Sir Isaac Newton was an English scientist. In the year 1686 he put forwarded three laws about the motion, which are known as Newton’s laws of motion. Here we discuss about the second law of Newton’s laws of motion.

**The second newton’s law of motion**

The liner momentum of a body of mass *m* moving with velocity *v* is define as the product of mass and velocity.

*Def : The rate of change of momentum of a body is directly proportional to the net external force applied on it and the change in momentum takes place in the direction in which the force is applied. *

We know from the 2^{nd} law of motion

- Concept of momentum
- Unit of force
- Measurement force

### Concept of momentum

*Def. **The product of mass and velocity of a body produce a motion of the body which is known as momentum of the body.*

Therefore, momentum is the product of the mass and velocity. It is a vector quantity because it has both value and direction. The direction of momentum is the same as the direction of velocity.

#### Unit of the momentum

S.I method : kg mtr/s

C.G.S method : gm cm/s

F.P.S method : pound foot/s

If two bus, one with full of passenger, another one is empty. Both are run in the same velocity then first one must be faster than the second one because the momentum of the first bus is bigger than the second one, as mass of the first one is greater than the second. Therefore, much more force need to stop the first bus.

### Unit of force

The SI unit of the force is **newton**. The symbol of newton is **N**.

In CGS system, the unit of force is **dyne**.

__Relationship between newton and dyne__

### Gravitational Unit of force

The SI unit of the force is **newton**. The symbol of newton is **N**.

In CGS system, the unit of force is **dyne**.

### Measurement force

We know, mass of a body new change. i.e. mas cannot increase or decrease, it is a constant property of any body. That is, change of momentum of a body is depending on the change of velocity of the body. Increasing of velocity means apply the force towards the direction of moving body (i.e. Acceleration). If we apply the force towards the opposite direction of the motion, the velocity decrease (i.e. Retardation).

Now, let initial velocity of a body is ** u** and final velocity of the body is

**. now we apply a force**

*v***for the**

*P***time and the mass of the body is**

*t***.**

*m*Therefore, force= mass x acceleration. This is the equation to find out the force.