In this topic you will learn about how Newton's laws of motion can be applied.

Newton's second law of motion says that the acceleration of an object is related to the object's mass and to the amount of force applied to the object. The law is written as F = ma, or "the net force on an object equals the object's mass times the object's acceleration." This tells us that for a given net force, objects with a greater mass have less acceleration. It also tells us that for objects of a given mass, a greater force results in greater acceleration.

According to Newton's third law of motion, any time one object exerts a force on another object, a pair of forces exists-an action-reaction pair of forces. The two forces have the same strength, but act in opposite directions. These action-reaction forces help explain why a rocket flies. Hot gases from burned fuels are pushed downward out of the engine. The reaction force is the push of the hot gases upward. The upward force propels the rocket aloft.

The quantity that measures both the mass of an object and how fast the object is moving is momentum. A baseball has more momentum than a tennis ball when the two are traveling at the same speed because the baseball has more mass. However, a tennis ball can have more momentum than a baseball if its speed is great enough.

Momentum is simply mass times velocity. Since velocity has both speed and direction, momentum also has direction. When an object's mass is measured in kilograms and its velocity is measured in meters/second, its momentum has units of kilogram-meters/second. The greater an object's momentum, the more force and time it takes to bring it to a stop.

Momentum is a very helpful quantity in studying motion. The total momentum does not change if there are no outside forces acting on the system. Scientists call this principle the conservation of momentum. Conservation of momentum explains why rockets fly. Before launch the total momentum of the rocket and the fuel is zero because they are at rest. After launch the total momentum must be zero. To cancel the momentum of the burned fuel shooting out of the engine in one direction, the rocket must be moving in the opposite direction.

Weight is not the same thing as mass. Weight is the force of gravity pulling down on objects located at the surface of Earth. The weight of an object depends on its location in the universe. However, the mass of the object does not change. For example, if your mass is 30 kg, you weigh about 294 N. If you were standing on the Moon, you would only weigh about 50 N, because the Moon has much less mass than Earth. Still, your mass would remain 30 kg on the Moon.