I'm having some trouble with some homework and I'm hoping someone can help me. PROBLEM: #6. Cart1, with mass m, is initially at rest and is struck by cart2, which has a mass of 336 kg and initial speed of 1.81 m/s. The collision is elastic and after the collision cart2 continues to move in its original direction at 0.807 m/s. Find m, the mass of cart1. #7.Find the speed of cart1 after the collision. ATTEMPT AT SOLUTION: I have calculated that m1v1+m2v2=m1v1'+m2v2' m1(0)+336kg(1.81m/s)=m1v1'+336kg(0.807m/s) m1v1'=336kg(1.81-0.807) m1v1'=337 kg*m/s I'm stuck here. I tried calculating the amount of energy the second cart should have 1/2m2(v2-v2')^2= 0.5(336)(1.81-0.807)^2=169J and then that 169J=0.5m1(v1)^2 so that m1=169*2/v1^2 and subbing that into m1v1'=337 kg*m/s (169*2/v1'^2)*v1'=337 kg*m/s 338/v1'=337 thus v1' =338/337 This however does not answer #7 and when working backwards to find the mass it also provides the wrong answer. Thank you for any assistance you can provide. Sincerely, Potato21 EDIT: Oops, dumb mistake, I got it!