# Newton's laws of motion

(Redirected from Newton's second law)

Isaac Newton (1642-1727), the father of the study of dynamics, – the study of motion – developed three sets of laws that are believed to be true because the results of tests done by scientists agree with the laws he produced.

## First Law

If a body is at rest it remains at rest or if it is in motion it moves with uniform velocity until it is acted on by a resultant force. (Duncan Falconer, 1995)

In other words, the first law says that an object that is not moving or moving with constant velocity will stay like that until something pushes it or blocks its path. From everyday experience we might complain that objects do not remain without motion (for example, a ball held above the ground does not stay there if it is released), nor do they remain in motion with a constant speed (a ball rolling down a hill moves faster and faster, while a ball rolling along a flat surface will eventually stop moving); however, if we were to remove external forces such as gravity and friction, we would observe the first law of motion.

## Second Law

Force is equal to mass times acceleration

This law provides the definition and calculation of force through mass and acceleration.

$F = ma$

For example, Weight is a force that we feel on Earth, caused by the gravity. Weight is calculated as

$W = mg$

where m is the mass of the object and g is the local gravitational acceleration (not to be confused with G, the universal gravitational constant), roughly equal to 9.8 meters per second2 (32 feet per second2) on Earth.

## Third Law

Newton's third law. The skaters' forces on each other are equal in magnitude, and in opposite directions

For every action, there is an equal and opposite reaction.

The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

A variety of action-reaction force pairs are evident in nature. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

## Sources

Duncan, Tom. Advanced Physics for Hong Kong: Volume 1 Mechanics & Electricity. John Murray Ltd, 1995.