A Simple Explanation of Principles of Motion and Gravity
Isaac Newton discovered three laws of motion and the law of gravity that explain motions observed on Earth and in space.
Isaac Newton was a 17th century English physicist who discovered three laws of motion and the law of gravity. We use these laws to explain motions we observe on Earth and in heavenly bodies. His other contributions to science include the study of light and calculus, a branch of mathematics.
Newton’s First Law of Motion
If something is moving at a uniform speed in a straight line, it has constant velocity. At rest, it has a constant velocity of zero. Either way, an outside force must act on the object to change its velocity. The tendency of objects to continue in their constant speed straight line motion is inertia. Newton’s first law is the inertial law.
If an object is at rest, it will remain at rest unless an outside force acts on it. If an object is moving, it will continue to move in a straight line at the same speed until an outside force acts on it.
Any change in velocity, which includes speed or direction, is an acceleration. Newton’s first law boils down to: Any acceleration requires a force.
Newton’s Second Law
Newton’s first law says that a force is required to accelerate something. Newton’s second law gives a formula telling us how much force is needed. You know from experience that it is harder to push a more massive object. Using physics words: It takes more force to accelerate a more massive object.
Newton’s second law says that the force needed to accelerate an object equals the mass of the object multiplied by its acceleration, or:
Force = Mass X Acceleration.
Newton’s Third Law
The statement of Newton’s third law is deceptively simple. For every action there is an equal and opposite reaction. The terms action and reaction refer to forces.
The key to getting this law right is understanding action-reaction pairs. Pair means two, so only two objects are in an action-reaction pair. If Earth’s gravity pulls you down, then the Newton’s third law reaction to that is your gravity pulls the Earth up. The reaction to Earth on you is you on Earth. The floor may exert a force on you equal and opposite to your weight, but that is not the Newton’s third law reaction. It involves three objects: you, Earth, and the floor; so it cannot be an action-reaction pair. The Newton’s third law reaction to the floor holding you up is you pushing the floor down. You on floor and floor on you are an action-reaction pair. That is why the carpet is squeezed.
Newton’s Law of Universal Gravitation
Gravity is the force holding the universe together. There is a gravitational force between any two objects in the universe, so it is called universal gravitation. Gravity pulls you down because there is a gravitational force between you and the Earth. There is also a gravitational force between you and the Sun and between you and the person sitting next to you. You don’t fall to the Sun, because it is too far away for its gravitational force to be strong. You don’t fall towards the people next to you because they are much less massive than Earth.
The gravitational force between objects is larger for more massive objects. The gravitational force is less as objects get farther away.
Using these three laws of motion and the law of universal gravitation, scientists can explain all motions we see around us on Earth and all motions of planets, stars, and other heavenly bodies.