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# Monthly Archives: August 2013

## A Question from Rotational Mechanics

A thin spherical shell lying on a rough horizontal surface is hit by a cue in such a way that the lines of action passes through the centre of shell. As a result the shell starts moving with a linear speed ‘V’ without any initial angular velocity. Find the linear speed of the shell after it starts pure rolling on surface.

## Communication GeoLocation

How to find the location of someone by using his/her phone number through the computer network?.

## A spring is held vertically downward and a current is passed through it . Then the length of the spring :

A spring is held vertically downward and a current is passed through it . Then the length of the spring :

A . increases     B. decreases    C . does not change    D the spring oscillates

Since the adjacent turns are carrying currents in the same direction (just like two parallel wires) there will be attraction between the turns and therefore the length of the spring

tends to decrease.

## How many bulbs can be connected in parallel

I need to know how many 15W bulbs can be connected in parallel If I use an extension cord in the 5A socket? If I use the 15 A socket then how many bulbs can I connect in parallel?

Assuming the voltage to be 220 V,

The current drawn by a single bulb is 15/220=0.068 A

So the no. of bulbs which can be connected in parallel = 5/.068=73 (avoiding the fractional part)

If you use 15A, then 220 bulbs can be connected. (follow the same procedure)

## Numerical Problems posted from Mechanics

1. AN 8000 KG ENGINE PULLS A TRAIN OF 5 WAGONS,each of 2000 kg, along a horizontal track.if the engine exerts a force of 4000 N and the track offers a frictional force of 5000 n , then calculate
a. the net accelerating force
b.the acceleration of train
c.the force of wagon 1 on wagon 2

(Posted by Fatima)

2. A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 second .find the force acting on it if its mass is 7 metric tones (posted by Muneem)

3. Derive an equation for the constant net force acting on an object in terms of the
object’s mass, its initial velocity, its final velocity, and the time interval during which
the net force is applied. (Collin Taylor Posted)

## The Physics of Accidents

Accidents

Why are road accident at high speeds very much worse than accidents at low speeds?

As you might have studied, greater the speed, greater is the momentum. When an accidents occurs, the greater momentum involved can cause greater damage.

1. http://outreach.phas.ubc.ca/phys420/p420_96/danny/danweb.htm

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