Ground continuing down when ball collide with ground. Help?

Im trying to make a basic game. I have coded the ground "basic cube" to move left and right via the "rigidbody" component with a script (java script). But the problem is that i have a ball that is gonna hit the ground. But when the ball falls down and collide with the ground, then the ground get forced down with the ball.

How do i make the ball collide with the ground, without the ground getting forced down?