## Momentum

I’ve just created this question for my undergraduated students that are going to apply for an engineering institute. I’m not quite sure if it’s a nice question, so I’d like to know your opinions or suggestions about it (and yours solutions, of course!)A flying saucer with mass 5m and radius R is at rest and preparing to destroy our beloved Earth. An aircraft is therefore sent to prevent it and shoots a missile with mass m and velocity v1 at the same plane of the flying saucer. The UFO reacts and launches an intercepting missile of mass m and velocity v2 that makes a 30o angle with the direction of the first projectile. If both projectiles meet after a distance h from the launching point of the second missile and then keep moving together, find the minimal value of v2 for which the aircraft will not hit the target.PS.: Ignore friction and gravitational forcesPlease check the attached picture.

Marcos Valle asked

## Newton’s law of gravitation

my question is that in the famous equation f= m1Xm2/r2 i understand the derivation but why do we take the product of the 2 masses and the distance between and not the sum ?

Asked Ojal

Answer: This is based on the observations. The law is experimentally verified.

## Problem from motion of connected systems

Two blocks of masses 10kg and 4kg are connected by a light spring lying on a smooth horizontal surface. An impulse gives a velocity of 14m/s to the heavier block in the direction of the lighter block. What is the velocity of the center of mass.

Posted by Sreedhar M

## Drift velocity and time taken to light up

As we know that the drift velocity of electron is 1mm/sec…if a bulb is at 50cm then how does it light up so fast within a fraction of second?

Asked Padhy