Newton's Laws of Motion
by Ron Kurtus
In 1687, Isaac Newton defined three laws of motion that concern the behavior of moving objects. 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 shows the relationship between force and acceleration. His third law is often called the Action-Reaction Law of Motion.
It is amazing that he was able to formulate these important laws of motion through his observations so many years ago.
Some questions you may have are:
- What is Newton's First Law?
- What is Newton's Second Law?
- What is action-reaction about?
This lesson will answer those questions. Useful tool: Units Conversion
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:
- If an object is motionless, it will stay motionless unless acted upon by some force.
- If an object is moving at a constant velocity, it will continue at that velocity unless acted upon by some force along the line of motion.
- 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.
(For more information, see Motion and the Law of Inertia.)
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 that 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, which is
F = ma
- F is the applied force
- m is the constant mass
- a is the resulting acceleration
- ma is m times a
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."
Pushing against something
Suppose you are push on a wall with a certain force. Since the wall is stationary, your force is reflected back against your hands. The force you apply is the same force that is applied to your hands.
Now if you are wearing roller skates, and you push on the wall with a certain force, you will move backward as if that same force was applied to you.
Suppose you push on a large box that is on the floor. If the friction between the box and the floor is greater than the force you are applying, then the equal force will be pushing on your hands. But if the resistive force of friction is less than your force, the box will slide along the floor. The force on will be the force you applied minus the force of friction.
Add Second Law to Third Law
We can also add Newton's Second Law to the Third Law. 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 = ma. 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 = ma. 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 MA = ma. If your weight (or mass) is twice that of your friend, then your friend would move back twice as fast as you.
M = 2m
2mA = ma
a = 2A
Note: If you weigh twice as much as your friend, then either:
- You should go on a diet,
- You should get bigger friends, or
- 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 = mg, 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 = MG, 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.
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.
Once you work toward a goal, your inertia moves you toward success
Resources and references
Newton's laws - HyperPhysics
Car Physics and Newton's Laws of Motion - Good explanation, along with useful links
Elevator Physics - Newton's Laws - Good links (thanks to Alice for resource)
Newton's laws of motion - Wikipedia
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