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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


Coulomb’s Law

in the equation as given in the attached file why do we take the constant as given and what is meant by f is inversely proportional to 4 pi e0 please explain




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

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April 2013

Schrodinger’s Cat in Daily Life

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