Sir. Isaac Newton proposed the laws to describe Force, which are now called as the Newton's Laws of Motion. Have you every thought, why does a football kicked by a person stops after traveling certain distance? When we push a toy car, it moves forward (or backward) and we say it has accelerated (or decelerated), how is this acceleration caused? You need to paddle the boat backward to actually displace forward. The boat-driver always ties the boat to a fixed support before the passengers get off from it, why? All these questions can be explained by the three Newton's Laws of Motion.

Newton's First Law

Newton's First Law states, "Every body continues its state of uniform motion or uniform rest until an external force is applied on it."

It gives the qualitative definition of force. When an object changes its state of uniform rest or uniform motion, we say that a net external force is acting on that object.

Now, we can get the answer to the stopping of football. When we kick a football along the ground, a friction force - force that opposes the relative motion - acts on the football. This is an external force and causes the ball to stop after travelling certain distance.

Newton's Second Law

Newton's Second Law states, "The acceleration produced on an object is directly proportional to the force applied and inversely proportional its mass ."

It gives the quantitative definition of force, i.e., it helps to measure the magnitude of the applied force. Therefore, we can calculate the acceleration and retardation produced on an object. The cause of these changes is the applied force, which is simply a push or a pull acting on those objects.

Magnitude of Force

From Newton's second law, we have,

a ∝ F    (1)

a ∝ 1/m    (2)

Combining statements (1) and (2), we get,

a ∝ F/m

or, F ∝ ma

or, F = k (ma)       (3)

where k is a constant of proportionality. Experimentally, the value of k has been found to be unity.

or, F = 1(ma)

So, F = ma    (4)

Hence, equation (4) is used to find the magnitude of net force acting on an object.

Newton's Third Law

Newton's Third Law states, "To every action, there is an equal and opposite reaction."

Action and reaction forces always occur in pair. However, they never cancel each other as they act on two different objects. Therefore, it gives the idea about the effect of force.

The questions about paddling and jumping off from the boat can be answered by the third law. When you paddle backwards, you apply a backward force on the water. Therefore, an equal and opposite force is exerted by the water on the boat which causes it to move forward. The case of jumping off from the boat can be explained in similar manner.