Newton's Three Laws of Motion

In the 17th Century some guy was sitting under a tree reading a book when an apple fell on his head and a revelation came to him. His name was Isaac Newton.

Newton established the fundamental principles of dynamics, the study of forces and motion. They're known as Newton's three laws of motion.

Newton's First Law of Motion

This law states that an object at rest will tend to remain at rest while an object in motion will tend to remain in motion unless acted on by an unbalanced force. The unbalanced force on the object will change the speed of object , the direction of the object, or both. The resistance from the object to change its state of motion is known as inertia. An object's inertia is dependent on its mass.

The box is moving at a constant speed in the direction of the blue arrow. When an unbalanced force, the pink arrow, is applied to the box it will accelerate.
Figure 1

If the unbalanced force is applied in the opposite direction, the box will decelerate.
Figure 2

If the unbalanced force is perpendicular to the motion that the object will move in a circular path.

Newton's Second Law of Motion

This law states that the unbalanced force exerted on the object is proportional to the object's mass and acceleration. Therefore the equation is:

F = ma

The unit used for force is newton.

Newton's Third Law of Motion

This law states that when an object exerts a force on a second object the second exerts an equal force on the first object, but in the opposite direction. (See Figure 3)


Person 1 is pushing person 2 with say a force of 150 newtons. Even though person 2 doesn't budge she is also pushing back 150 newtons. If she didn't she would topple over.
Figure 3


Figure 4

Try this: Get a friend and two wheel carts. Sit on it so you're facing each other, palm against palm like in figure 4. Mark where you are starting from, the space between the two carts. One of you push off. How far did you travel? Did you both go the same distance even though only one of you pushed off?

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