1. The problem statement, all variables and given/known data An object of mass m moving in a certain direction has a kinetic energy of 4.0 J. It hits a wall and comes back with half its original kinetic energy. 2. Relevant equations If the speed of the object on return is 2.0 m/s, what is the mass of the object? A) 2.8 kg B) 3.8 kg C) 1.0 kg D) 4.0 kg 3. The attempt at a solution let mass of object = m speed on return =2 kinetic energ on return = 0.5 m *v*v kinetic energy initial =0.5*m*v1*v1 kf=0.5 ki therefor v1= (2)^0.5 v v1 =2.828 m/s initial kinetic energy = 4 j 4=0.5*m*v1^2 8=m* 8 m=1 kg My problem: I don't understand the solution, nor what formula to use. Please help. Thanks. I need to know why Answer: C is correct.