Third Law

Newton’s third law of motion is often stated as, “To every action there is an equal and opposite reaction.” This means that when one object applies a force (the action) to a second object, the second object pushes back in the oppo­site direction with the same force (the reaction). When a rocket stands on its launch pad, for example, its weight
(the action) pushes down on the launch pad. The launch pad pushes up against the rocket with an equal and opposite force (the reaction).

Action-reaction forces should not be confused with the forces that make objects accelerate. Action-reaction forces are always equal and opposite. Whether or not an object accelerates depends on the forces acting only on that object.

The example of a rocket can be used again to illustrate this law. When a rock­et standing on a launch pad fires its engine, the jet of gas from the engine pushes up against the rocket (action), and the rocket pushes down against the gas with equal force (reaction). These forces are equal and opposite, but whether or not the rocket takes off depends on the forces acting only on the rocket itself.

Momentum is a property of all moving objects. It is equal to the mass of an object multiplied by its velocity. The more mass an object has and the faster it moves, the more momentum it has. A heavy airliner has more momentum than a small fighter plane flying at the same speed because the airliner has more mass. In addition, a plane flying at 1,000 miles per hour (1,600 kilometers per hour) has twice the momentum of the same plane fly­ing at 500 miles per hour (800 kilome­ters per hour).

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Planet’s Orbit

О Kepler’s first law stated that a planet’s orbit follows the shape of an ellipse with the Sun at one focus. Most planets have regular, almost circular orbits, but some comet orbits form very "eccentric" ellipses, as shown here.

The force of gravity pulls the rocket down, and the thrust of its engine push­es it up. If the thrust upward is greater than the force of gravity acting on the rocket, the resultant downward force will make the rocket accelerate upward.