Part III Class XI Physics Chapter 7 SYSTEMS OF PARTICLES AND ROTATIONAL MOTION

Part – III Class –XI Physics

Chapter – 7

SYSTEMS OF PARTICLES AND ROTATIONAL MOTION

Answer 22: The given situation can be shown as:

NB = Force exerted on the ladder by the floor point B NC = Force exerted on the ladder by the floor point C T = Tension in the rope

BA = CA = 1.6 m DE = 0. 5 m BF = 1.2 m

Mass of the weight, m = 40 kg

Draw a perpendicular from A on the floor BC. This intersects DE at mid-point H. ΔABI and ΔAIC are similar

∴BI = IC

Hence, I is the mid-point of BC. DE || BC

BC = 2 × DE = 1 m

AF = BA – BF = 0.4 m … (i) D is the mid-point of AB. Hence, we can write:

1

0.8 2

AD= ×BA= m ………….(ii) Using equations (i) and (ii), we get: FE = 0.4 m

Hence, F is the mid-point of AD.

FG||DH and F is the mid-point of AD. Hence, G will also be the mid-point of AH. ΔAFG and ΔADH are similar

FG AF DH AD ∴ = 0.4 1 0.8 2 FG DH = = 1 2 FG= DH 1 0.25 0.125 2 = × = In ΔADH: 2 2 AH = AD −DH

( ) (

2

)

2 0.8 0.25 0.76 m = − =

For translational equilibrium of the ladder, the upward force should be equal to the downward force.

c B

N +N = mg = 392 … (iii)

For rotational equilibrium of the ladder, the net moment about A is: 0 B c N BI mg FG N CI T AG T AG − × + × + × + × − × =

( )

0.5 40 9.8 0.125 0.5 0 B c N N − × + × × + × =

(

N_c−N_B

)

×0.5=49 98 c B N −N =

Adding equations (iii) and (iv), we get: 245 c N = N 147 B N = N

For rotational equilibrium of the side AB, consider the moment about A. 0 B N BI mg FG T AG − × + × + × = 245 0.5 40 9.8 0.125 T 0.76 0 − × + + × + × = 0.76T=122.5 49− 96.7 T N ∴ =

Answer 23: (a) 58.88 rev/min (b) No

(a)Moment of inertia of the man-platform system = 7.6 kg 2

m

Moment of inertia when the man stretches his hands to a distance of 90 cm: 2

2 mr×

= 2 5× ×

( )

0.9 2 = 8.1 kg m2

Initial moment of inertia of the system,I_i =7.6 8.1 15.7+ = kg m2 Angular speed,

ω

1=300rev/ min

Angular momentum, L_i =I_i

ω

_i =15.7 30× …………(i)

Moment of inertia when the man folds his hands to a distance of 20 cm: 2

2 mr×

= 2 5 0.2×

( )

2= 0.4 kg m2

Final moment of inertia, 2 7.6 0.4 8

f

I = + = kg m

Final angular speed = ω_f

Final angular momentum, L_f =I_fω_f =0.79ω_f … (ii) From the conservation of angular momentum, we have:

i i f f Iω =I ω 15.7 30 58.88 / min 8 f rev ω × ∴ = =

(b) Kinetic energy is not conserved in the given process. In fact, with the decrease in the moment of inertia, kinetic energy increases. The additional kinetic energy comes from the work done by the man to fold his hands toward himself.

Answer 24: Mass of the bullet, m= 10 g = 10 10 kg× –3 Velocity of the bullet, v= 500 m/s

Radius of the door, 1 2

r= m

Mass of the door, M= 12 kg

Angular momentum imparted by the bullet on the door: α =mvr

(

3

)

( )

1 2 1 10 10 500 2.5 2 kg m s − − = × × × = …….(i)

Moment of inertia of the door: 2 1 3 I= ML =1

( )

2 2 12 1 4 3× × = kg m But a=I

ω

a I ω ∴ = 1 2.5 0.625 4 rad s − = = Answer 25: (a)

Moment of inertia of disc I=I₁

Angular speed of disc I=

ω

1 Angular speed of disc II =I2 Angular momentum of disc II =

ω

1 Angular momentum of discI L, ₁=I₁

ω

₁

Angular momentum of disc II L, ₂=I₂

ω

₂

Total initial angular momentum, L₁=L₁

ω

₁+I₂

ω

₂

When the two discs are joined together, their moments of inertia get added up. Moment of inertia of the system of two discs, I = +I1 I2

Let ω be the angular speed of the system. Total final angular momentum, Lf =

(

I1+I2

)

ω

Using the law of conservation of angular momentum, we have: 1 f L =L

(

)

1 1 2 2 1 2 Iω +Iω = I +I ω 1 1 2 2 1 2 I I I I ω ω ω + ∴ = +

(b)Kinetic energy of disc I, 1 1 12 1 2

E = Iω

Kinetic energy of disc II, 2 1 12 1 2

E = Iω

Total initial kinetic energy,

(

2 2

)

1 1 2 2 1

2

i

E = Iω +Iω

When the discs are joined, their moments of inertia get added up. Moment of inertia of the system, I= +I₁ I₂

Final kinetic energyEf:

(

)

2 1 2 1 2 I I ω = +

(

)

1 1 2 2

(

1 1 2 2

)

1 2 1 2 1 2 1 1 2 2 I I I I I I I I I I ω ω ω ω +  ₊  = +  = + +   i f E E ∴ −

(

1 2

)

(

₍

₎

)

2 1 1 2 2 2 2 1 2 1 2 1 2 2 I I I I I I ω ω ω ω + = + − +

(

)

(

)

(

)

1 2 2 2 2 2 2 2 1 2 2 2 1 2 1 2 1 2 1 2 1 2 1 2 2 1 1 1 1 1 2 2 2 2 2 I I I I I I I I I I I I ω ω ω ω ω ω = + − − − + + +

(

)

1 2 2 2 2 2 2 2 2 2 2 2 1 1 2 1 1 2 2 1 1 2 2 1 2 1 2 1 2 1 1 1 1 1 1 1 2I 2I I 2I I 2I 2I 2I I I I I ω ω ω ω ω ω ω ω   = _ + + + − − − _ +  

(

1 2

)

12 22 1 2 1 2 2 2 I I I I ω ω ω ω   = _ + − _ +

(

)

2 1 2 1 2 1 2 ( ) 2 I I I I ω ω− = +

All the quantities on RHS are positive. 0 i f E E ∴ − > i f E −E

The loss of KE can be attributed to the frictional force that comes into play when the two discs come in contact with each other.

Answer 26:

(a)The theorem of perpendicular axes states that the moment of inertia of a planar body (lamina) about an axis perpendicular to its plane is equal to the sum of its moments of inertia about two perpendicular axes concurrent with perpendicular axis and lying in the plane of the body.

A physical body with centre O and a point mass m, in the x–y plane at (x, y) is shown in the following figure.

Moment of inertia about x-axis, Ix= mx2

Moment of inertia about y-axis, Iy= my2

Moment of inertia about z-axis, Iz=

(

)

2 2 2 m x +y 2 2 x y I + =I mx +my = m

(

2 2

)

x +y

=

(

)

2 2 2 m x +y x y z I + =I I

Hence, the theorem is proved.

(b)The theorem of parallel axes states that the moment of inertia of a body about any axis is equal to the sum of the moment of inertia of the body about a parallel axis passing through its centre of mass and the product of its mass and the square of the distance between the two parallel axes.

Suppose a rigid body is made up of n particles, having massesm m m₁, ₂, ₃, … ,m_n, at perpendicular distances r r r₁, , , _{2 3} … ,r_nrespectively from the centre of mass O of the rigid body.

The moment of inertia about axis RS passing through the point O: IRS= 1 12

n fmI

m r

∑

The perpendicular distance of mass mi, from the axis QP = a+ ri Hence, the moment of inertia about axis QP:

(

)

2 1 n QP i i i I m a r = =

∑

+

(

)

2 2 2 1 2 i n i i i m a r ar = =

∑

+ + 2 1 1 1 2 n n n i i i i i i i i m m r m ar = = = =

∑

+

∑

+

∑

2 2 1 1 2 n n RS i i i i i I m a m ar = = = +

∑

+

∑

Now, at the centre of mass, the moment of inertia of all the particles about the axis passing through the centre of mass is zero, that is,

1 2 0 n i i i m ar = =

∑

0 a ∴ ≠ 0 i i m r ∴

∑

= Also, ; n i i I m M = =

∑

M=Total mass of the rigid body 2

QP RS

I I Ma

∴ = +

Hence, the theorem is proved.

m= Mass of the body R = Radius of the body

K= Radius of gyration of the body v = Translational velocity of the body h =Height of the inclined plane g = Acceleration due to gravity

Total energy at the top of the plane, E1=mgh

Total energy at the bottom of the plane,Eb =KErot +KEtrans

2 2 1 1 2Iω 2mv = + But_{I mk and}2 v R ω = =

( )

2 2 2 2 1 1 2 2 b v E mk mv R   ∴ =  +   2 2 2 2 1 1 2 2 k mv mv k = + 2 20 2 1 1 2 k mv R   =  +   

From the law of conservation of energy, we have:

T b E =E 2 2 2 1 1 2 k mgh mv R   =  +   

(

2 2

)

2 1 / gh v k R ∴ = +

Hence, the given result is proved. Answer 28: v_A= R

ω

_°; vB= R

ω

°;

2

c

R v =_ _ω_°

  ; The disc will not roll Angular speed of the disc =

ω

_°

Radius of the disc = R

Using the relation for linear velocity, v =

ω

_°R For point A:

A

v = R

ω

_°; in the direction tangential to the right For point B:

B

v = R

ω

_°; in the direction tangential to the left For point C:

2

c

R v = ω_°

  ;in the direction same as that of vA

Since the disc is placed on a frictionless table, it will not roll. This is because the presence of friction is essential for the rolling of a body.

Answer 29: A torque is required to roll the given disc. As per the definition of torque, the rotating force should be tangential to the disc. Since the frictional force at point B is along the tangential force at point A, a frictional force is required for making the disc roll.

(a) Force of friction acts opposite to the direction of velocity at point B. The direction of linear velocity at point B is tangentially leftward. Hence, frictional force will act tangentially rightward. The sense of frictional torque before the start of perfect rolling is perpendicular to the plane of the disc in the outward direction.

(b) Since frictional force acts opposite to the direction of velocity at point B, perfect rolling will begin when the velocity at that point becomes equal to zero. This will make the frictional force acting on the disc zero.

Answer 30: Disc

Radii of the ring and the disc, r = 10 cm = 0.1 m Initial angular speed,

ω

_° =10 π rad s−1

Coefficient of kinetic friction,

µ

k= 0.2

Initial velocity of both the objects, u = 0

Motion of the two objects is caused by frictional force. As per Newton's second law of motion, we have frictional force, f = ma

µ

kmg= ma

Where,

a = Acceleration produced in the objects m = Mass

∴a =

µ

kg … (i)

As per the first equation of motion, the final velocity of the objects can be obtained as: v = u + at

= 0 +

µ

kgt

=

µ

kgt … (ii)

The torque applied by the frictional force will act in perpendicularly outward direction and cause reduction in the initial angular speed.

Torque, T= –Iα α = Angular acceleration xmgr

µ

=–Iα kmgr a I µ − ∴ = ……….(iii)

Using the first equation of rotational motion to obtain the final angular speed:

at

ω ω

= +° kmgr_t I µ ω_° − = + ………(iv)

0 kgmrt v r I µ ω   ∴ =  −    ………(v)

Equating equations (ii) and (v), we get: 0 k k gmrt gt r I µ µ = ω −    2 0 kgmr t r I

µ

ω

= − …….(vi) 2 : For the ring I=mr

2 0 2 k k gmr t gt r mr

µ

µ

ω

∴ = − 0 k r r

ω µ

gmt = − 0 2

µ

_kgt=r

ω

0 2 r k r t g ω µ ∴ = 0.1 10 3.14 0.80 2 0.2 9.8 s × × = = × × ………….(vii) 2 1 : 2

For the ring I = mr

2 0 2 1 2 k k d gmr t gt r mr µ µ ω ∴ = − 0 2 k r

ω

µ

gt = − 0 3

µ

_kgt_d =r

ω

0 3 d k r t g ω µ ∴ = 0.1 10 3.14 0.53 3 0.2 9.8 s × × = = × × ………(viii)

Sincet_d >t_r, the disc will start rolling before the ring. Answer 31: Mass of the cylinder, m= 10 kg

Radius of the cylinder, r= 15 cm = 0.15 m Co-efficient of kinetic friction, µk= 0.25

Angle of inclination, θ= 30°

Moment of inertia of a solid cylinder about its geometric axis, 1 2 2

I= mr

The various forces acting on the cylinder are shown in the following figure:

The acceleration of the cylinder is given as:

2 sin 1 mg a m r θ = +

2 2 2 sin 2 sin 30 1 3 mg g mr m r r θ ° = = + 2 2 9.8 0.5 3.27 / 3 m s = × × =

(a) Using Newton's second law of motion, we can write net force as:

net f = ma sin 30 mg °− =f ma sin 30 f =mg °−ma 10 9.8 0.5 10 3.27 = × × − × 49 32.7 16.3 N = − =

(b) During rolling, the instantaneous point of contact with the plane comes to rest. Hence, the work done against frictional force is zero.

(c) For rolling without skid, we have the relation: 1 tan 3 µ= θ tan

θ

=3

µ

= ×3 0.25

(

)

1 tan 0.75 36.87 θ − ° ∴ = =

Answer 32: (a) False

Frictional force acts opposite to the direction of motion of the centre of mass of a body. In the case of rolling, the direction of motion of the centre of mass is backward. Hence, frictional force acts in the forward direction.

(b) True

Rolling can be considered as the rotation of a body about an axis passing through the point of contact of the body with the ground. Hence, its instantaneous speed is zero. (c) False

When a body is rolling, its instantaneous acceleration is not equal to zero. It has some value.

(d) True

When perfect rolling begins, the frictional force acting at the lowermost point becomes zero. Hence, the work done against friction is also zero.

(e) True

The rolling of a body occurs when a frictional force acts between the body and the

surface. This frictional force provides the torque necessary for rolling. In the absence of a frictional force, the body slips from the inclined plane under the effect of its own weight. Answer 33: (a)Take a system of i moving particles.

Mass of the ith particle = mi

Velocity of the ith particle = v_i

Hence, momentum of the ith particle, p_i=m_iv_i

Velocity of the centre of mass = V

The velocity of the ith particle with respect to the centre of mass of the system is given as: 'i= i–

v v V… (1)

Multiplying mithroughout equation (1), we get:

' –

i i i i i

mv =mv mV

'_i = _i –m_i

Where, ' '

i =mi i

p v

= Momentum of the ith particle with respect to the centre of mass of the system '

i i mi

∴ =p p + V

We have the relation: p'i =mivi'

Taking the summation of momentum of all the particles with respect to the centre of mass of the system, we get:

i i i i i i i i dr p m v m dt ′ ′= ′=

∑

∑

∑

Where, f

r′= Position vector of its particle with respect to the centre of mass

i i dr v dt ′ ′ =

As per the definition of the centre of mass, we have: 0 i i f m r′=

∑

' 0 i f f dr m dt ∴

∑

= 0 f f P′ +

∑

(b) We have the relation for velocity of the particle as: ' i= i+ v v V l f l i l f f f m v = m v′+ m V

∑

∑

∑

… (2)

Taking the dot product of equation (2) with itself, we get:

(

)

(

)

. . l f l f l f l l f f f f m v m v = m v′ +V m v′+V

∑

∑

∑

∑

2 2 2 '2 2 2 2 2 . . i i i i i i i i i i M

∑

v =M

∑

v +M

∑

v v′+M

∑

v v +M V

Here, for the centre of mass of the system of particles, _i. _{i i}. _i

i i v v′= v v′

∑

∑

2 2 2 '2 2 2 i i i i M

∑

v =M

∑

v +M V 2 2 2 1 1 1 2 f l 2 f l 2 M

∑

v = M

∑

v′ + MV 2 1 2 K =K′+ MV Where, K = 1 2 2 f l

M

∑

v = Total kinetic energy of the system of particles K' = 1 2

2 f l

M

∑

v′ = Total kinetic energy of the system of particles with respect to the centre of mass

2 1