For now, momentum is especially useful in collision problems, speaking of full impact. When other forces are absent or balanced out, the total momentum is a constant. And yes, the rock and glass forces a few paragraphs ago are equal and opposite. Now that you know about energy and angular momentum, you might want apply these ideas to elliptical orbits . When you play golf on the moon, the ball follows an almost parabolic path, but to be exact it is a portion of an ellipse.
Planets follow elliptical paths.
Because planets attract each other, they deviate a little from ellipses. What the heck, as long as we're out in left field anyway, an Ogden Nash creation, entitled On the Antiquity of Microbes , was said to be the world's shortest poem. Send me a shorter one and I'll put it in here if it is clever enough. Another non-physics item is about a breakthrough in robotics . These are not necessarily equal.