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## 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)**.

## Conservation of energy Problem

A **ball** is dropped from a **height** of **10m**. If the kinetic **energy** of the **ball** reduces by 40% after striking the **ground**, how high can the **ball** bounce back? (**Elsy James** asked)

## Motion under gravity

Two stones of mass 5 kg and 10 kg are dropped from the same height above the ground level. Which one will hit the ground first? Why?

Answer: Both the stones will reach the ground simultaneously (provided we neglect the air resistance (the viscosity) and buoyant force.

This is because acceleration due to gravity is independent of mass of the **freely falling** body.

## A few numerical problems from laws of motion

- A worker pulls a 200. N packing crate with an applied force of 55.0 N. The crate accelerates at a rate of 0.250 m/s(squared). What is the coefficient of kinetic friction between the crate and the factory floor?
- A student pulls a 150. N sled up a 28 degree slope at a constant speed by applying a force of 100. N. Near the top of the hill he releases the sled. With what acceleration does the sled go down the hill?
- A 200. N crate rests on a ramp; the maximum angle just before it slips is 25 degrees with the horizontal. What is the coefficient of static friction between the crate and the surface of the ramp?
- A man pulls a sled with a weight of 200. N with a constant velocity across a horizontal snow surface. If a force of 80N is being applied to the sled rope at an angle of 53 degrees to the ground, what is the coefficient of friction between the sled and the ground?
- A jet plane is flying with a constant speed along a straight line at an angle of 30degree above the horizontal. The weight of plane is 86 500N. Its engine provides a forward thrust T of 103 000N. The lift of force L(directed perpendicular to the wings)and the force R of air resistance (directed opposite to the motion)act on the plane. Find L & R

## An interesting numerical problem from kinematics

George’s favorite food was bananas. One day he was walking with his friend, the man in the yellow hat, as he was eating a banana. They were in New York City and they were walking up the Empire State Building. Finally they made it to the top! George spied a nice young woman walking down 5th avenue balancing this hat on her head. She was very noticeable for she was six feet tall! George, being the troublemaker he was, wanted to see if he could drop his banana peel in her hat. He noticed that she was jogging at a constant pace of 6 miles per hour. George was on the 102nd floor observatory, which is 1,224 feet from the ground.*

Assuming that George lands the banana peel in the hat……..

A) How long will the banana peel be in the air? (Assuming there is no wind and the banana will drop straight down from the empire state building)

B) What will the velocity of the banana peel be right before it lands in the woman’s hat?

C) How far away must the woman’s hat be from the landing point of the banana peel (horizontally) when George makes his scandalous move and drops the peel.

D) If George decides to throw another peel one second later to land it on the ground and trip the jogger, at what velocity must he throw it at so that it hits the ground at the same the original peel lands in the hat.

## Kinematics Numerical

- A particle travels 20 m in 7th second and 24m in 9th sec. find initial velocity?
- in a projectile motion , a body thrown from the ground ,at what angle both the vertical height and range will be equal?
- The velocity v(cm/s) of a particle is given in terms of time t(in seconds) by the equation v = at+b/t+c. Dimension of a,b, and c are ?

## Horizontal projectile and freely falling body

A rifle at a height H aimed horizontally fires a bullet to the ground. At the same time , a bullet with the same mass in dropped from the same height. Neglecting air resistance, which one hits the ground first?Explain.

ED posted

**Answer**:

Both will hit the ground simultaneously.

When a body is projected horizontally, its initial vertical velocity is zero and vertical acceleration is g, the acceleration due to gravity.

The values of a velocity and acceleration of a freely falling body are also the same.

So, both will hit the ground simultaneously

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