Chapter 1 - Force, Work, Energy and Power- (Different Forms, Sources, Conservation and Transformation of Energy) Excercise 68
Define kinetic energy.
The energy possessed by a body by virtue of its motion is called kinetic energy.
Give two examples of kinetic energy.
Examples of kinetic energy:
(i) Air in motion has kinetic energy.
(ii) A swinging pendulum
Write the SI units of energy.
SI unit of energy is 'joule'.
Name the physical quantity which is equal to the energy of a body.
Name the physical quantity which is associated with the work done by a body.
'Energy' is the body's ability to do work.
Is kinetic energy a scalar or a vector physical quantity?
Energy is a scalar quantity.
If the speed of a body is doubled, what happens to its kinetic energy?
What are the physical parameters on which the kinetic energy of a body depends?
Kinetic energy of a body depends upon:
(i) Mass of the body
(ii) Speed of the body
Can a body possess zero kinetic energy?
Yes, if the body is not in motion; it has zero kinetic energy
Name the type of energy possessed by a moving bullet.
A moving bullet possesses kinetic energy
A body of mass 2 kg is moving with a speed of 20 ms-1. What is its K.E.?
A moving object of mass 30 kg has 60 J of kinetic energy. What is its speed?
What type of energy is possessed by a flowing water?
Flowing water possesses kinetic energy.
If the speed of a particle is increased four times, how will its kinetic energy be affected?
A bus and a car have the same K.E. which of the two should be moving faster? Explain.
The car should be moving faster because the mass of the car is less than that of the bus.
A body is thrown vertically upwards. Its velocity keeps on changing. What happens to its K.E. when does its velocity become zero?
Its kinetic energy changes with the change in velocity.
Velocity becomes zero at the highest point.
What change should be effected in the velocity of a body to maintain the same K.E., if its mass is increased four times?
The K.E. of a body is 5 J. What will be its K.E. when its speed is doubled?
Name the parameters that can change the K.E.
The parameters that can change the kinetic energy are:
Give an example of a body possessing no K.E.
A ball at lying at rest on the floor possesses zero kinetic energy.
Define potential energy.
The energy possessed by a body by virtue of its position, shape or change of configuration is called potential energy.
What are the factors on which P.E. of a body depends?
Potential energy of a body depends upon:
(i) Mass of the body
(ii) Acceleration due to gravity
(iii) Height of the body
Write the SI units of energy.
SI unit of energy is 'joules'.
What is the amount of energy spent in lifting 1 kg through 1 m? Take g = 10 ms-2.
Give two examples of P.E.
Examples of potential energy:
(i) Water stored at a height in a reservoir.
(ii) A stretched spring.
What is the P.E. of a body lying on the surface of the earth?
P.E of a body lying on a floor is zero.
What is the amount of energy spent by the heart in pumping blood through the body?
The energy spent by the heart in pumping blood through the body is 1 J per beat.
Name the type of energy possessed by a body due to a change in configuration.
How is energy stored in a clock?
The work done in winding a clock spring is stored as P.E. in the spring. This P.E. is then used to run the hands of the clock.
What is the P.E. of a body of mass M at a height H above the earth's surface?
P.E. of a body of mass M and at a height H above the earth's surface is:
P.E. = MgH ; here, g = acceleration due to gravity.
Is P.E. a vector or a scalar quantity?
P.E. is a scalar quantity.
Name the type of energy possessed by a
(i) stretched catapult (ii) hot iron
(iii) wound up clock
(i) Potential energy
(ii) Heat energy
(iii) Potential energy
An object is dropped from a height H. when is its
(a) P.E. maximum,
(b) K.E. maximum,
(c) P.E. = K.E.?
(i) At the height H because the height is maximum.
(ii) At the ground level because the velocity is maximum.
(iii) At half distance of the total path i.e. at height H/2, the P.E. is equal to the K.E.
A cricket ball is thrown up from the earth's surface. What happens to its P.E.
(a) during the motion
(b) at the highest point?
(a) As the height above the ground increases, the potential energy also increases.
(b) At the highest point, the height of the cricket ball is maximum and hence the potential energy is also maximum.
What is the difference between K.E. and P.E.?
P.E. is the energy possessed by a body by virtue of its position, shape or configuration but K.E. is the energy possessed by a body by virtue of its velocity or motion.
E.g. A ball kept on a cliff possesses P.E. but as soon as it is kicked it possesses K.E.
What is meant by the transformation of energy?
Whenever one form of energy dissipates or disappears, another form of equivalent amount of energy is produced; this is referred to as transformation of energy.
E.g. when a particular switch is pressed electric lamps light up owing to the heat produced in the filament. This is the transformation of electrical energy to heat and light energy.
Chapter 1 - Force, Work, Energy and Power- (Different Forms, Sources, Conservation and Transformation of Energy) Excercise 69
Does the total energy of a body always remain constant?
No, total energy of a body does not remain constant but the total energy of a closed system remains constant e.g. universe.
Is energy a scalar or a vector physical quantity?
Energy is a scalar quantity.
Can we destroy every energy associated with a body?
No, we cannot destroy energy associated with a body.
Define energy of a body.
Energy is the capacity or ability of a body to do work or energy is stored work.
State the law of conservation of energy.
The law of conservation of energy states that energy cannot be created or destroyed; the sum total of energy in a closed system remains unchanged. Energy only changes from one form to another.
Flowing water can rotate a turbine. Which type of energy is used by the turbine?
Potential energy of water stored at a height in dams get converted into the kinetic energy of the flowing water which is used by a turbine.
Name two uses of solar cells in our day to day life.
Two uses of solar cells:
(i) Solar cells are used as energy sources in calculators.
(ii) Solar panels made up of solar cells are used for the purpose of street lights.
What are the renewable sources of energy? Give two examples.
The renewable sources of energy are those sources of energy which can be used again and again. E.g. sun, hydro-energy.
What are the non-renewable sources of energy? Give two examples.
The non renewable sources of energy are those sources of energy, which once exhausted are not easily available again. E.g. coal, petroleum products.
What is the most abundant source of energy on the earth?
Sun is the most abundant source of energy on the earth.
Name the type of energy obtained from the Sun.
Solar energy is obtained from the sun.
A dry cell converts one form of energy into another. Name the two forms.
A dry cell converts chemical energy into electrical energy.
Name the device that converts electrical energy into mechanical energy.
A D.C. motor converts electrical energy into mechanical energy.
Name the device that converts mechanical energy into electrical energy.
A dynamo converts mechanical energy into electrical energy.
Name four different forms of energy.
Four different forms of energy:
(i) Solar energy
(ii) Mechanical energy
(iii) Wind energy
(iv) Nuclear energy
What kind of energy transformation takes place at a thermal power station?
At a thermal power station, nuclear energy is converted into electrical energy.
Name the energy changes for each of the following cases:
(i) electric ball (ii) bicycle brakes
(iii) pendulum (iv) human body
(i) Electrical energy to sound energy
(ii) Mechanical energy into heat energy.
(iii) Potential energy into kinetic energy and vice-versa.
(iv) Chemical energy obtained from food is converted into mechanical energy and heat energy in muscles.
(v) A dynamo converts mechanical energy into electrical energy.
What is meant by power?
The rate of doing work is called power.
Write the SI units of power.
'Watt' is the SI unit of power.
Define one horse power.
Horse power is the unit of power. The horsepower used for electrical machines is defined as exactly 746 watt.
Classically, a horse exerting 1 H.P. can raise 330 pounds of coal 100 feet in a minutes, or 33 pounds of coal 1,000 feet in one minute, or, 1,000 pounds 33 feet in one minute.
Define the unit 'watt'. How is it related to horse power?
If 1 joule of work is done in 1 second, the power is said to be 1 watt.
1 H.P. = 746 watt
A body does 20 J of work in 10 s. What is its power?
Name the physical quantity associated with the 'rate of doing work'.
Power is the physical quantity associated with the 'rate of doing work'.
An electric motor drives a machine which lifts a mass of 4 kg through a height of 10 m in 5 s at a constant speed. Assuming g = 10 ms-2, calculate
(i) the amount of work done, (ii) the power of the machine.
Name the practical unit of power.
Practical or commercial unit of power is kilowatt-hour.
In what form is energy stored in a wrist watch?
When we wind a watch, the configuration of its spring is changed. The energy stored in the spring is obviously potential in nature (elastic potential to be more accurate).
A log of wood cut by a saw becomes hot. From where does this heat energy come?
This heat energy comes from the mechanical energy.
Our hands become warm when we rub them against each other. Why?
When we rub our hands, mechanical energy is converted into heat energy.
The head of a nail becomes warm when it is hammered into a plank of wood. Explain the series of energy transformation taking place in the process.
When the head of a nail is struck with a hammer, the mechanical energy of the hammer pushes the nail into the plank of wood and in this process a part of energy is converted into heat energy.
A 100 W electric lamp emits energy in the form of light at the rate 10 J per second. What percentage of electric energy does the lamp transform into light energy?
A horse exerts a pull on a cart of 300 N so that the horse-cart system moves with a uniform speed 18 km/h on a level road. Calculate the power developed by the horse in watt and also find its equivalent in horse power.
If an electric bulb of 100 watt is lighted for 2 hours, how much electric energy would be consumed?
A woman pulls a bucket of water of total mass 5 kg from a well which is 10 s. calculate the power used by her.