Hi. I'm having trouble Googling this question. Take two equal sized objects and call them 'left' and 'right' a) If a force is applied for 1 second between them and they move apart at equal speeds then: b) what happens if we double the mass of the left one? Will it move at half the speed while the right one continues at the same speed it did in scenario 'a' (relative to the center point)? (in other words, are the left and right forces are independent of each other?) c) what happens when the left one is the Earth (ie, 'almost' doesn't move at all and ignoring gravity)? What will be the speed of the 'right' object? Will it still be the same speed as scenario 'a' or will it be travelling twice as fast as in scenario 'a'? "intuitively" I would expect that if the left object didn't move then the right one will accelerate off at twice the speed, but "theoretically" the Earth moves a tiny amount and still absorbs that side of the force.