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A Brief Review of Newton's Three Laws of Motion

Updated on May 17, 2016


Isaac Newton created three laws to describe the physics and mechanics of objects in motion. To understand these laws we must first understand some vocabulary. Firstly, in physics, velocity refers to an object’s speed with a given direction. A force is simply a push or pull enacted on an object, and is a vector meaning it can be negative or positive. The net force of an object is simply the combination of the individual forces acting on an object. These forces can be added together or cancel each other out to result in the final motion of the object. The mass of an object refers to a measure of resistance to acceleration, which is usually noticed by how heavy an object is. Lastly, acceleration is when an object’s velocity changes, meaning the object could speed up, slow down, or just change direction.

Newton's First Law

Newton’s first law of motion states that if the net force on an object is zero, then the velocity stays constant. This may seem like a simple and common knowledge observation, but it does explain many principles. If an object has no force acting on it then obviously it will not move, but if an object has two forces of equal strength but in opposite directions, then the object with either not move or continue to move in a constant velocity since the forces cancelled each other. For example, in deep space where there is no gravity, if an astronaut lets go of a ball the ball will float in its original position. Since there is no force acting on the ball it will continue to stay at rest since there is no force to change it’s velocity. Another example would be a jet airplane traveling at a constant velocity. There are multiple forces acting on the jet, including the jet’s engine, the air resistance, the lift force from the wings, and gravity. However, the force of the engine cancels out the force of the air resistance, and the lift force from the wings cancel the force of gravity. This means that all the forces cancel each other out, causing the net force to be zero, and causing the plane to continue at a constant velocity.

Newton's Second Law

Newton’s second law of motion sates that when forces are not equal, an object’s acceleration depends on the net forces and the mass of the object, illustrated by the equation: a=Fnet/m. This means that as the force acting upon an object is increased, the acceleration of the object is increased and as the mass of an object is increased, the acceleration of the object is decreased. An example displaying this is the equation when you double the mass of an object, and quadruple the net force: mass initial/2 mass initial x 4 net force/ net force = 4/2 = 2, which means as you double the mass and quadruple the force, you end up with double the acceleration. Newton’s second law is commonly used to describe terminal velocity. When an object is first released from a height the initial force is zero as the object is momentarily at rest. Soon the weight of the object causes gravity to act and the ball to fall. At this time the weight is greater than the net force on the object. After a period of time the force of the air resistance will equal the weight, and therefore the net force will become zero, and the ball will continue to fall at a continuous velocity known as terminal speed.

Newton's Third Law

Newton’s third law of motion may also be his most famously known law. It states that whenever an object exerts a force on a second object, the second object exerts a force on the first object of equal strength but opposite direction. Simply stated, every action has an opposite yet equal reaction. This means that when an object exerts a force on something, an equal force is exerted back on the object. This can be easily viewed when a person on roller skates pushed against a wall. When the person pushes on the wall, the wall pushes back on the person causing them to roll backwards. A slightly more complex example would be a rocket. As the rocket pushes on the exhaust molecules, the exhaust molecules push back on the rocket causing it to fly. It is not the air molecules that push the rocket but rather it’s own exhaust molecules that push back as they are pushed out.


In conclusion, almost every action of motion can be explained using Newton’s three laws of motion. If an object has no force acting on it, it will not move, or if it is already moving and the forces cancel, then the object will continue with it’s constant velocity. When forces are causing an object to move, its acceleration can be measured using the net force and the mass. Lastly, when an object pushes on another object the first object experiences an equal yet opposite force pushed back on it. This is what causes things such as rockets and skaters to move as they push on their surrounding environment.

Quick Overview

1st Law
2nd Law
3rd Law
Object in motion stays in motion unless a force acts on it.
Every action has an opposite and equal reaction


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