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# Monthly Archives: February 2011

### Subscribe to this site by email ## Numericals from Class XI Physics Conservation of Momentum)

1. A 41.0-kg boy, riding a 1.60-kg skateboard at a velocity of 5.70 m/s across a level sidewalk, jumps forward to leap over a wall. Just after leaving contact with the board, the boy’s velocity relative to the sidewalk is 6.00 m/s, 9.30° above the horizontal. Ignore any friction between the skateboard and the sidewalk. What is the skateboard’s velocity relative to the sidewalk at this instant?
2. A 1.20-g bullet, traveling at a speed of 478 m/s, strikes the wooden block of a ballistic pendulum. The block has a mass of 191 g. (a) Find the speed of the bullet/block combination immediately after the collision. (b) How high does the combination rise above its initial position?
3. After skiding down a snow-covered hill on an inner tube, Ashley is coasting across a level snowfield at a constant velocity of +3.0 m/s. Miranda runs after her at a velocity of +4.2 m/s and hops on the inner tube. How fast do the two of them slide across the snow together on the inner tube? Ashley’s mass is 71 kg, and Miranda’s is 58 kg. Ignore the mass of the inner tube and any friction between the inner tube and the snow.
4. A car (mass = 1170 kg) is traveling at 28.8 m/s when it collides head-on with a sport utility vehicle (mass = 2580 kg) traveling in the opposite direction. In the collision, the two vehicles come to a halt. At what speed was the sport utility vehicle traveling?
5. In an Atwood system mass 1= 2 kg and mass 2=7kg. The masses of the pulley and strings are negligible by comparison, the pulley turns without friction and the string does not stretch. The lighter object is released with a sharp push that sets into motion at v(initial)= 2.4 m/s downward.
How far will m1 descend below it’s initial level?
Find the velocity of m1 after 1.8 seconds

## Five Sure Shot portions for SA2 (Class X CBSE)

The students appearing for SA2 of CBSE class X are requested to practice the following portions of Physics very well so that they can get excellently good marks in Science.

1. Practice all ray diagrams showing image formation by concave mirror, convex mirror, convex lens and concave lens along with characteristics of image.
2. Defects of eye, cause and rectification
3. Structure of human eye (diagram), function of each part, power of accommodation and persistence of vision.
4. Refraction through a glass slab and a glass prism, lateral displacement and angle of deviation.
5. Scattering of light, why sky is blue, Tyndal effect.
6. Also Practice numerical based on mirror formula, lens formula, refractive index.

## Electromagnetic Waves :: Doubts continue …

why are oscillating electric and magnetic feilds created when an elctromagnetic wave is travelling?

Ans:

[wiki]Electromagnetic waves[/wiki] are produced by mutually perpendicular time varying electric and magnetic field mutually reinforcing each other. ## Questions normally not answered by Students in Viva

Here are some questions which are not properly answered by students during Viva.

1. Name the Physical quantities to be kept constant for Ohm’s law to be true.
2. State the Principle of a potentiometer. (The students say that potential drop is proportional to length but the constant quantities are not mentioned)
3. How can we increase the sensitivity of a potentiometer?
4. Define figure of merit of a galvanometer.
5. Which has more resistance – a galvanometer or a milliammeter?
6. How does an LED emit light?
7. What is the difference between an ordinary diode and an LED?
8. Define principal axis of a convex lens?
9. What happens to the focal length of a concave mirror if it is immersed in water?
10. What are the factors affecting the intrernal resistance of a cell?
11. What are the difference between primary and secondary cell?
12. Why can’t we use a dry cell for starting a car?

1. length, area of cross section,temperature
2. The potential drop across any length of a conductor of uniform cross section and composition carrying a constant current is directly proportional to the length.
3. increasing the length of potentiometer wire, decreasing the current, decreasing the potential gradient
4. Current for unit deflection
5. galvanometer
6. The energy released during recombination of electrons and holes across the junction is responsible for the release of light by LED
7. In ordinary diode the energy emitted during recombination of electrons and holes is in the invisible region of the em spectrum but in the case of LED, the energy is in the visible region.
8. Straight line joining the centres of curvature of the lens..
9. The focal length of mirror does not change by changing the medium. Their is a pure geometrical relationship between the radius of curvature and focal length and the relation does not include any term depending on refractive index.
10. the nature of electrolyte, the concentration of electrolyte, temperature, distance between electrodes, area of electrodes
11. Primary cell cannot be recharged, secondary cells can be recharged; secondary cells have less internal resistance than primary cells.
12. Due to the high internal resistance of a dry cell, it won’t be able to provide the current sufficient to start the car

## A numerical from Centre of mass

Two objects, of mass 1 kg and 2 kg, are moving with velocities equal to +2 m/sec and -3 m/sec. The center of mass of the two objects is moving at velocity =(1×2+2x(-3)/(1+2)=(2-6)/3=-4/3=-1.33 m/s (-ve sign shows that the centre of mass will be moving in the direction of the second body)

## A numerical Problem from optics

When an object is kept at a distance of 60cm from a concave mirror, the magnification is 1/2. Where should the object be placed to get magnification of 1/3??

Ans:

In the first part, u=-60, m=(-)1/2 implies v=-30 cm (Since the image must be real as concave lens form virtual image of bigger size only)

we get f = –20 cm

In II case

m=-1/3

v=u/3

f=-20cm

we get u=4f=4 x (-20)=-80 cm

## i want score max marks in physics. So how much time must be given to study it ?

“i want to score max marks in Physics. So how much time to be given to study it”

Mathew Abraham responds:

First of all I would like to say that “Don’t study just for marks!! “. You have to concentrate on concepts and ideas and don’t leave until you fully grasped the idea. If you have understood the basic concept and how to apply it in different situations, learning Physics is just fun.

The time you should spend for studying Physics depends on how easy you can grasp the ideas.

Having a master plan is very important.

Depending on the time available to you before exams, arrange a daily plan of revision and ensure that you reach the target.

Have a regular plan for solving numerical problems systematically. Clarifying your doubts and questions related to the topic is important. (AskPhysics is always ready to give you an answer)

If you are a candidate for the AISSCE 2011, Then the following strategy will help you.

• Have a collection of all 3 marks and 5 marks questions and their answers prepared in the most presentable manner. (show it to your teacher for any corrections or modifications) You can consult some text books for preparing the first draft. The 3 and 5 marks alone constitute 60 % of the total marks.
• Solve and practice all the exercises from NCERT text (including examples and exercises) Many publishers have released solutions for NCERT exercises. If you can get one, it will be helpful.
• As a last step practice at least 5 Previous Board Papers. (The more the best)
• If you do the above, you can easily score more than 90% marks or more.

Once again I would like to remain, It’s not time; but the task in hand is more important.

Wish you all the best.

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