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Flash of lightning is seen before the sound of thunder. Why

   Both sound and light energy are released at the same time. In air the speed of light is 300000000 m/s and speed of sound is 340 m/s. Hence the light reaches the observer instantly and sound after some time.     For example, if an electric discharge  between the clouds takes place at a distance of 1km from the observer then lightning flash is seen instantly but sound of thunder will be heard after about 3 seconds.

NEWTON'S SECOND LAW OF MOTION

The Second law states that the rate of change of momentum is directly proportional to the applied force and the change takes place in the direction of the force applied. If u= initial velocity, v=final velocity, t= time taken to change the velocity from u to v, Then Initial momentum = mu Final  momentum=mv So change in momentum = mv-mu =m(v-u) Hence the rate of change of momentum = m(v-u)/t Therefore F= m(v-u)/t   F=ma.