#### ritwik06

1. Polishing of the surfaces in contact beyond a limit increases friction because:
i)irregularities are increased ii)surface projections are sharpened iii)area of actual contact is increased iv)none of these

My ans. (ii)

2. A chain is placed on a frictionless table such that 1/5 part is hanging down over the edge of the table. If the length of the chain be 'l' and mass be m, then the work done in pulling up the hanging part of the chain back on the table is _________

My ans. (mgl/5)

3. A bottle filled with water at 30 degree celsius and fitted with a cork is taken to the moon. If the cork is opened at the surfacce of the moon then-
water will freeze. Am I right?

4. A body of mass 2kg slides down on a curved track which is a quadrant of a circle of radius 1 meter. All the surfaces are frictionless, if the body starts from rest, its speed at the bottom of the track is __________

My calculation :4.43 m/sec

5. The bob of a simple pendulum of length 'l' is negatively charged. A positively charged metal plate is placed just below it and the pendulum is made to oscillate. Then what will be the effecton the time period?

6. Tank containing water has an orifice in one verticle side. If the centre of orfice is 4.9 m below the surface level in the tank, the volcity of discharge is-
Ans: 9.8 m/sec

7. The magnetism in a magnet is mainly due to:
i) the orbital motion of the electrons ii) the spin motion of the electrons
iii) the nuclear charge iv) none of these

8. Solar spectrum is a band spectrum. True or false?
Ans: True.

9. A faulty balance is in equilibriumwhen its pans are empty. A body weighs 100 gf and 121 gf on the two pans. Then its actual guess will be _____
My ans: 110 gf

10. Magnifying power of a simple microscope can be increased if we use an eyepiece of higher focal length. true or false?

Please tell me whether I am right, some suggestions etc. You can answer for any question you know. Thanks in advance!!

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#### Hootenanny

Staff Emeritus
Gold Member
1. Polishing of the surfaces in contact beyond a limit increases friction because:
i)irregularities are increased ii)surface projections are sharpened iii)area of actual contact is increased iv)none of these

My ans. (ii)
I'm afraid I would have to disagree with you there. Why would you think that polishing results in sharper surface projections?
2. A chain is placed on a frictionless table such that 1/5 part is hanging down over the edge of the table. If the length of the chain be 'l' and mass be m, then the work done in pulling up the hanging part of the chain back on the table is _________

My ans. (mgl/5)
Your on the right track, however, you should consider the centre of mass of the hanging chain.
3. A bottle filled with water at 30 degree celsius and fitted with a cork is taken to the moon. If the cork is opened at the surfacce of the moon then-
water will freeze. Am I right?
Consider the pressure decrease relative to the decrease in temperature. (http://en.wikipedia.org/wiki/Vapour_pressure" [Broken])
5. The bob of a simple pendulum of length 'l' is negatively charged. A positively charged metal plate is placed just below it and the pendulum is made to oscillate. Then what will be the effecton the time period?

Are you sure about that? You may wish to draw a FBD to examine the forces acting on the pendulum.
7. The magnetism in a magnet is mainly due to:
i) the orbital motion of the electrons ii) the spin motion of the electrons
iii) the nuclear charge iv) none of these
What do you think?
8. Solar spectrum is a band spectrum. True or false?
Ans: True.
I would agree
10. Magnifying power of a simple microscope can be increased if we use an eyepiece of higher focal length. true or false?
Are you willing to hazard a guess. Forgive me for not checking your calculations.

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#### ritwik06

Originally Posted by ritwik06
1. Polishing of the surfaces in contact beyond a limit increases friction because:
i)irregularities are increased ii)surface projections are sharpened iii)area of actual contact is increased iv)none of these

My ans. (ii)

I'm afraid I would have to disagree with you there. Why would you think that polishing results in sharper surface projections?
I think so because when a solid is tried to polish beyond limit, maybe the edges of the projections present on the solid may get sharper, dont you think?

3. A bottle filled with water at 30 degree celsius and fitted with a cork is taken to the moon. If the cork is opened at the surfacce of the moon then-
water will freeze. Am I right?

Consider the pressure decrease relative to the decrease in temperature. (Vapour Pressure)
that shows it will freeze, if I am not wrong. thanks!

5. The bob of a simple pendulum of length 'l' is negatively charged. A positively charged metal plate is placed just below it and the pendulum is made to oscillate. Then what will be the effecton the time period?

Are you sure about that? You may wish to draw a FBD to examine the forces acting on the pendulum.
I am sure about that 'cause time period depends on length and acceleration due to gravity. Whats an FBD diagram by the way?

7. The magnetism in a magnet is mainly due to:
i) the orbital motion of the electrons ii) the spin motion of the electrons
iii) the nuclear charge iv) none of these

What do you think?
I would guess it to be the first option, ie., orbital motion. 10. Magnifying power of a simple microscope can be increased if we use an eyepiece of higher focal length. true or false?

Are you willing to hazard a guess. Forgive me for not checking your calculations.
I would say true. Just a guess Thanks for the effort sir.

#### Hootenanny

Staff Emeritus
Gold Member
I think so because when a solid is tried to polish beyond limit, maybe the edges of the projections present on the solid may get sharper, dont you think?
Not that I am aware of. As far as I know, once the surface becomes sufficiently smooth, surface roughness is no longer the dominant factor in determining the coefficient of friction. At this point molecular interactions become the dominant factor (electrostatic interactions). The strength of these interactions are proportional to the contact area; the smoother the surface is the greater the contact area between the two surfaces and hence the greater the electrostatic interactions.
I am sure about that 'cause time period depends on length and acceleration due to gravity. Whats an FBD diagram by the way?
This is true of course, provided that the weight of the pendulum is the only force acting. FBD is short for a 'Free Body Diagram'.
I would guess it to be the first option, ie., orbital motion. Out of the options I would go with (ii) although magnetism is not exclusively due to the spin of the electrons.
I would say true. Just a guess It is (often surprisingly) false. The magnification of the eyepiece in a microscope is of the form (I can't remember exactly);

$$M \approx \frac{1}{f}+1$$

Thanks for the effort sir.
And there's no need for the sir 