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# Tag Archives: Ans

## Gravity and something more

**Jihin** Asked:

Some Things That I Have Always Wondered OF : (i hope it may be answered soon …)

1)is it possible that our human body can withstand pressures twice or more than the atmospheric pressure without a suite or is there any artificial instruments or means in which our heart can be made to pump into a level in which we can attain this ability !!!

2)when more gravity is experienced , it is said that time travels slowly – is it relative ??? caz time goes at a constant rate !!! it is only the position or view that makes the observer feel the situations ???

3) Is there any gravity chambers successfully made out … in which the gravity can be changed accordingly to observe diff. phenomena ??? Cant It be possible to do that ????

**Ans**:

To be posted soon.

However, I agree that I may not be able to give you a satisfying answer to all your queries, as “Science says the first word of everything and the last word of nothing.” We are learning new things everyday. When we explore more and more, we find that we know less and less and our knowledge is too meagre.

By the time I prepare a well fitting answer to this, visitors are also requested to respond to the question by posting answers and comments.

## A Problem from Motion

Sandeepan Asked:

“Q. Velocity of a moving particle is given by v = t

^{2}– 4. Find out:-

(i) Time at which velocity is zero.

(ii) Acceleration at which t= 1s

(iii) What type of motion uniform or non-uniform.”

Ans:

In the equation, put v=0 to find the time at which velocity equals zero

Differentiate the equation and then put t=1 to find acceleration at t=1s since a = dv/dt

The motion is non uniform as its velocity varies with time.

## TENSILE FAILURE STERSS

john Alexander asks

Is the **energy** required to break a sample of string proportional to the **length** of string or is the **energy** requirednto break the sting constant?

**Ans**

The **energy** required to break a string depends on the stress known as tensile failure stress.It is not related to the **length** of the string .for more **knowledge** ref. Mohr’s theory

## WAVE MOTION QUESTIONS AND ANSWERS FOR CLASS XI

**One mark questions with answers**

**Q1.****body** of **mass** 2kg. is at restand is hit by a **mass** of 4kg. moving with 3m/s, find fraction of the momentumretained by the moving **body** assuming the collision to be elastic and head-on.

**Ans1.**_{2}/m_{1}

(n – 1)/(1 + n) is the fraction of the momentum retained by the moving body so

(n – 1)/(1 + n) = (2 – 1)/(1 + 2) = 1/3.

**Q2.****mass** of the moving **body** ismuch greater than the **mass** of the **body** at **rest** than what is the approximatevilocity of the moving **body** after head-on collision?

**Ans2. ****mass** is muchgreater than the **mass** at **rest** then after the collision the heavier **mass** keepson moving with the same **velocity** and in the same direction.

**Q3.****point** the potentialenergy of a **body** is taken to be zero?

**Ans3.****energy** of the bodyat the surface of the earth is taken to be zero (Potential **energy** = mgh, whereh is the **height** of the **body** from the surface.).

**Q4.****work** done on a **body** by aforce depend upon the path followed by it?

**Ans4. ****force** isconservative then it does not depend but if it is non-conservative (friction)then it depends.

**Q5.****body** hits the **ground** from aheight h_{1} and rebounds to a **height** h_{2} after havinginelastic collision with the **ground** then what is the coefficient ofrestitution?

**Ans5. **_{2}/h_{1})

**Q6.****body** hits the **ground** with 50m/s **velocity** and has inelastic collision with the **ground** then with whatvelocity it will rebound if the coefficient of restitution is 0.2.

**Ans6. **_{2}/v_{1}where v_{1} is the **velocity** with which the **body** hits the **ground** and v_{2}is the **velocity** of rebound.

e = 0.2 = v_{2}/50, so v_{2} = 10 m/sec.

**Q7.****body** at **rest** explodes in threefragments. Is it possible that two equal parts move in mutually perpendiculardirections with the same **velocity** and third **mass** moves midway between the two?

**Ans7. **

**Q8.****speed** of a moving vehicleis increased by 200% then how much should be the change in the retarding forceto stop the vehicle over half the previous distance?

**Ans8. **^{2} = F.S where Fis the retarding **force** and S is the distance over which the vehicle comes torest. When ‘v’ is increased by 200% then K.E. increases by 800%. As S is halvedthen F should be made 16 times.

**Q9.****work** is done in compressinga spring from 0 cm to 6 cm then find the **work** done in compressing the same from3cm to 6 cm.

**Ans9.**_{2}^{2}– x_{1}^{2}). This is the **work** done when the spring iscompressed from x_{1} to x_{2}.

20 = (1/2)K (36 – 0)

W = (1/2)K (36 – 9)

Solving the above equations we get W = 15 J.

**Two mark questions with answers**

**Q1.****ball** is dropped from **rest** at aheight of 20m. If it loses 30% of its kinetic **energy** on striking the **ground**,what is the **height** to which it bounces? How do you account for this loss inkinetic **energy**?

**Ans1.****ball** acquires avelocity ‘v’ after falling through a **height** of 20m.

Because the ball is dropped from rest, hence u = 0.

Hence, v^{2} = u^{2} + 2as

= 0 + (2 ** x** 10

**20) = 400**

*x*So, v = 20 m/s

Kinetic energy of the

**ball**just before hiting the ground

= (1/2)mv

^{2}= (1/2)m(400) = 200m Joule

Because the

**ball**loses 30% of the kinetic

**energy**on striking the

**ground**, hencekinetic

**energy**retained by the

**ball**after striking the

**ground**= 70% of 200m J

= 140m J

The

**energy**loss is due to the inelastic collision with the

**ground**.

**Q2.****energy** being consumedin planetary motion.

**Ans2.****body** whichrevolves round the star (the sun). The **force** which is responsible for circularmotion, is called centripetal **force**. The direction of the centripetal **force** isalways towards the centre. Thus, the angle between **force** F and displacement Sis q^{o} at every **point**.Work done in moving planet, W = **F.S** = FSCos^{o}.So, W = 0. Hence,no energyis being consumed in planetary motion.

height=82src=”./xi%20work,%20power%20and%20energy_files/image001.gif”alt=”workpowerandenergyf06q16i.gif (1336 bytes)” v:shapes=”_x0000_i1027″>

**Q3.****energy**?

**Ans3.****energy** (Power).The potential **energy** of water storedat a **height** is converted into K.E. when water is made to rush down. This fallof water is used to rotate the turbine and the coil and armature of generatoris rotated and electricity is produced . Thus, the K.E. of the fall of water isconverted into electrical form of **energy**. Hence the hydroelectric power-plantis an example of law of conservation of **energy**.

**Q4.****body** is dropped from aheight of 40m then after 3 inelastic collisions with the **ground** to which heightthe **body** will rise? (given: Coefficient of restitution = 0.5)

**Ans4.****body** is dropped from aheight of ‘H’ and ‘e’ is the coefficient of restitution then after ‘n’inelastic collisions with the **ground** the **body** rises to a **height** ‘h’ given by

h = H.e^{2n}.

h = 40 ** x** (1/2)

^{2 x 3}= 40

**(1/2**

*x*^{6}) = 40/64 = 0.625 m

**Q5.**

**Ans5.**_{1}v_{1}^{2}= (1/2)m_{2}v_{2}^{2} ………….(*i*)

If force of brakes be the same then m_{1}a_{1} = m_{2}a_{2}……….(*ii*)

If truck stops over a distance S_{1} then v_{1}^{2} =2a_{1}S_{1} ……..(*iii*)

If car stops over a distance S_{2} then v_{2}^{2} = 2a_{2}S_{2}………(*iv*)

From (*i*) and (*ii*)

(1/2)m_{1}v_{1}^{2} = (1/2)m_{2}v_{2}^{2}………..(*v*)

From (ii) and (v)

v_{1}^{2}/a_{1} = v_{2}^{2}/a_{2}…………(*vi*)

From (*iii*), (*iv*) and (*vi*)

2a_{1}S_{1}/a_{1} = 2a_{2}S_{2}/a_{2}.

S_{1} = S_{2}

Hence distances covered S_{1} and S_{2} are equal.

**Q6.****kg**/minute can it raise a **height** of 20m? (g = 10 m/s^{2})

**Ans6.**

If mass of water raised in one second = m kg.

Total work done in lifting water,W = mgh

Power P = W/t, but t = 1 minute = 60 sec.

4000 = mgh/60

4000 = (m ** x** 10

**20)/60**

*x*m = 1200

**kg**.

**Q7. ****length** 3m is suspendedvertically from a fixed **point**. It is given an angular displacement of 60^{o}in the vertical plane. If its **mass** per unit **length** is 2 **kg** then find the workdone?

**Ans7. ****mass** of the rod and’l’ be its **length** then

m = 2 ** x** 3 = 6

**kg**

If the rod is displaced through an angle qthen the

**work**done on it, W = mg(l/2)(1 – Cosq).

The effective length of the rod is taken to be (l/2) because in uniformdistribution of

**mass**the centre of

**mass**is at the geometric centre so

W = 6

**10**

*x***(3/2)(1 – Cos60) =45 J.**

*x*

**Three mark questions with answers**

**Q1.****mass** m* _{2}*is at

**rest**and

**mass**m

_{1}moving with

**velocity**u

_{1}hits itelastically, show that the fraction of the momentum transferred to the

**mass**atrest is 2n/(1 + n) where n is ratio of the masses.

**Ans1.**_{1} = (m_{1} -m_{2})u_{1}/(m_{1} + m_{2}).

v_{2} = 2m_{1}u_{1}/(m_{1} + m_{2}).

This is from the theory of conservation of momentum.

Momentum of the **mass** m_{2} after collision,

P_{2} = m_{2}v_{2} = (2m_{1}m_{2}u_{1})/(m_{1}+ m_{2})

Fraction of momentum transferred to m_{2}.

= (2m_{1}m_{2}u_{1})/(m_{1} + m_{2})m_{1}u_{1}= 2m_{2}/(m_{1} + m_{2})

= 2n/(1 + n) ….[because m_{2}/m_{1} = n]

**Q2.****work** to bepositive negative or zero? "Explain with example.

**Ans2. ****force** applied F and the displacement S movedby **body** *i.e.*

W = F.S,

W = FS Cos q

If q**work** is maximum.

It remains +ve for the angle q between 0^{o}to 90^{o}

height=45src=”./xi%20work,%20power%20and%20energy_files/image002.gif”alt=”tutor2phyworkpowerandenergyf06q2i1.gif (1062 bytes)” v:shapes=”_x0000_i1037″>**or**, q^{o} and360^{o} *i.e.*, if the displacement is in a directionoppisite to which the **force** is applied.

height=59src=”./xi%20work,%20power%20and%20energy_files/image003.gif”alt=”tutor2phyworkpowerandenergyf06q2i2.gif (1097 bytes)” v:shapes=”_x0000_i1038″>

Thus **work** is +ve if Cos q**work** done will be -veif Cosq*i.e.* ^{o} to 270^{o}.

If q^{o} then Cos 90^{o}= 0.

Hence **work** done W = FS Cos 90^{o} = 0.

Thus, W may be +ve, -ve or zero

**Q3.****length** of the pendulum is 2m, whatis the **speed** with which the bob arrives at the lowermost **point**? Given that itdissipates 10% of its initial **energy** against air resistance?

**Ans3.****energy** atthe highest position

= mg ** x** 2 Joules = 2mg Joules

Kinetic

**energy**at lowest position

= Potential

**energy**at the highest position – the

**energy**dissipitated againstair resistance or friction

= [mg

**2 – (10/100)**

*x***mg**

*x***2] Joule**

*x*= mg

**18/10 J**

*x*^{2}= mg

**18/10**

*x*or, v = 1.9 ms

^{-1}.

**Q4.****work**. Give their relations also.

**Ans4. **

W = FS**(1)** In S.I system,

If F = 1 **kg** weight or 1 **kg** **force** and S = 1m then,

W = (1 **kg** wt)(1m) = 1 **kg** m ……………(*i*).

Hence, one kgm is the gravitational unit of **work** in S.I (M.K.S) system and isdefined as the amount of **work** done if 1 **kg** **force** displaces a **body** through 1m inthe direction of the applied **force**.**(2)** In C.G.S system,

F = 1 gmwt and S = 1cm,

W = (1 gm wt) (1 cm) = 1 gm cm ………………(*ii*).

Hence, one gm cm is the gravitational unit of **work** and is defined as the amountof **work** done, if 1 gm **force** displaces a **body** through 1 cm in the direction ofthe applied **force**.

1 gm cm = 980 ergs.**NOTE**: 1 **kg** m = 9.8 Joules.

**Q5.**

**Ans5. **

One electron volt is the **energy** acquired by one elctron in moving it betweentwo **point** having a P.D of 1V.

Thus, 1eV = (1.6 ** x** 10

^{-19}) C

**1J/C = 1.6**

*x***10**

*x*^{-19}Joules.

**NOTE:**The other practical units used are

1 Million electron volt = 1 MeV = 10

^{6}eV, 1 MeV = 10

^{6}

**1.6**

*x***10**

*x*^{-19}J, 1 MeV = 1.6

**10**

*x*^{-13}Joules and

1 Billion eV = 10

^{9}eV, 1 BeV = 1.6

**10**

*x*^{-10}joules.

**Q6.****velocity** changes by 5%, find the change in the power of water?

**Ans6.**** x**Velocity = Rate of change of momentum

*x***velocity**={(

**mass**/time)

*x***velocity**}

*x***velocity**= {(adv)

**v}**

*x***v =adv**

*x*^{3}where ‘a’ is area of cross section, ‘d’ is the density of waterand ‘v’ is the

**velocity**of flow of water.

Therefore, Power of water is directly proportional to the cube of velocity ofwater so let P = Kv

^{3}(k is a constant and is equal to ‘ad’.)Taking log on both sides

log P = 3log v + log k

Differentiating on both sides

D

percentage change in power, DP/P

**100 = 3**

*x***5%**

*x*= 15%.

**Q7.****energy** of rushing outwater from a dam is used in rotating a turbine. The pipe through which water isrushing is 2.4 meters and its **speed** is 12 m/sec. Assuming that whole of kineticenergy of the water is used in rotating the turbine, calculate the currentproduced if efficiency of the dynamo is 60% and the station transmits power at240 kV. Density of water = 10^{3} **kg**/m^{3}.

**Ans7.**

r = radius of pipe = 1.2m, average speed of water v = 12 m/s

V = 240 kV = 240 ** x** 10

^{3}volt, density ofwater p = 10

^{3}

**kg**/m

^{3}.

Now, kinetic

**energy**of rushing water per second i.e.

Power P = (1/2)(

**mass**flowing per sec)

**v**

*x*^{2}

= (1/2)p

^{2}(l/t)

^{2}

= (1/2)p

^{2}

^{3}

= (1/2)

**3.14**

*x***(1.2)**

*x*^{2}

**10**

*x*^{3}

**(12)**

*x*^{3}watt

= 3.9

**10**

*x*^{6}watt

current = output power/voltage

= (60% of power P)/(240

**1000)**

*x*= [(60/100)

**3.9**

*x***10**

*x*^{6}]/(240

**1000) = 9.75 amp.**

*x*

**Five mark questions with answers**

**Q1.****mass** m isaccelerated from **rest** when a constant power P is supplied by its engine; showthat :

(a) The **velocity** is given as a function of time by

v = (2Pt/m)^{1/2}

(b) The position is given as a function of time by

s = (8P/9m)^{1/2}t^{3/2}.

(c) What is the shape of the graph between **velocity** and **mass** of the vehicle ifother factors remain same?

(d) What is the shape of the graph between displacement and power?

**Ans1.***i.e.*, m ** x** (dv/dt)

**v =P [as F = ma = m**

*x***(dv/dt)]**

*x*After rearranging and integrating on both sides

ò

**dt**

*x*(v

^{2}/2) = (P/m)

**t + C**

*x*_{1}

Now as initially the

**body**is at

**rest**,

*i.e.*, v = 0 at t = 0, so C

_{1}= 0.

v = (2Pt/m)

^{1/2}…………(1)

(b) By definition v = (ds/dt),

Using eq (1) above,

ds/dt = (2Pt/m)

^{1/2}

On integration we get

ò

^{1/2 }dt

s = (2P/m)

^{1/2}

**(2/3)**

*x***t**

*x*^{3/2}+ C

_{2}.

Now, as at t = 0, s = 0, so, C

_{2}= 0

s = (8P/9m)

^{1/2}t

^{3/2}.

(c)

height=104src=”./xi%20work,%20power%20and%20energy_files/image004.gif”alt=”5a1.gif (1325 bytes)” v:shapes=”_x0000_i1046″>

(d)

height=102src=”./xi%20work,%20power%20and%20energy_files/image005.gif”alt=”5a1i.gif (1265 bytes)” v:shapes=”_x0000_i1047″>

**Q2.****mass** m and lengthl swings back and forth up to a maximum angle q_{0} with the vertical. When at an angle q, what is its (a) potential **energy**, (b) kinetic **energy**, (c) **speed**, and(d) tension?

**Ans2.**

height=149src=”./xi%20work,%20power%20and%20energy_files/image006.gif”alt=”5a2.gif (2041 bytes)” v:shapes=”_x0000_i1048″>

Taking the reference level at the lowest point R, we have

h_{P} = l – l cos q_{0}_{0}

h_{Q} = l – l cos q = l(1 – cos

So (a) potential **energy** at Q relative to R will be

PE = mgh_{Q}

PE = mgl(1 – cos q

(b) PE at P = mgh_{P} = mgl(1 – cos q_{0})

KE at P = 1/2 ** x** mv

^{2}= 0

so, total mechanical

**energy**at P = mgl(1 – cos q

_{0}) …….(

*i*)

Now, if K

_{Q}is the KE at Q,

then using eq. (

*i*)

mechanical

**energy**at Q = K

_{Q}+ mgl(1 – cos

*ii*)

But by conservation of mechanical

**energy**between P and Q

K

_{Q}+ mgl(1 – cos q) = mgl(1 -cos q

_{0}

*i.e.*, K

_{Q}= mgl(cos q– cos q

_{0}

(c) If v is the

**speed**at

**point**Q, from eq. (b)

1/2

**mv**

*x*^{2}= mgl(cos

_{0})

*i.e.*, v = height=17src=”./xi%20work,%20power%20and%20energy_files/image007.gif”alt=”i2.gif (1049 bytes)” v:shapes=”_x0000_i1049″>.

(d) If ‘E’ is the

**energy**at Ðq, then itis equal to mgl(1 – Cosq

^{2}.

Since the

**energy**remains constant throughout, E = E

_{o}.

mgl(1 – Cosq

^{2}= mgl(1 -Cosq

_{o}

or mv

^{2}= 2mgl(Cosq – Cos

_{o}

Therefore, tension ‘T’ at q would begiven by

T = mv

^{2}/l + mgCosq = mg Cos

_{o}

or T = 3mgCosq

_{o}

**Q3.**What do youmean by **work** in the language of physics? Give its absolute and gravitationalunits. Give two illustrations of zero **work**, negative **work** and positive **work**.

** Ans.(Try yourself)**.

**Q4.**How will youfind **work** done by a variable **force** mathematically and graphically?

** Ans.(Try yourself)**.

**Q5.**What do youmean by conservative and non-conservative forces? Give their importantproperties.

** Ans.(Try yourself)**.

**Q6.**What do youmean by gravitational potential **energy**? Show that gravitational potentialenergy is independent of the path followed.

** Ans.(Try yourself)**.

**Q7.**If a **body** iskept on the top of a rough inclined plane, find the expression for

(i) work done in bringing it down to the bottom of the plane with constantvelocity

(ii) **work** done in moving it up the plane with constant acceleration

(iii) **work** done in moving it down the plane with constant acceleration.

** Ans.(Try yourself)**.

## HYSTERISIS

Namrata Asked:

What is hysteresis? I only know its related to the frequency.

Ans:

Hysterisis mens “Lagging Behind”

Hysteresis phenomena occur in magnetic materials, (as well as in the elastic, electric, and magnetic behavior of materials,) in which a lag occurs between the application and the removal of a field (or force) and its subsequent effect

The lack of retraceability of the magnetization curve is the property called hysteresis

#### Magnetic hysteresis

When an external magnetic field is applied to a ferromagnetic substance, the atomic dipoles align themselves with the external field. Even when the external field is removed, part of the alignment will be retained: the material has become magnetized.

The relationship between magnetic field strength (H)(EXTERNAL) and magnetic flux density (B) (ACQUIRED) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation.

If the magnetic field is now reduced linearly, the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or remanence (**RETENTIVITY**).

If this relationship is plotted for all strengths of applied magnetic field the result is a sort of S- shaped loop. (See the diagram) The ‘thickness’ of the middle bit of the S describes the amount of hysteresis, related to the **coercivity** of the material.

## Speed of Light

Saafa asked:

“what is the speed of light?”

**Ans**:

Speed of Light in Vacuum is 3 x 10^{8} m/s

The various experiments conducted to determine the speed of light are

- [wiki]Foucault[/wiki]’s method
- [wiki]Fizeau[/wiki]’s method
- [wiki]Michelson[/wiki]’s method
- … and many more