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# Monthly Archives: January 2011

## A problem from Hydraulic

“a stone of [wiki]density [/wiki] 4gm/cm3 is dropped freely in a liquid of density 0.8 gm/cm3 .what will be the acceleration of the sinking stone?”

Ans:

the force acting are

weight =mg= $\frac{4}{3}\pi r^{3}\rho g$ downwards (where $\rho$ is the density of the body)

[wiki]Buoyant force[/wiki] =$\frac{4}{3}\pi r^{3}\sigma g$

SO, THE NET DOWNWARD [wiki]FORCE [/wiki]IS
$\frac{4}{3}\pi r^{3}(\rho -\sigma ) g$

Therefore, acceleration = net downward force / mass

=$\frac{\frac{4}{3}\pi r^{3}(\rho -\sigma ) g}{\frac{4}{3}\pi r^{3}\rho }$
=$\left (1-\frac{\sigma }{\rho } \right )g$

Now substitute the values and calculate

## In a pn junction, if diffusion current equals drift current, then why is there a potential barrier?

“In a [wiki]pn junction[/wiki], if diffusion current equals drift current, then why is there a potential barrier?”

Ans: When a [wiki]diode [/wiki]conducts, there is no [wiki]potential barrier[/wiki]

## Magnetic Field of Earth

“could u please explain me what is earth’s [wiki]magnetic field[/wiki]? in detail and easily in brief?”

Ans

Please refer to http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html for details. If you have any specific doubts related to it please ask as comment here.

[wiki]Geomagnetism[/wiki]

## Why Water is transparent?

“why [wiki]water [/wiki]has no colour?”

Ans:

Simple questions are always difficult to answer.

An object appear coloured or colourless depending on the colours of the spectrum absorbed, transmitted or reflected by it. We see an object in the colour we receive from it. Water allows all the colours to pass through them and therefore we see it as [wiki]transparent[/wiki].

(From another point of view “Water is one of the most essential substance for the existence of water. [wiki]Transparent [/wiki]and colourless appearance of water tels about its [wiki]purity[/wiki].”)

## A few Numerical Problems from Mechanics

ninzyy posted:

1. A 12 kg block sliding at 6.5 m/s along a level floor encounters rough area where the coefficient of friction is 0.20. The rough area is 5 meters long. How fast will the block be moving as it leaves the rough area? Where did the energy go?

2. A 800 kg car traveling southward at 9.2 m/s collides with a 900 kg car traveling northward at 4.5 m/s.?
The cars become entangled. What is the speed and direction of the cars immediately following the collision?

3. A 6kg block slides down a 2.0 m long frictionless ramp which is inclined 40 degrees above the ground.?
a) find the height of the top of the ramp and then determine the speed at the bottom of the ramp.

b) If the block collides with, and sticks to a 8.2 kg block which had been at rest at the bottom of the ramp, how fast would they now move?

c) Determine how much energy the two blocks have after the collision.

1. The problem can be solved using work energy theorem

i.e; work done against friction = change in KE
$\mu mg x = \frac{1}{2}m\left (v_{1}^{2}-v_{2}^{2} \right ) \Rightarrow v_{1}^{2}-v_{2}^{2}= 2\mu g x$
Substitute the values and you will get the answer.

2. This is an example for inelastic collision.

According to the law of conservation of momentum,

total momentum before collision = total momentum after collision

Taking northwards as positive and southwards as negative,

Substitute in the formula
$m_{1}v_{1}+m_{2}v_{2}=\left ( m_{1}+m_{2}\right )v$

Substitute the values of m1, m2, v1 and v2 and you can calculate the value of v.

If v is positive, it will be towards north and if negative it will be towards south.

## Stone falling in a well

“what is the distance a rock will fall in 5 seconds inside a well bore?”

Ans:

Using

$S=ut+\frac{1}{2}at^{2}$
and taking u = 0 (if falling from rest)
and g= 10 m/s2

S= 125 m

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