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## A Numerical from Kinematics

Advait Asks:

“A test projectile is fired horizontally into a viscous liquid with a velocity u.The retarding force is propotional to the square of the velocity, so that the acceleration becomes a=-k(v)^2 .v is the velocity at that point. Derive expressions for the distance D travelled in the liquid and corresponding time t required to reduce the velocity to u/2.
Neglect any vertical  motion”

CAN YOU ANSWER?

## Why does a test tube seem silvery when put in water?

Amritha Asked:

Why does a test tube seem silvery when put in water?
Answer:

This is due to Total Internal Reflection. When an empty test tube is inserted in water, the light trying to enter the test tube undergoes total internal reflection and shines like silver.

The air bubbles trapped in a glass paper weight a;lso shines like silver due to TIR.

## Why don’t we feel Earth’s Rotation?

Sreeda Asked:

“Why are we not feeling Earth’s rotation while it is actually happening”

Answer:

It is just like we don’t feel like moving when we are inside  a train moving with constant velocity and especially when we are not looking out of the window.

When we look out from earth, we see stars and other heavenly bodies which are very much far away and the relative change in position is very much less to be noticeable.

The most celebrated test of Earth’s rotation is the Foucault pendulum first built by physicist Léon Foucault in 1851, which consisted of an iron sphere suspended 67 m from the top of the Panthéon in Paris. Because of the Earth’s rotation under the swinging pendulum the pendulum’s plane of oscillation appears to rotate at a rate depending on latitude. At the latitude of Paris the predicted and observed shift was about 11 degrees clockwise per hour. Foucault pendulums now swing in museums around the world.

## MAGNETISM : Why repulsion is told to be the test for magnetism?

Sreeda sked:

“Can a magnet lose its magnetic properties?How?
Why ‘repulsion’ and not ‘attraction’  is suggested to be a good test of finding whether a substance is a magnet or not?”

Answer:

A magnet can lose its magnetism.

A magnetic substance is made a magnet by making the molecules which are individually possessing magnetic properties align in a particular direction. In the unmagnetized state they are arranged randomly and hence do not have magnetic behaviour.

If a magnet is heated, beaten, mishandled etc, it will lose its magnetism.

Why repulsion is the sure test for magnetism?

A magnet can attract the opposite pole of other magnet as well as the magnetic substance (iron cobalt nickel). SO, if A magnet attracts another object, we cannot say that the other object is a magnet. But if the magnet repels the object, then we can surely say that the other object is a magnet. Because “Like poles of a magnet repel each other”

## AIEEE Will Finally Go Online For The First Time in 2011

All aspiring candidates of engineering and architecture courses prefer to take online test instead of regular pen-paper based format. AIEEE is finally about to conduct its online entrance exams on 1st May, 2011. The online exams will follow the standardized pattern of paper-based test which is being used for nine years. However, Central Board of Secondary Education (CBSE), the organizer of the exam, will only allow a limited number of students to appear in the online exams in 2011. Students from 20 cities will be able to give their exams online with a maximum of 5,000 students per city. The students will be allotted seats for online exams on first-come-first-served basis.

## Difference between emf and pd

ananthesh asked:

“can you please explain about emf and PD of a cell?”

Answer:

EMF stands for electromotive force.

EMF of a cell is equal to the potential difference between the two terminals of a cell when no current is drawn from it.

Pd (potential difference) between any two points on a circuit is defined as the work done per unit charge in carrying a positive test charge from one point to the other. With reference to a cell, the potential difference between the terminals of a cell when a current is drawn from it is called terminal voltage.

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