Physics Help Forum free falling problem (might be impossible?)

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Sep 15th 2009, 05:07 PM #1 Junior Member   Join Date: Sep 2009 Posts: 2 free falling problem (might be impossible?) I'll type out the question exactly how it's written: A pole vaulter comes down after barely clearing the hurdle. the hurdle is at a height "H" above the top of the soft mat on the floor. The mat has a thickness of "h". when the athlete lands on the mat he slows down at a constant rate so that his speed is 0 just before he hits the ground (suppose the mat can be squeezed to a negligible thickness). a) Find the speed of the athlete just before he hits the mat. b) find the acceleration of the athlete while he is in the mat. I tried the Vf^2=Vo^2+2ad, subbing in H for d and then getting Vf=(19.62H)^1/2 figured that wouldn't work though... and i just don't think they give me enough information to be able to find anything. Haven't tried b) yet, but i'd be glad if you guys could lead me on the right path for that as well.
 Sep 15th 2009, 10:38 PM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 Well you have got part a correctly. With the given data, you are expected only to find the result in terms of H. Use the vel from part a as the initial vel for part b, with the final vel =0 and distance = h. Find a in terms of H and h again using $\displaystyle v^2 = u^2 + 2.a.s$.
 Sep 16th 2009, 05:08 PM #3 Junior Member   Join Date: Sep 2009 Posts: 2 Okay... I didn't mention that i m=then have to graph y-t v-t and s-t graphs.. i know how they'll look (y-t is somewhat like y=x^2, v-t is like y=x, and a-t is y=9.8) I don't know what point to plot because i don't have a way to figure out time. Or velocity. Or distance. do i just plot the points with the why axis also being in terms of H(or h)?
 Sep 16th 2009, 10:06 PM #4 Physics Team   Join Date: Feb 2009 Posts: 1,425 Assume reasonable values for the height of the hurdle (say 3 to 4 m) and the thickness of the mat say 0.25 to 0.5 m. Then find the time taken to touch the mat, and then the time taken to stop after touching the mat. Use suitable scales and then plot.

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