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Homework Help: Net Force?

  1. Nov 23, 2011 #1
    A bullet of mass 20 g strickes a fixed block of wood at a speed of 320m/s. The embeds itself into the block of wood, penetrating to a depth of 6.0cm. Calculate the average net force acting on the bullet while it is being brought to rest.
  2. jcsd
  3. Nov 23, 2011 #2
    The block will stop all of the force applied by the bullet; when it just enters the block, the force is equal to that of the force of the bullet, when it stops there is no force. So the average force would be the average of those two forces.
  4. Nov 23, 2011 #3
    How do I find that force?
  5. Nov 23, 2011 #4

    Andrew Mason

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    What is the force of the bullet? By your reasoning, the average force = initial force/2. Why would that be? You seem to be mixing up average velocity with average force.

    (Instantaneous) Force = dp/dt = mdv/dt; Average force =?

  6. Nov 23, 2011 #5
    Well this is a question from a worksheet and it does give the answer, but my teacher wants us to show how we got there! The answer is 1.7 x 10^4. Also How would I find the initial force using those information???
  7. Nov 23, 2011 #6
    You could apply the Work-Energy Theorem ...
  8. Nov 23, 2011 #7
    Can you tell me what it is? I don't really remember it!
  9. Nov 23, 2011 #8
  10. Nov 23, 2011 #9
    And work can be calculated with the following formula:
    See if you can put those two together ...
    Last edited by a moderator: May 8, 2017
  11. Nov 23, 2011 #10
    What do I substitute the energies with, since I only have a speed of 320m/s? Do sub. that? And the 1/2m(v(2)^2 - v(1)^2 , I don't have v(1)^2!
  12. Nov 23, 2011 #11
    d is for distance right?
  13. Nov 23, 2011 #12
    The only unknown is the net force. Read the question again, it says that it is being brought to rest. What does that indicated about the final velocity of the bullet?

    And yes, d is distance, in meters.
  14. Nov 23, 2011 #13
    How do I find the F in W= F*d? Sorry I am over thinking this question! And I am panicking since I still have more physics homework to do!!!!
  15. Nov 23, 2011 #14
    The work-energy theorem states that work is equal to the change in kinetic energy. However, we also know that work is equal to the net force multiplied by the distance. Therefore:

    F/\d = 1/2m(v2^2 - v1^2).

    To solve for the net force, just divide by the distance.

    F = (1/2m(V2^2 - v1^2))/(/\d)

    But since the final velocity is zero:

    F = (1/2m(-v1^2))/(/\d).

    So make sure all the units are correct, substitute and done.
  16. Nov 23, 2011 #15
    OMG....YOU ARE THE BEST! Even though i don't know you! THANKS A BUNCH. Can you also check out my other post. I haven't gotten any help for that.....!!!
  17. Nov 23, 2011 #16
    Wait what is v1???
  18. Nov 23, 2011 #17
    The initial speed, before it hits the block. 320m/s.
  19. Nov 23, 2011 #18
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