We shall now study about the exact relation between the force applied and the motion of the object on which the force is applied. This was studied by Sir Issac Newton. He gave the three basic laws of motion concerning objects in motion.
Sir Isaac Newton formulated Galileo's observations into a law. This law is known as Newton's first law of motion.
A body continues to be in a state of rest or of uniform motion along a straight line unless an external force acts upon it.
This means that every object has a tendency to resist any change in its state of rest or motion. This tendency is inertia. This law is also known as law of inertia.
Newton's first law of motion gives a qualitative definition of force.
Momentum
A cricket ball and a tennis ball are moving with a constant velocity v. You have to apply more force to stop the cricket ball than you have to apply to stop the tennis ball, since the mass of the cricket ball is more than that of the tennis ball.
The force required to stop a moving object depends on the mass of the body.
Now, suppose you have to stop two cricket balls A and B moving with velocities v1 and v2 respectively (v1 > v2) then you have to exert a greater force on A, because it is moving with a greater velocity.
The force required to stop a moving body is directly proportional to its velocity. Thus the quantity of motion in a body depends on mass and velocity of the body. This quantity of motion defines a new physical quantity called momentum.
The momentum of an object is defined as the product of its mass and its velocity. Momentum is a vector quantity and its direction will be same as that of velocity. It is represented by p.
p = mv where, m is the mass of the object, v is its velocity.
SI unit of momentum is kg m/s.
Note
p = mv. SI unit of mass is kg and SI unit of velocity is m/s.
Unit of momentum p is kg m/s
