Physics Help Forum question on kinematics

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Jan 10th 2013, 06:10 AM #1 Junior Member   Join Date: Dec 2012 Posts: 5 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
 Jan 10th 2013, 07:46 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,326 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.
 Jan 10th 2013, 07:55 AM #3 Junior Member   Join Date: Dec 2012 Posts: 5 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 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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