Newton’s first law states, “Every body [object) continues in its state of rest, or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it.” This was not an original idea—Galileo first thought of it nearly fifty years earlier. But Newton incorporated Galileo’s theory to involve the behaviour of matter, namely the movement of the planets.
According to the first law, however, the planets would not circle the sun but rather move in a continuous straight line out into space. The second law explains why this doesn’t happen.
“The change of motion,” the second law reads, “is proportional to the motive force impressed: and is made in the direction of the straight line in which that force is impressed.” Simply stated, the reason the planets do not continue in a straight line is that they are continually pulled toward the sun by the sun’s immense gravity.
This is demonstrated by swinging a ball on a string. The ball keeps circling around because of the string’s pull, or effect, on it. In this example, the string represents gravity.
Newton’s third law states, To every action, there is always opposed an equal reaction: or, the mutual action of two bodies upon each other are always equal, and directed to contrary parts.” Newton devised this law in its entirety.
In simpler terms, the third law states that if one body pulls on another, the other body is pulling with the same amount of force. For example, Earth pulls on the moon, and in turn, the moon pulls on Earth with an equal amount of force.
The reason the moon orbits the Earth rather than the other way around is due to the fact that Earth is so much more massive than the moon. Therefore, Earth has more gravity.
Newton’s Law of Universal Gravitation
In addition to the three laws of motion, Newton’s law of universal gravitation states that every object in the universe is acting on every other object. As Newton writes in Proposition VII of Book III: Every particle of matter attracts every other particle with a force proportional to the products of the masses and inversely proportional to the square of the distances between them.
This theory declares that the gravity of everything in the universe affects the movement and position of every other object.