Ans :- According to this law - The change in momentum of a body per unit time is directly proportional to the unbalanced force acting on the body and the change in momentum takes place in the direction of the unbalanced force on the body.

That is, F is directly proportional to dp/dt

Where, dp = Change in momentum,And, dt = time taken for this change in momentum

MATHEMATICAL FORMULATION OFSECOND LAW OF MOTION

Consider a body of mass m moving with initial velocity u. Let a force F acts on the body for time t so that the velocity of the body after time t is v.

Initial momentum = muFinal momentum = mvNow,Change in momentum = mv - mu= m(v-u)

Time taken to change this momentum = (t-0)= t

Therefore,

Rate of change of momentum = Change in momentum/ Time taken

= m(v-u) / t

According to the definition of Newton's second law of motion,

Force applied is directly proportional to rate of change of momentum

Or, F is directly proportional to m(v-u)/t.................(1)

Since v= u+at or (v-u/t) = a

Therefore, eqn. 1 can be written as

F is directly proportional to maOr, F= kma...........(2)

Where k is constant of proportionality

If F = 1 unitm = 1 unitAnd, a = 1 unit

Then from eqn (2),

1 = k or k= 1

Put this value of k = 1 in eqn 2, we get,

F= ma

Thus, force acting on the body is directly proportional to

1. It's mass2. It's acceleration