# 561 - Physics Questions Answers

Q  IF Al DIPPED IN H2O AT 10*C THEN FORCE OF BUOYANCY WILL INCREASE OR DECREASE?

Q TEMP. OF A SOLID OBJECT IS OBSV.  CONST .DURING A PERIOD . IN THIS PERIOD

a)HEAT MAY HAVE BEEN SUPPLIED TO IT

b) H MAY HAVE BEEN EXTRACTED FROM IT

c) no H SUPPLIED TO IT

d)NO H EXTRACTED FROM IT

more than one opt is correct . pls explain your each opt with expln.

Joshi sir comment

It depends that either Al or water will change its density more rapidly, because Bouyancy is based on densities of solid and liquid

Answer : a, because it may be an isothermal process

b, because in isothermal process, surrounding can also give heat to the system.

c, may be the process is adiabatic

d, may be adiabatic.

Q gas of certain mass at 273K is expanded to 81 times its vol. under adia. condition. if gamma = 1.25 for gas then its final temp. = ?

Q for a const. vol. gas thermometer one should fill gas at high temp & low pressure WHY ?

Q  iF TEMP of uniform rod is slightly increase by del T its moment of inertia about 90* bisector increase BY = ? (ANS 2alpha I del T ).

Q Al sphere is dipped into H2O at 100*C if temp is increased forse of buoyancy will inc. OR decreased WHY =?

DENSITY OF A SUB. IS 10g/cc at 0*C &at 100*C d= 9.7 g/cc . coeff. of linear expantion is =?

Joshi sir comment

Answer :    For adiabatic process TVγ-1 = K

so 273*V0.25 = T*(81V)0.25

so 273/3 = T

Answer :   Because real gases perform ideal behaviour in this range.

Answer :    Inertia about perpendicular bisector = 1/12 ML2

on increasing temperature by dT new inertia = 1/12 M[L(1+αdT)]2

so increment in inertia = 1/12 ML2 [(1+αdT)2-1-] = 1/12 ML2 [ (αdT)2 + 2αdT ]                 now neglect (αdT)because it is small term

Answer :   Since temperature is already 100 degree centigrade so on increasing it further density of Al will increase rapidly in comparision to density of water so Buoyancy force will be increased.

Answer :   use the formula d100 = d/ (1+γ100)   here γ=3α                    calculate α

Q Agraph having temp *C along xaxis& *F along y axis. if graph is straight then it intercept +ve y axis. WHY ?

Joshi sir comment

(F-32)/9 = t/5

so F = 9t/5 + 32

on taking t = 0 we will get F = 32

thats why it makes an intercept of 32 in y axis

## sir,  can u please post a topic on rigid body rotation ? mainly heip to solve question please.  with some examples.

Asked By: SHUBHAM VED
Joshi sir comment

Yes, I will do very soon.

Q 1-when current passes through a solenoid .then what happens to the length of solenoid?

Q2-a current carrying wire carrying current i normally into the paper at the centreof current carring circular ring.the magnetic force acting on the circular ring is ?

Q3- To increase the current through the solenoid from 1A TO 2A ,30J work is done by thr battery.work done  by the same battery in increasing the current from 3A TO 4A IS?

Q4- When a closed looop is held stationary in  magnetic field b/w north & south pole of 2 permanent magnets of different dipole moments , then current will be induced in the loop or not?

Asked By: TANVI GROVER
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Joshi sir comment

1. the length will decrease as the current in the two consecutive turns of the solenoid are in same direction. which will create an atrraction.

2. since B produced by the wire is parllel to the direction of dl so net force will be 0

3.dw = Lidi first take limit 1 to2 and w=30 to find the L (INTEGERATE)

After than again use the above formula  taking limit 3 to 4 .put L here ,L= 20

the answer is 70J.

4. I think no emf will be induced as the flux is not changing.

These answers by NIKHIL VARSHNEY are correct

## a uniformrope of lenth 10 m and mass 12 kg hangs verticaly from a rigid support .  a block of mass 4 kg is attached to the free end  of  the rope . atrans verse wave of wavelenth 00.03 m is prodused at the lower end .  wavelenth of pulse when it reaches upper end  will be?

Asked By: SHUBHAM VED
Joshi sir comment

at top point tension in the string = 16g

and at bottom point T = 4g

and v = √(T/m)   here m is mass per unit length which is same throughout the rope.

first calculate v at both the points

frequency will be same throughout

so use the formula v = nλ twice

once at bottom and once at top point

then by diving these 2 formulae we will get the result

try it.

why is electric field of earth is in order of 100v/m any reason by formulae

Asked By: ABHISHEK JAIN
Joshi sir comment

emf generated between the extremes of equator line = vBl = RωBl

so E = RωBl/l = RωB = 2πRB/T

now calculate

please,  write tibs about tidal effect  and it's fourmula ?   Is it depend inversly on the cube of the distance? and is  tidal effect of the moon's pull grater than the tidal effect of Sun?

Asked By: SHUBHAM VED
Joshi sir comment

The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side farther away. Stated differently, the tidal force is a differential force. Consider three things being pulled by the moon: the oceans nearest the moon, the solid earth, and the oceans farthest from the moon. The moon pulls on the solid earth, but it pulls harder on the near oceans, so they approach the moon more causing a high tide; and the moon pulls least of all on the far oceans (on the other side of the planet), so they stay behind more, causing another high tide at the same time. If we imagine looking at the Earth from space, we see that the whole Earth was pulled, but the near oceans more and the far oceans less; the far oceans stayed behind since they are pulled less (since they are farther away).

Tidal effects become particularly pronounced near small bodies of high mass, such as neutron stars or black holes, where they are responsible for the "spaghettification" of infalling matter. Tidal forces create the oceanic tide of Earth's oceans, where the attracting bodies are the Moon and, to a lesser extent, the Sun.

Formula $\vec a_t$(axial) $~ \approx ~ \pm ~ \hat r ~ 2 \Delta r ~ G ~ \frac{M}{R^3}$

why the direction of electric dipole moment is from -ve to +ve

Asked By: HIMANSHU MITTAL
Joshi sir comment

It is the direction of electric field due to a dipole at axial point.

a hollow sphere of mass M (in K.G.) radius R (in m.)  is rotating with angular  frequency  omega(in radsec)  . it suddenly stops rotating and 75% of  the K.E. converted  into heat energy .   if S joulekg-kelvin is the spesific heat of material of the sphere, the rise of temperature of the sphere is ?

Asked By: SHUBHAM VED
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Joshi sir comment

Rotational K. E. = 1/2 I ω2

so 0.75 [1/2 I ω2] = M S dT

and I = 2/3 M R2

now solve