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Homework Help: Newton's Laws of Motion - Force Problem

  1. Oct 1, 2013 #1
    1. The problem statement, all variables and given/known data
    A 0.075-kg arrow hits the target at 21 m/s and penetrates 3.8cm before stopping. (a) What average force did the target exert on the arrow? (b) What average force did the arrow exert on the target? (c) An identical arrow strikes the target at 42m/s. If the target exerts the same average force as before, what's the penetration depth?

    Known: mass: .075kg velocity (?) 21m/s, displacement: 3.8cm

    2. Relevant equations
    Here I am a little confused. (Or a lot confused)
    Fnet=m x a

    a=velocity final squared - velocity initial squared / 2(Δ)? Not sure if this equation applies

    3. The attempt at a solution
    I used the equation to figure out the acceleration since I have the mass. So here is where I am at:
    a=0-(21m/s)2 / 2(.038m) = -5.8 x 10^3 m/s2

    Then I used that to find Fnet=m x a = (.075kg)(-5.8 x 10^3m/s2)= -435N

    I feel like I am on a completely wrong track and am not sure how to answer the question. Is the target applying a force of -435N?

    Any help in the right direction is greatly appreciated, thank you for your time.

  2. jcsd
  3. Oct 1, 2013 #2


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    Homework Helper
    Gold Member

    Looks good! Yes , if the arrow is moving left to right, the target applies a force on the arrow of 435 N to the left. What force and in what direction does the arrow apply on the target?
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