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# Tag Archives: system

## why in all the system of units time is always in second?

Please tell me why in all the system of units time is always in second? -Pankaj Sharma

## Sequence of Learning Physics

In what sequence should these chapters be prepared for my exams:-

units and dimensions,kinematics,laws of motion,work power and energy,motion of system of particles and rigid body dynamics,gravitation.

Apoorv Katoch asked

Answer:

It is better to follow the same pattern as given in NCERT text book

## A Question from Electromagnetic Waves

Rajat asked: –

we know that electric and magnetic field are associated with charged particle but when light travells,it produces electric and magnetic field both but it consests of photons,which is a uncharged particle.why it did so?

Master will soon answer this.

By the time, if any visitor wants to answer, please use the commenting system

## Numerical from centre of mass

Two skaters, one with Mass 45kg, 2’nd with Mass of 65kg stand on an ice rink holding a pole with L=10m with negligible mass. starting from the ends of the pole, the skaters pull themselves along the pole until they meet. How far does the 45kg skater move?

**Hint: **They will meet at the centre of mass of the system.

If the 45 kg skater moves by x m, then the second one travels (10-x)m

then,

45 x = 65(10-x)

Solve and get the value for x

## Conservation of Angular Momentum and Kinetic Energy

Imagine a rod 2 meters long fixed at its center, 2 identical masses are placed 0.5 meters away on opposite sides , as the rod spins they slide towards the edges , according to the formula k.E.=0.5*I*ω^{2} the kinetic energy decreases , but since its a closed system why does the kinetic energy decrease ?

**Answer**:

The decrease in KE accounted by the work done in displacing the masses

## Centre of mass of an accelerated body

“What is the effect on centre of mass of system of particles when it is placed in accelerated train?” Vimal asked