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Homework Help: What is the deceleration of the ball?

  1. Mar 29, 2017 #1
    1. The problem statement, all variables and given/known data
    The question didn't fit there.
    *A boy drops a metal ball from rest on sand. It hits the sand bed in one second and makes an impression of maximum depth 8mm in the sand. What is the average deceleration of the ball as it hits the sand? Assume air resistance is negligible.

    vi= 0m/s
    t= 1 sec
    S= 8 mm or 0.008 m
    2. Relevant equations

    I used S=vit+1/2(at^2)
    3. The attempt at a solution
    By using that formula I got the answer 0.016 m/s^2 is it correct? Please tell me how to do it
  2. jcsd
  3. Mar 29, 2017 #2
    Welcome to Physics Forums.
    You are using 1 second as the time, but that is the time the ball is falling - not the time that the ball is decelerated by the sand.
  4. Mar 29, 2017 #3
    This isn't correct: The ball is dropped from rest, but by the time it hits the sand, it already has a certain speed, as it is accelerated by gravity. So, what is ##v_i## when the ball reaches the ground?
  5. Mar 29, 2017 #4
    Which formula should I use now?
  6. Mar 29, 2017 #5
    The deceleration starts when the ball touches the sand. You need to know 3 things to solve. What is the velocity when the ball first touches the sand? What is the distance over which the deceleration occurs? What is the final velocity of the ball? You should use a formula which utilizes those 3 knowns and allows you to find the unknown (acceleration).
  7. Mar 29, 2017 #6


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    Science Advisor
    Homework Helper
    Gold Member

    That is surely what the question intends, but the question is flawed. Average acceleration is Δv/Δt, so it would have to be possible to find the deceleration time. But there is not enough information in the question to find that.
    We could find it by pretending the deceleration is constant from the instant of touching the sand to that of coming to rest, but that is clearly not the case, and the question would not need to ask for average deceleration.
    A better wording would be "what is the deceleration as it presses into the sand, pretending it is constant?"
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