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Momentum and movement basic GCSE Quest

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  1. Mar 16, 2012 #1
    when a gun is fired a very large force acts on the bullet for a very short time.

    formula that's given: force x time = change in momentum (kg m/s)

    an average force of 4000 acts for 0.01 seconds on a bullet of 50g mass.

    DONE THIS PART

    b. the bullet is fired horizontally. in the short time that it takes for it to reach it's target, its horizontally has fallen by 80% of its initial speed.

    explain why the speed of the bullet has decreased so quickly.

    c. calculate the percentage of its original Kinetic energy the bullet still has when it reaches its target.

     
  2. jcsd
  3. Mar 16, 2012 #2

    tiny-tim

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    hi nonthesecond! :wink:
    show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:
     
  4. Mar 16, 2012 #3
    I'm a little confused. What part have you done already? Also, please show us your attempt at a solution.
     
  5. Mar 16, 2012 #4
    for b i think because it already has reached it's fastest speed it must slow down from then on. what's the best way to word this?

    c. for this question i think it's o.5(0.005) x velocity^2 but i don't know how to work out velocity i know it's v=d/t.
     
  6. Mar 16, 2012 #5
    i've finished the first part (a)

    i can't do b and c

    for b i think because it already has reached it's fastest speed it must slow down from then on. what's the best way to word this?

    c. for this question i think it's o.5(0.005) x velocity^2 but i don't know how to work out velocity i know it's v=d/t.
     
  7. Mar 16, 2012 #6

    tiny-tim

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    hi nonthesecond! :smile:
    but why?
    you know that vf/vi is 0.8 …

    so how much KE is lost? :wink:
     
  8. Mar 16, 2012 #7
    sorry but you have to dumb this down i'm doing GCSEs and i don't understand what you mean by the Vs, i also have no idea why it slows down
     
  9. Mar 16, 2012 #8
    vf = final velocity, vi = initial velocity.
     
  10. Mar 16, 2012 #9

    tiny-tim

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    ah, the question says that the horizontal speed has fallen to 20% of its initial value

    ie vf/vi = 0.2, where vi means initial speed and vf means final speed

    (i put vf/vi = 0.8 before, i misread the question)
     
  11. Mar 16, 2012 #10
    is the answer 40% ?

    (0.5x 0.005) x 40^2
     
  12. Mar 16, 2012 #11
    You have to model air friction right? If not I don't see how the bullet is supposed to loose speed (momentum)

    Once the bullet is flying, the only horizontal force acting on it is air friction. This force is a function of the bullet's speed, and since the speed decreases constantly, the force also decreases constantly. The model you're supposed to use for air friction I don't know, but it's most probably a linear model, something like F= k · v

    Use Newton's law of conservation of momentum to obtain an ODE (an easy one), solving that ODE will have your problem solved.

    For the kinetic energy, I might be confusing the question because it's too easy, if they say the speed has decreased by 80%, than the energy has decreased by:

    v1= v -> v2= 0,2 · v

    E1 proportional to v1^2= v^2 -> E2 prop. to v2^2 = (0,2·v)^2 = 0.04·v^2

    Thus the kinetic energy has decreased by 96% if I understood the question properly.
     
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