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Newton's Laws of Motion

Isaac Newton defined three laws concerning the behavior of moving objects in the 1687. These scientific statements help to explain the nature of matter and space. Newton's first law of motion is often called the Law of Inertia. His second law concerns forces and acceleration. His third law is often called the Action-Reaction Law of Motion. It is amazing that he was able to formulate these laws of motion through his observations so many years ago.

Some questions you may have are:

  • What is the Law of Inertia?

  • What is Newton's Second Law?

  • What is action-reaction about?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

Newton's First Law

Newton's First Law was actually formulated by Galileo many years previous. It is called the Law of Inertia and states:

Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Another way of stating this law in more detail is:

  1. If an object is motionless, it will stay motionless unless acted upon by some force.

  2. If an object is moving at a constant speed or velocity, it will continue at that speed unless acted upon by some force along the line of motion.

  3. If an object is moving, it will move in a straight line unless acted upon at an angle by some force.

The Law of Inertia assumes there is no friction or other resistive force that can slow down an object. Inertia can be best demonstrated in outer space.

Object will stay motionless

Objects that are not moving will remain motionless, unless you apply a force such as a push or pull. This law of nature makes sure things will stay where you put them.

A trick using inertia

There is the parlor trick of quickly pulling a tablecloth from under a setting of heavy dishes or some other objects, leaving them on the table. This trick works because the inertia of the heavy objects tends to keep them in place. By quickly pulling the tablecloth, the force of friction is easily overcome. If the tablecloth was pulled slowly, the friction would be greater than the inertia, and the dishes would follow along.

(Note: The tablecloth must be pulled down at the edge, otherwise the dishes may fly upward.)

Object will continue motion

Once you start an object moving, it will keep moving unless you apply a force in the opposite direction to slow it down. Typically, the force of friction will slow things down. But in order space, where friction is almost zero, an object will move at its given velocity forever unless acted upon by some external force.

If you push a moving object in the direction of motion, it will accelerate to a new velocity. Once you stop pushing, the object will continue at the velocity it had once you stopped pushing.

Things move in straight lines

The Law of Inertia states that moving objects go in a straight line. You must apply a force on an object to make it go in a circular motion. For example, when you spin an object around on a string, you are applying a force on that object from the string to make it go around. Once you let the string (or the force) go, the object will fly off in a straight line.

Likewise, the Moon is attracted to the Earth by the force of gravity. That force is just enough to make the Moon spin around the Earth. If gravity would stop, the Moon would fly off in a straight line into outer space.

Newton's Second Law

The second law is sometimes called the Law of Dynamics, because it concerns forces and what causes objects to move. It can be stated as:

The acceleration of an object of constant mass is proportional to the force acting upon it.

Acceleration is the changing of the velocity of the object. Usually, we are talking about the object speeding up. The word "deceleration" is usually used when the object is slowing down, but it also is acceleration or changing of the velocity.

A force is a push or pull on the object. It may pushing in direct contact or pulling at a distance in the case of gravity.

This law determines the relationship between force, mass and acceleration:

F = m*a


a = F/m

Note that the force F and acceleration a are in the same direction. Since they have a direction, they are called vectors.

What this law says is that while you are applying a force on an object, it will continue to accelerate or change its velocity. It also states that the greater the force on an object, the greater the acceleration.

Newton's Third Law

Newton's Third Law is sometimes called the Law of Reciprocal Actions or the Action-Reaction Law:

Whenever one body exerts force upon a second body, the second body exerts an equal and opposite force upon the first body.

This is often stated as: "For every action there is an equal and opposite reaction," which can be confusing and even incorrect in some situations. Although it is easier to remember, it probably should be avoided.

Pushing against something

One example of this law is if you push against a door with a certain force, the door is also pushing with the same force against you.

Adding Newton's Second Law to this, if you and a friend are on roller skates or ice skates and facing each other, and then you push on your friend with a certain force, your friend will be accelerated backwards according to F = m*a. But because of Newton's Third Law, your push causes an equal opposite push on you. So you will also accelerate backwards.

The force you apply on your friend is F = m*a. So, the acceleration (a) of your friend's motion is dependent on the force you push (F) and his or her mass (m).

But also, that same force is applied on you. Suppose we call your mass (M) and your acceleration (A). Then since the forces are the same M*A = m*a. If your weight (or mass) is twice that of your friend, then your friend would move back twice as fast as you.

2*m*A = m*a

a = 2*A

Note: If you weigh twice as much as your friend, then either:

  1. You should go on a diet,

  2. You should get bigger friends, or

  3. You should not push little people around


The action-reaction law also applies to the force of gravity, especially combined with Newton's Law of Dynamics. If you jump off a ladder, the force of gravity will pull you to the Earth according to F = m*g, where m is your mass and g is the acceleration due to gravity.

But that same force is working in an opposite direction on the Earth, pulling it toward you according to F = M*G, where M is the mass of the Earth and G is its acceleration. Since the mass of the Earth is so much greater than your mass, its movement is extremely small.

In conclusion

Isaac Newton defined his three laws of motion, which are the Law of Inertia, the Law of Dynamics and the Law of Reciprocal Actions. These laws can be verified in many common experiments, and they explain how and why objects move when forces are applied to them.

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