Newton posited three laws of motion, and all three are present in every problem related to motion, even though the emphasis may be on only one of the three in a given problem.

The first law, commonly known as the law of inertia, basically means that objects want to keep doing what they are already doing. If an object is in motion, it wants to keep moving and if an object is at rest it wants to remain at rest. To change what an object is doing you must apply a net force > 0 to the object. If you apply a net force to an object the object will change its motion. This means that the object is acceleration - either speeding up or slowing down if moving, and/or changing direction - or that an object at rest will start moving. Inertia is an inherent property of all objects and is directly related to the mass of the object. The greater the mass the greater the inertia. think of trying to stop a large truck, vs. a toy car.

The second law quantifies the relationship between the force applied to an object and how much the object will accelerate depending on its mass. The second law is: F = ma. The second law can also be rewritten by recognizing that a = delta v/delta t. The second law then becomes:

F = m delta v/delta t

This is rewritten as the impulse-momentum equation as:

Fdelta t = mdelta v

This form is particularly useful in problems related to collisions such as in sports where a ball is struck (tennis, golf, volleyball, baseball, soccer, etc.)

The third law essentially states that forces act in pairs, equal in magnitude and opposite in direction. For example, if you apply a force of 100 N to the floor with your foot, the floor also applies a force of 100 N to your foot.

Newton's Law of motion:

1. A force will continue in constant uniform motion until an opposite force of equal or greater magnitude acts upon it. This law is also known as the law of Inertia.

e.g. you're driving your car and you see a squirrel crossing the road. You slam on the brakes so you don't hit it. Your body's reaction is to keep moving forward even though the car has stopped. The only thing that keeps you flying through the windshield is the seat belt. The seat belt applies an opposite force, greater than what you are being hurled towards the windshield at, thus stopping motion.

2. The rule of Force. F = Mass (m) x acceleration (a) - (F=m x a)

But, in cases where the acting force is gravity, we know that the Earth's acceleration due to gravity is 9.8 ms^-2. , so we can substitute Gravity (g) for (a)

So, F = m x a

becomes F = m x g

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

So, you decide to stand on your desk at school. The force you exert on the desk is your mass times the acceleration due to gravity (F=MxA). You don't fall through the desk because the desk has been built so that an equal or greater force holds you. This force is opposite the force you are putting on the desk.

But, if you were to stand on the desk, but the desk had been eaten away by termites and gives way, sending you crashing to the floor, we know that the force of the table was not strong enough to support your downward force.

Make sense?