NCERT Solutions For Class 9 Science Chapter 11 Work and Energy

Yes. When an object is moving with constant velocity, the net force acting on it is zero. However, there is a displacement along the motion of the object. Therefore, there can be a displacement of an object without any force acting upon it.

On pushing a huge rock, no muscular energy is transferred to the stationary rock. Here, muscular energy is transferred into heat energy which makes the body feel warm. Hence, there is no loss of energy.

In the case of bicycle riding, the muscular energy of the rider changes into heat energy and kinetic energy of the bicycle. Heat energy gives heat to the rider’s body while kinetic energy provides a velocity to the bicycle. In this whole process, the total energy remains conserved.

The statement of Soni is correct. Acceleration in an object may be zero even when several forces are acting on it. This condition occurs when all the forces cancel out each other. For a uniformly moving object, the net force acting on the object is zero. Hence, the acceleration of the object is zero.

When the person holds a bundle of hay over his head, then bundle of hay is not displaced. However, force of gravity is still working on the bundle but the person is not applying any force on it. Thus, work done by the person on the bundle is zero.

\begin{array}{l}\text{Here},\text{ 1}\text{unit}=\text{1}\text{kWh}\\ \text{Also},\text{ 1}\text{kWh}=3.6\times 10^6\text{J}\\ \therefore \text{25}0\text{units}\text{ of}\text{energy}=250\times 3.6\times 10^6\text{J}=9\times 10^8 \text{J}\end{array}

No. When the body falls freely from a height, then its potential energy converts into kinetic energy with time. The decrease in the potential energy is equal to the increase in the kinetic energy of the body. Thus, total mechanical energy of the body remains conserved.

The chemical energy of battery is changed into electrical energy when a bulb is connected between the terminals of a battery. Later on, this electrical energy changes into light and heat energy. Hence, the transformation of energy in the given situation can be shown as:

\text{Chemical}\text{energy}\to \text{Electrical}\text{Energy}\to \text{Light}\text{Energy}\text{+}\text{Heat}\text{Energy}

Work done by the force of gravity on an object depends only on vertical displacement. Vertical displacement can be calculated by the difference in the initial and final heights of the object. Here, vertical displacement is zero. Hence, work done by gravity W = mgh

Here, h = 0

Therefore, W = mg × 0 = 0

In free fall, potential energy of an object decreases and kinetic energy increases. When the object comes in contact with ground, potential energy converts into kinetic energy. As the object hits the hard ground, all its kinetic energy gets converted into heat energy and sound energy. The object can also deform the ground depending upon the nature of the ground and the amount of kinetic energy possessed by the object.

1. In first case, the direction of force acting on the block is perpendicular to the displacement. Thus, work done by force on the block will be zero.

2. In the second case, the direction of force acting on the block is in the direction of displacement. Hence, work done by force on the block will be positive.

3. In the third case, the direction of force acting on the block is opposite to the direction of displacement. Hence, work done by force on the block will be negative.

There are two conditions when the work is said to be done.

(i) A force acts on the body.

(ii) There is a displacement of the body due applied force in or opposite to the direction of force.

If the direction of force is perpendicular to displacement, then the work done is zero.

When a satellite revolves around the Earth, then the direction of force of gravity on the satellite is perpendicular to its displacement. Thus, the work done on the satellite by the Earth is zero.

Work done by gravity depends only on the vertical displacement of the body. The expression for work done by gravity is given by,

W = mgh

Here, vertical displacement (h) is zero.

Hence,        W = mg × 0 = 0

Thus, work done by gravity on the body is zero.

According to  law of conservation of energy, energy can be neither created nor destroyed. It can only be converted from one form to another.

When a pendulum oscillates from its mean position O to either of its extreme positions P or Q, it rises through a height h above the mean level P. At this point, the kinetic energy of the bob changes completely into potential energy. The kinetic energy becomes zero, and the bob possesses only potential energy. As it goes towards point O, its potential energy decreases progressively. Accordingly, the kinetic energy increases. As the bob reaches point O, its potential energy becomes zero and the bob possesses only kinetic energy. This process is repeated as long as the pendulum oscillates.

After some time, bob comes to rest because air resists its motion. The pendulum loses its kinetic energy to overcome this friction and stops after some time.

Here, the law of conservation of energy is not violated because the energy lost by the pendulum to overcome friction is gained by its surroundings. Thus, the total energy of the pendulum and the surrounding system remain conserved.

$\begin{array}{l}\text{Kinetic energy }\mathrm{E}\text{ = }\frac{1}{2}{\mathrm{mv}}^2\\ \mathrm{Where},\text{ mass, m = 20 kg, v = velocity}\\ {\text{(i) Kinetic energy E}}_{\text{1}}{\text{, when velocity is 5 ms}}^{\text{-1}}\\ {\text{ K}}_{\text{1}}\text{ = }\frac{1}{2}\left(20\mathrm{kg}\right){\left(5{\mathrm{ms}}^{-1}\right)}^2=250\text{ J}\\ {\text{(ii) Kinetic energy (K}}_2{\text{) when velocity is 2 ms}}^{\text{-1}}\\ {\text{ K}}_2=\frac{1}{2}\left(20\mathrm{kg}\right){\left(2{\mathrm{ms}}^{-1}\right)}^2=\text{ 40 J}\\ \text{Work done is equal to the change in the kinetic energy}\\ {\text{Hence, W = K}}_2-{\text{K}}_{\text{1}}=40-250=\text{ -210 J}\end{array}$

• While swimming, Suma exerts a force on the water in backward direction. Hence, she swims in the forward direction caused by the forward reaction of water. Thus, the force causes a displacement. Hence, Suma does the work while swimming.

• When the donkey carries a load, it applies a force in the upward direction while displacement of the load is in the forward direction. In this case, displacement is perpendicular to the force. Hence, the work done is zero.

• A wind mill pulls water against the gravitational force. Thus, work is done by the wind mill to lift the water from the well.

• In photosynthesis process, there is no displacement of the leaves of the plant. Hence, the work done is zero.

• A force is applied by engine on the train. Therefore, the train moves in the direction of applied force. Here, displacement is in the direction of force applied. Hence, work is done by the engine on the train.

• Food grains do not move in the presence of solar energy. Hence, the work done is zero during the process of food grains getting dried in presence of the Sun.

• Wind exerts a force on the sailboat to push it in the forward direction. Thus, there is a displacement in the boat in the direction of force. Hence, work is done by wind on the boat.