# Newton's Laws of Motion

Sir Isaac Newton was an English mathematician, astronomer and physicist who gave three laws which proved to be fundamental laws for describing the motion of a body.  These are generally known as Newton’s laws of motion. We will discuss each law of motion one by one in detail.

### First Law:

Newton’s first law of motion states that “A body at rest or uniform motion will continue to be at rest or uniform motion until and unless a net external force acts on it”. Suppose a block is kept on the floor, it will remain at rest until we apply some external force to it. Also, we know that it takes us more effort or force to move a heavy mass. This is directly related to a property known as Inertia. This law is also known as the law of inertia.

### Second Law:

The first law has already given us a qualitative definition of force. Now we are interested in finding out its magnitude. According to Newton’s second law of motion, the net force experienced by a body is directly proportional to the rate of change of momentum of the body. It can be written as:

$$f\alpha \frac{dP}{dt}\\ \\ \Rightarrow f\alpha \frac{mv-mu}{t}\\ \\ \Rightarrow f\alpha\frac{m(v-u)}{t}\\ \\ \Rightarrow f\: \alpha\: ma\\ \\ \Rightarrow f=k\, ma$$
Where k is the constant of proportionality and it comes out to be 1 when the values are taken in SI unit. Hence the final expression will be,

F = ma

Numerical:

Suppose a body of mass 10kg is having an acceleration of 5m/s2. What is the net force experienced by it?

Solution:

From second law of motion, we know,

F = ma

Putting the values we get,

F = (10 x 5) N

F = 50N

So the body is experiencing a net force of 50N.

### Third Law:

According to Newton’s third law of motion, for every action, there is an equal and opposite reaction. Forces are always found in pairs. For instance, when you sit on a chair, your body exerts a force downward and that chair needs to exert an equal force upward or else the chair will collapse.