Newton's Laws

Newton's First Law - Law of Inertia
(Inertia: the tendency of an object to resist changes in its state of motion.)

An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

Basically, an object will 'keep doing what it was doing' unless acted on by an unbalance force. If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving. It takes force to change the motion of an object.

The first law states that all objects have inertia. The more mass an object has, the more inertia is has (and the harder it is to change its motion)

Examples of Inertia

A powerful train begins to pull a long line of carriages that were sitting at rest. Since the carriages are so big, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.

What is meant by unbalanced force?

If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. They are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

Example
 * A soccer ball is sitting at rest, it takes an unbalanced force of a kick to change its motion.



If objects in motion tend to stay in motion, why don't moving objects keep moving forever?

Things don't keep moving forever because there's almost always an unbalanced force acting upon it. In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed will continue to travel at that speed unless stopped by an unbalanced force.

Inertia explains why you sometimes 'feel' lighter or heavier when in a lift as it first moves off or slows to a stop. It also tells why you 'move sideways' when a car corners, you keep trying to travel in a straight line.

Newton's Second Law
''Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object). Heavier objects also require more force to move the same distance as lighter objects.''

F = MA

Force = Mass times Acceleration

E.g.



Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s/s. Using Newton's Second Law, you can compute how much force Mike is applying to the car.



Answer = 50 newtons

Newton's Third Law
For every action there is an equal and opposite re-action.

What does it Mean?

This means that for every force there is a reaction force that is equal in size, but opposite in direction. That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard.



The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.