Need some help

[rootbear75]

i am currently taking AP Physics C and this problem stumped both me and my tutor... i dont see my teacher for a while, so i was wondering if anyone could help.

A 7.00-g bullet, when fired from a gun into a 1.00-kg block

of wood held in a vise, penetrates the block to a depth of

8.00 cm.

What If?

This block of wood is placed on a frictionless

horizontal surface, and a second 7.00-g bullet is fired from

the gun into the block. To what depth will the bullet

penetrate the block in this case?

[★ Blocks]

You can answer this one with knowing next to nothing about physics, just by reading the problem carefully. That's my hint. Edited March 21, 2007 by Blocks

[rootbear75]

uh no, because it will not penetrate to 8 cm... remember the block is on a frictionless horizontal surface. and you also have the law of the conservation of momentum

[★ Blocks]

I argue that the block is massive enough, the bullet is light enough, and the impact happens suddenly enough that the bullet "doesn't care" whether the block is in a vise or not. I could be wrong. Edited March 21, 2007 by Blocks

[rootbear75]

I argue that the block is massive enough, the bullet is light enough, and the impact happens suddenly enough that the bullet "doesn't care" whether the block is in a vise or not. I could be wrong.

trust me on this... you are wrong in this case....

they wouldn't put this in a textbook if it was that easy

[★ candygirl]

7.94cm

[★ Blocks]

Meh, poor question. The idea of firing a bullet into a wood block and measuring its depth to that precision is absurd. Edited March 21, 2007 by Blocks

[jacob hunter!]

Hence why you are a block head

[★ candygirl]

Well it is Physics

(m1+m2)*a*h1 = (m1+m1+m2)*a*h2 =>h2 = (m1+m2)h1/(m1+m1+m2)=1007*8/1014 = 7.94

[rootbear75]

this is why i hate AP Physics and i am just barely passing....

[Shock]

Well it is Physics

(m1+m2)*a*h1 = (m1+m1+m2)*a*h2 =>h2 = (m1+m2)h1/(m1+m1+m2)=1007*8/1014 = 7.94

My head hurts.

[★ Dav]

surely its 0 because the block will just be pushed by the bullet. You need resistance to cause the pushing of the block to be slowed and the bullet to penetrate.

 

The vice is a 100% resistance example

[Smong]

You could say using the opposite equal reaction thing that the block moves 4cm back and the bullet penetrates the remaining 4cm (so it would have travelled the same distance as before).

 

I think the bullet must penetrate, imagine a paper bag inflated. If you blow on it, it will deform and move away from you. Obviously the wood is not as weak as air surrounded by paper, but from the example you could say all materials deform. That then leads us to look at the hardness properties of the block and the bullet. What if the bullet was made of paper and the block from diamond? I don't think it would penetrate in that case.

[★ candygirl]

The only way to find out is to do it.

[rootbear75]

surely its 0 because the block will just be pushed by the bullet. You need resistance to cause the pushing of the block to be slowed and the bullet to penetrate.

 

The vice is a 100% resistance example

common sense says that the bullet will penetrate AND the block will move backward...

 

i have an idea...

get a gun

and 2 blocks of wood

put one in a vice, shoot it, and figure out how far it goes.

then put the other one on a super greasy table or greased up flat surface, and shoot that one with the gun, and figure out how far it went.....

[★ candygirl]

done "the bullet when in helf way and the wood move backward and off the table and hit the wall.

[MikeTheNose]

done "the bullet when in helf way and the wood move backward and off the table and hit the wall.

 

Are you having fun shooting?

[★ candygirl]

yes i am lol did it today at the fireing range. I go there everyday after my shift.