Question: A car of mass 1500kg collides head-on with a truck of mass 9000kg. After the collision, the wreckage is at rest. The speed of the car just before the collision was 30m/s. What was the speed of the truck just before the collision? Relevant Equation(s): m1v1+m2v2 = m1v1'+m2v2' (?) My Attempt: (1500kg)(30m/s)+(9000kg)( ___m/s) = (1500kg)(0m/s)+(9000kg)( 0m/s) (1500kg)(30m/s)+(9000kg)( ___m/s) = 0 (45000 kg*m/s)+(9000kg)(___m/s) = 0 Problem: It's 1 in the morning and I'm not understanding what I'm doing wrong. Am I using the right equation? Do I need to set up west and east on the x-axis??