Identical twins, each with mass 52.6 kg, are on ice skates and at rest on a frozen lake, which may be taken as frictionless. Twin A is carrying a backpack of mass 12.0 kg. She throws it horizontally at 3.40 m/s to Twin B. Neglecting any gravity effects, what are the subsequent speeds of Twin A and Twin B? I think I'd use conservation of linear momentum wouldn't I? m1v1i + m2v2i = m1v1f + m2v2f Would I do it twice? Once for when the backpack is thrown and then again for when it is caught?