A bullet from speeding to full stop.

  • Thread starter chawki
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  • #1
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Homework Statement


A bullet (mass 15 g) moving at a speed of 425 m/s is brought to a stop in 3.5 cm in a block of wood.

Homework Equations


Find the force on the bullet, assuming that it is constant.


The Attempt at a Solution


what law should we use?
0 - 1/2*m*V^2 = m*g*0.035 + F*0.035 ?
 

Answers and Replies

  • #2
PhanthomJay
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Homework Statement


A bullet (mass 15 g) moving at a speed of 425 m/s is brought to a stop in 3.5 cm in a block of wood.

Homework Equations


Find the force on the bullet, assuming that it is constant.


The Attempt at a Solution


what law should we use?
0 - 1/2*m*V^2 = m*g*0.035 + F*0.035 ?
Explain why you think that the gravity force does work.
 
  • #3
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It's a law..isn't ? HELP
 
  • #4
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Wow. I think i was wrong...it doesn't have work because the motion is on X axis, not Y axis.
so i think we will have only: - 1/2*m*V^2 = F*0.035 ?
 
  • #5
tiny-tim
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so i think we will have only: - 1/2*m*V^2 = F*0.035 ?

yup! :smile:
 
  • #6
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But then we will get F=-38705.35N
How is possible to have a negative value!
 
  • #7
PhanthomJay
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Force is a vector, that has both magnitude and direction. The negative sign implies that if the bullet is moving from left to right, in the positive x direction, then the force acting on it is in the opposite (negative x) direction, from right to left, acting to slow it down.
 
  • #8
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Ok, so F=-38705.35N is correct ?
And thank you everyone, i'm learning new things :)
 
  • #9
tiny-tim
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Find the force on the bullet, assuming that it is constant.
Ok, so F=-38705.35N is correct ?

no, the question is either asking for the magnitude of F, which is always positive, or it's asking for a "vector" description of F, which means a magnitude and a direction

(oh, and far too many significant figures :redface:)
 
  • #10
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ok, so it is 38705.35N!
 
  • #11
tiny-tim
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yeees …

but what did i say about the significant figures?
 
  • #12
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38705?
 
  • #13
tiny-tim
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hmm …

how many significant figures are there in the question?

so how many do you think there should be in the answer? :smile:
 
  • #14
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I never ever used these things of significant figures...that's why i'm all confused
 
  • #15
tiny-tim
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ok …*look at signifcant figures in the PF Library :smile:
 
  • #16
510
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38705 Yes or NO
 
  • #17
510
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hmm …

how many significant figures are there in the question?

so how many do you think there should be in the answer? :smile:

There are three? so the answer would be 38705.357N?
 
  • #18
tiny-tim
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There are three?

no, the question gives m v and d to 3 and 2 significant figures …

so the answer is probably best in only 2 sig figs
… so the answer would be 38705.357N?

are you even trying? :redface:

how would you write 38705.357 to 3 sig figs?

how would you write 38705.357 to 2 sig figs?

(if you don't know how, then look it up)
 
  • #19
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39000n?
 
  • #20
tiny-tim
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yes, to 3 significant figures it's 38700, to 2 it's 39000
 
  • #21
510
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we can write also: - 1/2*m*V^2 = -F*0.035 knowing that F will be from right to left, opposite the bullet direction and then we will find F the same. no need to introduce the magnitude.
 

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