the car pushes on me. The only way for me to move the car / accelerate it is if i push on the ground really hard.
 
  • #19
A.T.
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i just dont get why theres acceleration if the force of the hammer is the same and opposite the force on the nail. To me it seems like the force should just go away if thats the case
They don't cancel, because they act on different objects.
 
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  • #20
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When i push on a car the car pushes on me. The only way for me to move the car / accelerate it is if i push on the ground really hard.
This is not correct. If the ground were frictionless so that neither you nor the car could push on the ground then when you push on the car you would accelerate in one direction and the car would accelerate in the opposite direction.
 
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  • #21
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so then how does someone push something like a car or a box
 
  • #22
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last year when i took physics my physics teacher said if you have a game of tug of war, the only way to win is if one side pushes harder on the ground because when you pull on the other side the other side pulls with the same exact magnitude in the opposite direction. Is my physics teacher completely misinformed???
 
  • #23
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i dont understand why there is any acceleration at all. In my mind there has to be a net force for there to be acceleration but i see no net force here
 
  • #24
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i see no net force here
Draw the free-body diagrams for the problem and label all of the forces acting on each body. Then you should clearly see net forces.
 
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  • #25
I think eulerddx4's confusion is pretty understandable, and most of you really aren't helping by acting like the answer to his problem is so obvious.

So I'm going to try to actually help him out, please correct me if I make any mistakes...


The term "net force" has to do with what you make your system. Here your system is the hammer and the nail, and there are external forces on the system: your hand, gravity, and contact forces in the wood you're nailing. These EXTERNAL forces are transmitted through the hammer and nail, and cause acceleration. The force between the hammer and the nail is an internal force, and causes no acceleration of the system as a whole. So if there was truly no net force - no external force - that is the hammer was flying at the nail in space - it becomes a question of conservation of momentum. The hammer knocks into the nail, giving it a certain amount of momentum. In this case, the center of mass of the system remains at a constant velocity (no force), but remember, your system is NOT just the nail, its the hammer AND the nail. If you're only looking at the nail, you have an external force - the impulse given by the hammer.

But in the case of "driving" the nail, what's really happening is your hand is applying an external force to the system - the force between the nail and hammer does "cancel out" in the sense that it does not affect the motion of the center of mass of your system.


Does this help dude?
 
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