Student Doctor Network Forums > Pre-Medical Forums > MCAT Discussions > MCAT Study Question Q&A > Conservation of energy + momentum question PDA View Full Version : Conservation of energy + momentum question fashafosho04-06-2008, 01:50 PM(From Kaplan) A bloc of mass "m" starts from rest and slides down a frictionless semi-circular track from a height "h". When it reaches the lowest point of the track, it collides with a stationary piece of putty also having mass "m". If the block and the putty stick together and continue to slide, the maximum height that the block-putty system could reach is: a) h/4 b) h/2 c) h d) independent of h The answer is h/4. And the solution goes through a super long substitution (that I don't really get), re-arrangement of kinetic energy and momentum formulas..I was wondering if there is an easier way to do this problem (faster too). Thanks tncekm04-06-2008, 02:04 PMv1=sqrt(2gh) m1v1 = (m1+m2)v2 v2=v1(m1/(m1+m2))v2 = 1/2 * v1 This means KE has decreased by a factor of 4 since v went down by a factor of 1/2 (KE = 1/2mv^2). When the KE goes down by a factor of 4 the total E goes down by a factor of 4, and at the max height all KE goes to PE, which means. h = KE/4mg or, h = (1/4)KE/mg which suggestst that the new height is only 1/4 of the original height. fashafosho04-06-2008, 03:42 PMAwesome, that simplifies it a lot. Thank you!