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# neeed help on this questiionn!!

Thu 18 Mar, 2010 01:31 pm
When using a hammer to drive a nail into wood, you have toswing the hammer through the air with a certain velocity. Because the hammer has both mass and velocity, it has momentum. Describe what happens to the hammer's momentum after the hammer hits the nail.
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Type: Question • Score: 4 • Views: 686 • Replies: 8
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boomerang

1
Thu 18 Mar, 2010 02:43 pm
My guess is that the momentum is transferred to the nail.
0 Replies

1
Thu 18 Mar, 2010 02:50 pm
i'm sure engineeeeeeeeeeeeeeeeer will be right along...
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realjohnboy

1
Thu 18 Mar, 2010 05:13 pm
@swaylorwwe1110,
Please tell us why we should do your homework for you.
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fresco

1
Thu 18 Mar, 2010 05:16 pm
@swaylorwwe1110,
The force on the nail is proportional to the rate of change of momentum. This is large since the velocity ceases in a very short time period. Not all the momentum is transferred to the nail since the hammer,nail and receiving surface act as a combined body after impact.
The energy/ work done by the hammer =
force x distance travelled by hammer+nail
+ energy lost due to heat, and sound
+ energy lost in deformation of pierced body.
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realjohnboy

1
Thu 18 Mar, 2010 05:35 pm
@swaylorwwe1110,
swaylorwwe1110 wrote:
Describe what happens to the hammer's momentum after the hammer hits the nail.

Eventually it stops would be my guess. That should be good for a B+ I think.
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ebrown p

2
Thu 18 Mar, 2010 06:24 pm
Momentum is conserved... meaning the total amount of momentum is always the same... meaning that if something loses momentum, something else needs to gain it.

In this case, the hammer puts force on the nail. The nail starts moving. But then the wall puts force on the nail, called friction, which slows the nail down.

Now for the interesting part... Newton's third law says that if the wall puts force on the nail, the nail puts an equal and opposite force on the wall, this of course means that the wall speeds up-- and it must speed up in order to fulfill the law of the conservation of momentum. But the wall is connected to the Earth (which also speeds up in this example.

This all makes sense when you consider that Momentum is mass times velocity. The hammer is pretty small. The wall connected to the Earth is very very big. This means that the Earth only changes an tiny little insignificant amount compared to the big change you see in the change of the hammer.

Furthermore, course, you pushed on the Earth (backwards) in order to get the hammer moving in the first place-- meaning you changed the speed of the Earth in a tiny little way so that the momentum of the moving hammer was counteracted... so the change after the hammer moved means that nothing really changed.

The basic problem thinking about this is that we are connected to such a massive planet, that we don't think about the insignificant ways our little motions affect it.

If you did this in a tiny spaceship far from Earth, the impact of your actions would be much more clear. When you started swinging the hammer, you would fly backwards. When you hit the nail, you would fly backwards faster (which would throw the spaceship forwards). When you hit the wall of the spaceship you would stop, and the spaceship would stop.

(the point being, this similar thing happens with the Earth but the effect is far too small with the big Earth to even notice).

1
Thu 18 Mar, 2010 06:28 pm
@ebrown p,
fascinating stuff
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JTT

1
Thu 18 Mar, 2010 09:07 pm
@ebrown p,
Quote:
The hammer is pretty small. The wall connected to the Earth is very very big. This means that the Earth only changes an tiny little insignificant amount compared to the big change you see in the change of the hammer.

Not when Pan swings his 22 ouncer!
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