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# Monthly Archives: November 2012

## Body at incline

1. In first case we have the box with weight, say, 10 Newtons on incline. We calculate two components (perpendicular and parallel)of weight. We calculate normal force and force of friction. The box is at rest.

2. In second case we have ball on incline, made of same material as box from first case. Weight of the ball is the same as in first case, components of weight are the same as in first case. Normal force and friction have the same amount as in the case of box. But,unlike the box, ball is moving – it slides down. How’s that possible? If calculated net force is zero?

It is not true that the ball slides,  but it will roll down.

In the first case, the forces balance each other and there is no motion.

In the second force, the frictional force acting tangentially backwards (up the incline) and the component of weight of the ball acting parallel to the plane and through the centre of the ball constitute a couple and tends to rotate it. Now there is no sliding; it rolls.

(If any further clarification is required please post as comment to this post)

Please refer to http://www.real-world-physics-problems.com/rolling-without-slipping.html for detailed treatment of the Physics of rolling without friction.

## Can the process of Pair Production be used as a source for energy generation in space?

Can the process of Pair Production

be used as a source for energy generation in space?  Can Gamma Rays in space become the source for generating  energy using Pair Production process?  EM radiation >1.02 MeV enery, Pair Production is the dominant absorption process.

(Posted By Chintan Shah)

Have an answer? Post as comment now!

## Why earlier scientists could not guess about the gravitational force?

It’s rightly said that nature reveals its secrets to those who observe closely and think deeply. Newton is credited with the Universal law of gravitation, but Newton was not the only one who thought so.

Robert Hooke published his ideas about the “System of the World” in the 1660s, when he read to the Royal Society on 21 March 1666 a paper “On gravity”, “concerning the inflection of a direct motion into a curve by a supervening attractive principle”, and he published them again in somewhat developed form in 1674, as an addition to “An Attempt to Prove the Motion of the Earth from Observations”. Hooke announced in 1674 that he planned to “explain a System of the World differing in many particulars from any yet known”, based on three “Suppositions”: that “all Celestial Bodies whatsoever, have an attraction or gravitating power towards their own Centers” [and] “they do also attract all the other Celestial Bodies that are within the sphere of their activity”; that “all bodies whatsoever that are put into a direct and simple motion, will so continue to move forward in a straight line, till they are by some other effectual powers deflected and bent…”; and that “these attractive powers are so much the more powerful in operating, by how much the nearer the body wrought upon is to their own Centers”. Thus Hooke clearly postulated mutual attractions between the Sun and planets, in a way that increased with nearness to the attracting body, together with a principle of linear inertia. (Courtesy: Wikipedia]

Newton had his discussion with Sir Christopher Wren too.

Newton’s dignity was somewhat like The Pope (The one who cannot err). Everyone believed “What Newton says is right”

## Gravitational potential and Gravitational potential Energy – A doubt

I am very confused with the difference between U=GM/r which has a unit J/kg and U=GMm/r which is work done and have a unit of Joules. What are exactly their differences?

There’s a question “The gravitational potential difference between two points is 3.0×10^3 J/kg. Calculate the work done in moving mass of 4.0 kg between the two points.”

What I did was seeing their units and then equate since they’re J/kg and kg, I multiplied both magnitude which will give me ‘Joules’ in the end.

Like 3.0×10^3 J/kg = Workdone/4.0 kg

Please help, my print out book has some missing pages, which didn’t allow me to see the answer at the back of the book.

Posted by Taffs

Gravitational potential difference is the work done per unit mass in carrying a body from one place to other.

Work done has the unit J and therefore the potential difference (i.e the work done per unit mass) should have the unit J/kg

## Doubt from relativity

Here’s one problem….i am inside a train, the train is moving… and there is one electron inside a train … and I am looking at that electron. In my perspective the electron is not moving and (as my high school teacher told) only moving charged particle  creates magnetic field…so i will not see any magnetic field..but suppose my freind is looking that electron sitting outside the train..then he will see a magnetic field is created…then here is a problem…what is happening in reality..

Is there a magnetic field or not, does a independent reality exist in this case

Posted by Aawaz

## Doubts from Physics for Class XI – Collisions, Spring Constant, Gravitation, Measurement

1. A billiard ball A moving with an initial speed of 1 m/s, undergoes a perfectly elastic collision with another identical ball B at rest. A is scattered through an angle of 30˚. What is the angle of recoil of B? What is the speed of ball A after the collision?
2. Two springs have force constants K1 and K2 (K1> K2).On which spring is more work will be done if : (i) when they are stretched by the same force? and (ii) they are stretched through the same distance?
3. Show that the acceleration of free fall g at the surface of earth and the gravitational constant G are related by the expression
g = 4\3 Π p G R,

4. The radius of curvature of a concave mirror ,measured by a spherometer ,is given by

R = l2 /6h + h/2 The value of l and h are 4.0 cm and 0.065 cm respectively, where l measured by a meter scale and h by a spherometer.Find the error in the measurement of R.

## CLASS 11 – HALF YEARLY EXAMINATION SAMPLE PAPER -1

HALF YEARLY EXAMINATION SAMPLE PAPER

(QUESTIONS ARE FROM THE PART 1 NCERT TEXT BOOK)

CLASS: XI SUB: PHYSICS MM: 70 TIME: 3hrs.

BLUE PRJNT

 Sl.No Unit VSA(1) SA I (2) SA II(3) LA (5) Total 1 Physical World& Measurement 2(2) 4(2) 3(1) ————- 9(5) 2 Kinematics 1(1) 4(2) 6(2) 5(1) 16(6) 3 Laws of Motion 1(1) 2(1) 6(2) ———– 9(4) 4 Work,Energy and Power 1(1) 4(2) 3(1) 5(1) 13(5) 5 Motion of System of Particles&Rigid Body 2(2) 4(2) 3(1) 5(1) 14(6) 6 Gravitation 1(1) 2(1) 6(2) —————- 9(4) Total 8(8) 20(10) 27(9) 15(3) 70(30)

General instructions:

(i) All questions are compulsory.

(ii) There are 30 questions in total. Questions 1 to 8 carry one mark each, questions 9to18carry two marks each, questions 19to27carry three marks each and questions 28to30 carry five marks each.

(iii) There is no overall choice. However, an internal choice has been provided in one question of two marks, in one question of three marks and all three questions of five marks each. You have to attempt only one of the given choice in such questions

1. Name the fundamental force which is responsible for the stability of the nucleus. (1)

2. State the no. of significant figures in the following (more…)

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