Momentum Versus Impulse

hello friend,could u tell me why specifically (deltaM)....is it change in mass??why we have to take so....

Not delta M, but delta MV.

∆(delta) preceding any quantity, as you seem to already know, means 'the change in' the following quantity.

MV, the product of mass and velocity, is called momentum, which is a measure of the tendency of an object to keep moving with the same velocity (meaning both speed and direction). When an unbalanced force acts on an object, it accelerates (changes the speed and/or direction) of the object. The final momentum of the object is the sum (vector) of the initial momentum and the change in momentum caused by the impulse.

A good example is launching a satellite. The launching rocket gets the satellite up to the orbital altitude, but if that is the only force, the satellite will just fall back down to Earth. A small orbital insertion rocket must then burn (exert a force) for the correct time to increase the velocity of the satellite to the higher value required to stay in orbit at that altitude. The product of the force and time of burn of the insertion rocket is the impulse of the rocket, which changes the already considerable momentum of the satellite to the higher value required to stay in orbit.

Once in orbit (assuming a circular orbit), Earth's gravity exerts a force of constant magnitude but constantly changing direction, of the correct value to keep changing the momentum (again constant magnitude but constantly changing direction) of the satellite, so it's velocity is always perpendicular to the line between the center of the satellite and the center of the Earth. Here the time may be less than obvious. If you choose a time equal to half the orbital period, the net effect of all the little impulses (like the impulse exerted by the Earth each minute, for example) exerted during that half orbit is to reverse the direction of the satellite.

Hope this helps...

Dick