Newton's laws of motion
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.
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[change] 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, 1995)
In other words, the first law says that an object that is not moving or moving in a constant speed in a straight line, will stay like that until something pushes it or blocks its path. As we all know, nothing in the visual world ever stays in constant speed, but the object itself is moving at constant speed, while a force is stopping it from moving at constant speed, friction. That does not change the law. However, in space, an object can move in a constant speed as long as it does not get close to any other objects, and stays in open space. This is why rockets use less fuel in space than they do getting to it.
[change] Second Law
Force is equal to mass times acceleration.
This law provides the definition and calculation of force through mass and acceleration.
For example, Weight is a force that we feel on Earth, caused by the gravity. Weight is calculated as
where m is the mass of the object and g is the local gravitational field (not to be confused with G, the universal gravitational constant), equal to 9.8 meters per second2 (32 feet per second2) on Earth. Simply put this law states that "energy cannot be created or destroyed but may be changed from one form to another"
[change] Third Law
Newton's third law is:
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.
[change] Sources
Duncan, Tom. Advanced Physics for Hong Kong: Volume 1 Mechanics & Electricity. John Murray Ltd, 1995.
