# A particle that defies classical mechanics ! 1

A photon has no mass but energy and it has speed and it has momentum although it does not have mass. Thats impossible in Classical Mechanics. Because Classical Mechanics associates with every mass; momentum and kinetic and potential energy. All those would be zero if mass were to be zero.

Also the speed of the photon is constant while its energy and momentum are changing. This is unimaginable in classical mechanics. In CM first momentum can’t be defined from mass and speed if mass is zero, then while speed is not changing how is momentum changing? So both variables speed and mass fail to define a momentum for the photon, and not just the failure to have a mass. How then momentum is defined for this particle? Its not defined as long as photon is just a particle. Thats impossible. More…

# 40 interesting problems in elementary mechanics Reply

This post has 40 interesting problems in elementary mechanics from IE Irodov, Problems in general physics that I solved in last couple weeks. Note that another 30 problems which has been solved will be uploaded after they are scanned, shortly. More…

# An interesting problem in elementary mechanics. Reply

Here is a problem on mechanics.

Problem; Two particles move in a uniform gravitational field with an acceleration g. AT the initial moment the particles were located at one point and moved with velocities v1=3.0 m/s and v2=4.0 m/s horizontally in opposite directions. Find the distance between the particles at the moment when their velocity vectors become mutually perpendicular.

Here is my ans. (Got to fig. it out in a concise way after scribbling through pages unsuccessfully. )

An interesting problem in mechanics

1. Aristotle Fallacy; A notion that objects need force for their movement. It contradicts the idea of inertia. Newton corrected this by introducing the first law, things continue in their state of motion, a quality called as inertia, without requiring force and the motion changes due to application of force.

2. Earth is flat; that there is a boundary where you fall off its edge. [I am not going to explain or tell you how and when we found this was a horrendously hilarious and misleading notion we had. But it might have been used in the past by parents to discipline their teen-age kids. Don’t go out, you will fall off earth. That would have kept them in check.]

3. Rotational Dynamics; Earth is accelerating in a near circle in addition to about itself, so additional forces are acting that changes our observation about the world. Newton tried to understand this (not successful) in his last days, by rotating a bucket full of water, his laws could not explain the effects observed. His laws needed to be modified slightly. The same thing makes objects feel weightless by a given amount if they are accelerating towards a gravitational field (eg merry go round, satellites) This is the basis of many works of Einstein. First came Mach’s Principle which says observations made from objects that are accelerating in circular paths are to be corrected by fixing frames of references to stars that are so far away that the rotational motion is neglected. [if you shake your head while looking at stars and shake your head by looking at nearby objects such as a light post, evidently the light post shakes more and the stars less]. This helps in correcting observed phenomena from earth. Earth moves at 30 kms/second wrt sun … More…