# Newton's Laws of Motion

The laws of motion help us understand the behaviour of objects when they are moving, standing still and their behaviour when forces act upon them. There are three laws of motion. Here in this article, we have provided a brief description of each of the laws clearly.

## Newton’s Law 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.

• ### Newton’s First Law of Motion:

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.

• ### Newton’s Second Law of Motion:

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.

• ### Newton’s Third Law of Motion:

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.

#### Practise This Question

A physicist is investigating the effect that different conditions have on the force of friction.  The material used is an ordinary brick, with a mass of 1.8 kg.  It is pulled across the surface of a wooden table.  Friction is measured by pulling the brick with a string attached to a spring scale, calibrated in newtons (N).  When the brick is pulled at constant  speed, the reading on the scale is equal to the force of friction between the brick and the table top.
Experiment 1 :  The brick is placed on the table in three different positions. First, it is allowed to rest on its broad face (area =180 cm2), then on its side (area =130 cm2 ), and finally on its end (area =56 cm2 )
Table 1
Area (cm2)18013056Friction (N)7.17.37.2

Experiment 2:  A wooden block of mass 0.6 kg is made to the same dimensions as the brick, and the experiment is repeated.
Table 2
Area (cm2)18013056Friction (N)1.21.11.2

Experiment 3: The time, the wooden block is loaded by adding 1.2 kg of extra mass on top of it, to give it the same weight as the brick.
Table 3
Area (cm2)18013056Friction (N)3.53.63.7
Which combination of experiments shows that the amount of friction depends on the weight of the object?