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Old Jan 10th 2013, 06:10 AM   #1
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question on kinematics

A metal ball is dropped from rest over a bed of sand. It hits the sand bed one second later and makes an impression of maximum depth 8.0mm in the sand. Air resistance is negligible. On hitting the sand, what is the average deceleration of the ball?

Can anyone tell me why can't I use s=ut + 1/2at^2 ? I put these values inside: s=8.0mm, u=0, a=9.81, t=1s but I didn't get the answer. Ans is 6.0 x 10^3 m/s^2
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Old Jan 10th 2013, 07:46 AM   #2
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Your approcah doesn't work because it doesn't comply with the facts given. For the deceleration phase you know s = 0.008m, but you don't know t (you tried to use 1 second but that is not the time of deceleration) and at the instant the ball hits the sand its velocity is not 0 but rather v=gt, where t = 1 second.

Mu suggestion is (1) calculate the velocity of the ball when it hits the sand, then (2) find the rate of deceleration using the equation v_2^2 - v_1^2 = 2ad, where v_2 is the final velocity (0), v_1 is the initail velocity (calculated in step 1), and d = 0.008m.

Last edited by ChipB; Jan 10th 2013 at 03:17 PM.
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Old Jan 10th 2013, 07:55 AM   #3
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Oh okay thanks for explaining it so well! Really appreciated it!
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a boy drops a metal ball from rest on sand .it hits the send bed in one second and makes an impression of maximum depth 8 mm in the sand.what is the average deceleration of the ball as it hits the sand. assume air resistance is negligible
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solution of problem a metal ball is dropped from rest over a beg of sand it hits the sand bag one second later and makes an impression of maximum depth 8.0 mm in the sand .air resistance is negligible on hitting the sand what is average acceleration of ba

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solution of problem a metal ball is dropped from rest over a beg of sand it hits the sand bag one second later and makes an impression of maximum depth 8.0 mm in the sand .air resistance is negligible on hitting the sand what is average acceleration of ba

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solution of problem a metal ball is dropped from rest over a beg of sand it hits the sand bag one second later and makes an impression of maximum depth 8.0 mm in the sand .air resistance is negligible on hitting the sand what is average acceleration of ba

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a boy drops a metal ball from rest on sand. it hits yhe sand bed in one second and makes an impression of maximum depth 8 mm in the sand. what ks the average deceleration of ball as it hits the sand?( assume air resistance is negligible)

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