NCERT Grade 11-Work Energy and Power-Answers

NCERT Solutions for Class 11 Physics

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1.    (a) Positive

In the given case, force and displacement are in the same direction. Hence, the sign of work done is positive. In this case, the work is done on the bucket.

(b) Negative

In the given case, the direction of force (vertically downward) and displacement (vertically upward) are opposite to each other. Hence, the sign of work done is negative.

(c) Negative

Since the direction of frictional force is opposite to the direction of motion, the work done by frictional force is negative in this case.

(d) Positive

Here the body is moving on a rough horizontal plane. Frictional force opposes the motion of the body. Therefore, in order to maintain a uniform velocity, a uniform force must be applied to the body. Since the applied force acts in the direction of motion of the body, the work done is positive.

(e) Negative

The resistive force of air acts in the direction opposite to the direction of motion of the pendulum. Hence, the work done is negative in this case.

2.    Mass of the body, m= 2 kg

Applied force, F = 7 N

Coefficient of kinetic friction, µ = 0.1

Initial velocity, u = 0

Time, t = 10 s

The acceleration produced in the body by the applied force is given by Newton’s second law of motion as:

Frictional force is given as:

f = µmg = 0.1 × 2 × 9.8= -1.96 N

he acceleration produced by the frictional force:

Total acceleration of the body:

a = a’ + a”

= 3.5 + (-0.98) = 2.52 m/s2

The distance travelled by the body is given by the equation of motion:

(a) Work done by the applied force, Wa = F × s = 7 × 126 = 882 J

(b) Work done by the frictional force, Wf = F × s = -1.96 × 126 = -247 J

(c) Net force = 7 + (-1.96) = 5.04 N

Work done by the net force, Wnet = 5.04 ×126 = 635 J

(d) From the first equation of motion, final velocity can be calculated as:

v = u + at

= 0 + 2.52 × 10 = 25.2 m/s

Change in kinetic energy = Final Kinetic Energy – Initial Kinetic Energy

3.    (a) x > a; 0

Total energy of a system is given by the relation:

Energy = Potential Energy + Kinetic Energy

Therefore, Kinetic Energy = Energy – Potential Energy

Kinetic energy of a body is a positive quantity. It cannot be negative. Therefore, the particle will not exist in a region where K.E. becomes negative.

In the given case, the potential energy (V0)  of the particle becomes greater than total energy

(E) for x > a. Hence, kinetic energy becomes negative in this region. Therefore, the particle will not exist is this region. The minimum total energy of the particle is zero.

(b) All regions

In the given case, the potential energy (V0) is greater than total energy (E) in all regions. Hence, the particle will not exist in this region.

(c) x > a and x < b; -V1

In the given case, the condition regarding the positivity of K.E. is satisfied only in the region between x > a and x < b.

The minimum potential energy in this case is -V1. Therefore, K.E. = E-(-V1) = E + V1. Therefore, for the positivity of the kinetic energy, the total energy of the particle must be greater than -V1. So, the minimum total energy the particle must have is -V1.

(d) -b/2 < x < a/2; a/2 < x < a/2; -V1

In the given case, the potential energy (V0) of the particle becomes greater than the total energy (E) for -b/2 < x < a/2; a/2 < x < a/2; -V1. Therefore, the particle will not exist in these regions.

The minimum potential energy, in this case, is -V1. Therefore, K.E. = E-(-V1)= E + V1. Therefore, for the positivity of the kinetic energy, the total energy of the particle must be greater than -V1. So, the minimum total energy the particle must have is -V1.

4.    Total energy of the particle, E = 1J

Force constant, k= 0.5 N m-1

Kinetic energy of the particle, 

According to the conservation law:

At the moment of ‘turn back’, velocity (and hence K) becomes zero.

Hence, the particle turns back when it reaches x = ±2 m.

5.    (a) Rocket

The burning of the casing of a rocket in flight (due to friction) results in the reduction of the mass of the rocket.

According to the conservation of energy:

Total Energy (T.E.)= Potential energy (P.E.) + Kinetic energy (K.E.)

The reduction in the rocket’s mass causes a drop in the total energy. Therefore, the heat energy required for the burning is obtained from the rocket.

(b) Gravitational force is a conservative f Since the work done by a conservative force over a closed path is zero, the work done by the gravitational force over every complete orbit of a comet is zero.

(c) When an artificial satellite, orbiting around the earth, moves closer to earth, its potential energy decreases because of the reduction in the heig Since the total energy of the system remains constant, the reduction in P.E. results in an increase in K.E. Hence, the velocity of the satellite increases. However, due to atmospheric friction, the total energy of the satellite decreases by a small amount.

(d) In the second case

Case (i)

Mass, m = 15 kg

Displacement, s = 2 m

Work done,

Where, θ = Angle between force and displacement

= mgs cos θ = 15 × 2 × 9.8 cos 90°

0 (∵ cos 90° = 0)

Case (ii)

Mass, m = 15 kg

Displacement, s = 2 m

Here, the direction of the force applied on the rope and the direction of the displacement of the rope are same.

Therefore, the angle between them, θ = 0°

Since cos 0° = 1

Work done, W = FScosθ = mgs

= 15 × 9.8 × 2 = 294 J

Hence, more work is done in the second case.

6.    (a) Decreases

(b) Kinetic energy

(c) External force

(d) Total linear momentum

Explanation:

(a) A conservative force does a positive work on a body when it displaces the body in the direction of f As a result, the body advances toward the centre of force. It decreases the separation between the two, thereby decreasing the potential energy of the body.

(b) The work done against the direction of friction reduces the velocity of a body. Hence, there is a loss of kinetic energy of the body.

(c) Internal forces, irrespective of their direction, cannot produce any change in the total momentum of a body. Hence, the total momentum of a many- particle system is proportional to the external forces acting on the system.

(d) The total linear momentum always remains conserved whether it is an elastic collision or an inelastic collision.

7.    (a) False

(b) False

(c) False

(d) True

Explanation:

(a) In an elastic collision, the total energy and momentum of both the bodies, and not of each individual body, is conserved.

(b) Although internal forces are balanced, they cause no work to be done on a body. It is the external forces that have the ability to do work. Hence, external forces are able to change the energy of a system.

(c) The work done in the motion of a body over a closed loop is zero for a conservation force only.

(d) In an inelastic collision, the final kinetic energy is always less than  the  initial  kinetic energy of the This is because in such collisions, there is always a loss of energy in the form of heat, sound, etc.

8.    (a) No

In an elastic collision, the total initial kinetic energy of the balls will be equal to the total final kinetic energy of the balls. This kinetic energy is not conserved at the instant the two balls are in contact with each other. In fact, at the time of the collision, the kinetic energy of the balls will get converted into potential energy.

(b) Yes

In an elastic collision, the total linear momentum of the system always remains conserved.

(c) No; Yes

In an inelastic collision, there is always a loss of kinetic energy, i.e., the total kinetic energy of the billiard balls before collision will always be greater than the total kinetic energy after the collision.

The total linear momentum of the system of billiards balls will remain conserved even in the case of an inelastic collision.

(d) Elastic

In the given case, the forces involved are conservation. This is because they depend on the separation between the centres of the billiard balls. Hence, the collision is elastic.

9.     (ii) t

Mass of the body = m

Acceleration of the body = a

Using Newton’s second law of motion, the force experienced by the body is given by the equation:

F = ma

Both m and a are constants. Hence, force F will also be a constant.

F = ma = Constant … (i)

For velocity v, acceleration is given as,

Where, α is another constant

v ∞ t …(iii)

Power is given by the relation:

P = F.v

Using equations (i) and (iii), we have:

P ∞ t Hence, power is directly proportional to time.

10.   (iii) 

Power is given by the relation:

Integrating both sides:

For displacement ‘x’ of the body, we have:

Where New constant On integrating both sides, we get:

11.   Force exerted on the body, 

Displacement, s = 

Work done, W = F.S

 

Hence, 12 J of work done by the force on the body.

12.   Electronics faster; Ratio of speeds is 13.54: 1

Mass of the electron, me = 9.11 × 10-31 kg

Mass of the proton, me = 9.11 × 10-27 kg

Kinetic energy of the electron, EKe = 10 keV = 104eV

= 104 × 160 × 10-19

= 160 × 10-15 J

Kinetic energy of the proton, EKe = 100 keV = 105eV = 1.60 × 10-14 J

For the velocity of an electron ve, its kinetic energy is given by the relation:

For the velocity of a proton VP, its kinetic energy is given by the relation:

Hence, the electron is moving faster than the proton. The ratio of their speeds:

13.   Radius of the rain drop, r = 2 mm = 2 × 10-3 m

Volume of the rain drop, 

Density of water, p = 103 kg m-3

Mass of the rain drop, m = pV

Gravitational force, F = mg

The work done by the gravitational force on the drop in the first half of its journey:

W1 = Fs

= 0.082 J

This amount of work is equal to the work done by the gravitational force on the drop in the second half of its journey, i.e., WII = 0.082 J

As per the law of conservation of energy, if no resistive force is present, then the total energy of the raindrop will remain the same.

∴ Total energy at the top:

Due to the presence of a resistive force, the drop hits the ground with a velocity of 10 m/s.

∴ Total energy at the ground:

14.    Yes; Collision is elastic

The momentum of the gas molecule remains conserved whether the collision is elastic or inelastic.

The gas molecule moves with a velocity of 200 m/s and strikes the stationary wall of the container, rebounding with the same speed.

It shows that the rebound velocity of the wall remains zero. Hence, the total kinetic energy of the molecule remains conserved during the collision. The given collision is an example of an elastic collision.

15.   Volume of the tank, V = 30 m3

Time of operation, t = 15 min = 15 × 60 = 900 s

Height of the tank, h = 40 m

Efficiency of the pump, = 30%

Density of water, p = 103 kg/m3

Mass of water, m = pV = 30 × 103 kg

Output power can be obtained as:

For input power Pi, efficiency η, is given by the relation:

16.   Case (ii)

It can be observed that the total momentum before and after collision in each case is constant.

For an elastic collision, the total kinetic energy of a system remains conserved before and after collision.

For mass of each ball bearing m, we can write:

Total kinetic energy of the system before collision:

Case(i)

Total kinetic energy of the system after collision:

Hence, the kinetic energy of the system is not conserved in case (i).

Case(ii)

Total kinetic energy of the system after collision:

Hence, the kinetic energy of the system is conserved in case (ii).

Case(iii)

Total kinetic energy of the system after collision:

17.   Bob A will not rise at all

In an elastic collision between two equal masses in which one is stationary, while the other is moving with some velocity, the stationary mass acquires the same velocity, while the moving mass immediately comes to rest after collision. In this case, a complete transfer of momentum takes place from the moving mass to the stationary mass.

Hence, bob ‘A’ of mass m, after colliding with bob ‘B’ of equal mass, will come to rest, while bob B will move with the velocity of bob A at the instant of collision.

18.   Length of the pendulum, l = 1.5 m

Mass of the bob = m

Energy dissipated = 5%

According to the law of conservation of energy, the total energy of the system remains constant.

At the horizontal position:

Potential energy of the bob, EP= mgl

Kinetic energy of the bob, EK = 0

Total energy = mgl … (i)

At the lowermost point (mean position):

Potential energy of the bob, EP = 0

Kinetic energy of the bob, 

Total energy  … (ii)

As the bob moves from the horizontal position to the lowermost point, 5% of its energy gets dissipated.

The total energy at the lowermost point is equal to 95% of the total energy at the horizontal point, i.e.,

19.   The sand bag is placed on a trolley that is moving with a uniform speed of 27 km/h. The external forces acting on the system of the sandbag and the trolley is zero. When the sand starts leaking from the bag, there will be no change in the velocity of the trolley. This is because the leaking action does not produce any external force on the system. This is in accordance with Newton’s first law of motion. Hence, the speed of the trolley will remain 27 km/h.

20.   Mass of the body, m = 0.5 kg

Velocity of the body is governed by the equation, 

Initial velocity, u (at x = 0) = 0

Final velocity v (at x = 2 m) = 10√2m/s

Work done, W = Change in kinetic energy

21.   Area of the circle swept by the windmill = A

Velocity of the wind = v

Density of air = p

Volume of the wind flowing through the windmill per sec = Av

Mass of the wind flowing through the windmill per sec = pAv

Mass m, of the wind flowing through the windmill in time t = pAvt

(b) Kinetic energy of air = 1/2 mv2

(c) Area of the circle swept by the windmill = A = 30 m2

Velocity of the wind = v= 36 km/h

Density of air, p = 1.2 kg m-3

Electric energy produced = 25% of the wind energy

22.   (a) Mass of the weight, m = 10 kg

Height to which the person lifts the weight, h = 0.5 m

Number of times the weight is lifted, n = 1000

∴ Work done against gravitational force:

(b) Energy equivalent of 1 kg of fat = 3.8 × 107 J

Efficiency rate = 20%

Mechanical energy supplied by the person’s body:

Equivalent mass of fat lost by the dieter:

23.   (a) 200 m2

(a) Power used by the family, P = 8 kW = 8 × 103 W

Solar energy received per square metre = 200 W

Efficiency of conversion from solar to electricity energy = 20 %

Area required to generate the desired electricity = A

As per the information given in the question, we have:

(b) The area of a solar plate required to generate 8 kW of electricity is almost equivalent to the area of the roof of a building having dimensions 14 m × 14 m.

24.   Mass of the bullet, m= 0.012 kg

Initial speed of the bullet, ub = 70 m/s

Mass of the wooden block, M = 0.4 kg

Initial speed of the wooden block, uB = 0

Final speed of the system of the bullet and the block = v

Applying the law of conservation of momentum:

For the system of the bullet and the wooden block:

Mass of the system, m’ = 0.412 kg

Velocity of the system = 2.04 m/s

Height up to which the system rises = h

Applying the law of conservation of energy to this system:

Potential energy at the highest point = Kinetic energy at the lowest point

The wooden block will rise to a height of 0.2123 m.

Heat produced = Kinetic energy of the bullet – Kinetic energy of the system

25.   No; the stone moving down the steep plane will reach the bottom first

Yes; the stones will reach the bottom with the same speed

vB = vC = 14 m/s

t= 2.86 s; t2 = 1.65 s

The given situation can be shown as in the following figure:

Here, the initial height (AD) for both the stones is the same (h). Hence, both will have the same potential energy at point A.

As per the law of conservation of energy, the kinetic energy of the stones at points B and C will also be the same, i.e.,

Where,

m = Mass of each stone

v = Speed of each stone at points B and C

Hence, both stones will reach the bottom with the same speed, v.

For stone I:

Net force acting on this stone is given by:

Fnet = ma1 = mg sinθ1

a1 = g sin θ1

For stone II:

a2 = g sin θ2

∵ θ2 > θ1

∴ a2 > a1

Using the first equation of motion, the time of slide can be obtained as:

Hence, the stone moving down the steep plane will reach the bottom first.

The speed (v) of each stone at points B and C is given by the relation obtained from the law of conversation of energy.

26.   Mass of the block, m= 1 kg

Spring constant, k = 100 N m-1

Displacement in the block, x = 10 cm = 0.1 m

The given situation can be shown as in the following figure.

At equilibrium:

Normal reaction, R = mg cos 37°

Frictional force, f = µ R = mg sin 37°

Where, µ is the coefficient of friction

Net force acting on the block = mg sin 37° – f

= mg sin 37° – µmgcos 37°

= mg (sin 37° – µcos 37°)

At equilibrium, the work done by the block is equal to the potential energy of the spring, i.e.,

27.   Mass pf the bolt, m = 0.3 kg

Speed of the elevator = 7 m/s Height, h = 3 m

Since the relative velocity of the bolt with respect to the lift is zero, at the time of impact, potential energy gets converted into heat energy.

Heat produced = Loss of potential energy

= mgh = 0.3 × 9.8 × 3 = 8.82 J

The heat produced will remain the same even if the lift is stationary. This is because of the fact that the relative velocity of the bolt with respect to the lift will remain zero.

28.   Mass of the trolley, M = 200 kg

Speed of the trolley, v = 36 km/h = 10 m/s

Mass of the boy, m = 20 kg

Initial momentum of the system of the boy and the trolley

= (M+ m)v

= (200 + 20) × 10

= 2200 kg m/s

Let v’ be the final velocity of the trolley with respect to the ground.

Final velocity of the boy with respect to the ground = v’ – 4

Final momentum = Mv’ + m (v’ – 4)

= 200v’ + 20v’ – 80

= 220v’ – 80

As per the law of conservation of momentum:

Initial momentum = Final momentum

2200 = 220v’ – 80

Length of the trolley, l = 10 m

Speed of the boy, v” = 4 m/s

Time taken by the boy to run, 

∴ Distance moved by the trolley = v” × t = 10.36 × 2.5 = 25.9 m.

29.   (i), (ii), (iii), (iv), and (vi)

The potential energy of a system of two masses is inversely proportional to the separation between them. In the given case, the potential energy of the system of the two balls will decrease as they come closer to each other. It will become zero (i.e., V(r) = 0) when the two balls touch each other, i.e., at r = 2R, where R is the radius of each billiard ball. The potential energy curves given in figures (i), (ii), (iii), (iv), and (vi) do not satisfy these two conditions.

Hence, they do not describe the elastic collisions between them.

30.   The decay process of free neutron at rest is given as: n → p + e

From Einstein’s mass-energy relation, we have the energy of electron as Δmc2

Where,

Δm = Mass defect = Mass of neutron-(Mass of proton + Mass of electron)

c = Speed of light

Δm and c are constants. Hence, the given two-body decay is unable to explain the continuous energy distribution in the β- decay of a neutron or a nucleus. The presence of neutrino νon the LHS of the decay correctly explains the continuous energy distribution.

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