CBSE Class 9 Science Work and Energy assignment

Question : What is energy of a body?

Question : Define kinetic energy.

Question : What is potential energy?

Question : What will be the work done by a force if displacement of the body is zero?

Question : When is work done on a body positive?

Question : Can energy be converted from one form to another?

Question : Define power.

Question : What will be the new kinetic energy of a body if its velocity is doubled?

Question : What will be the potential energy of an object of mass 5 kg kept at a height of 10 metres from the ground? Take g = 10 m/s2.

Question : A green plant is carrying out photosynthesis. What is the work done in this process?

Question : What is mechanical energy of an object?

Question : How we can define one watt?

Question : Give a mathematical expression for the work done when a force is acting on an object in the direction of its displacement.

Question : How average power can be calculated?

Question : When a ball is thrown in upward direction, work is done by the force of gravity. When a ball comes down freely towards the earth, in this case too work is done by the force of gravity. What is the difference in the nature of work done in the two cases?

Question : When work done is positive on a body?

Question : What will be the new kinetic energy of a body if its velocity is doubled?

Question : A constant force of 10N displaces a body through 5 m. Find the work done by the force?

Question : What will be the work done if a stone of mass 2 kg is raised through a height of 10cm?

Question : An object of mass 10kg is moving with speed 4 m/sec. What is the kinetic energy of the object?

Question : An object of mass 10kg is at a certain height above the ground. If the potential energy of the object is 200J. Find the height of the object from the ground.

Question : A person is holding a heavy bag on his head for 20 minutes and gets tired. Has he done some work or not? Explain.

Question : A body of mass5 kg is kept on a table. If it is displaced by a force of 10N by 2 m on the table on the same horizontal line, find the work done by the gravitational force.

Question : What is law of conservation of energy?

Question : A man of mass 50kg climbs a tower of height 45m in 5 seconds with the help of a rope. Find the power of the man. 

Question : If a particle falls through a height; its potential energy decreases. Does this violate the law of conservation of energy? Explain why?

Question : A ball of mass 2kg is kept on a tower of height 30m. Find its potential energy at this point. If it is allowed to fall freely, find its kinetic energy when it just touches the ground.

Question : What is the work done by the force of gravity on a satellite moving round the earth?

Question : What will be the work done to stop a moving cycle of mass 30kg which is moving with speed 54km/hr ?

Question : A freely falling body stops when it hits the ground. What happens to its kinetic energy?

Question : Prove that the kinetic energy of a body moving with speed v is equal to 1/2 mv2.
 

Exam Questions NCERT Class 9 Science Chapter 11 work And Energy

Question. Name the transformation of energy involved in the following cases :
(a) When a body is thrown upwards.
(b) When a body falls from the top of a hill.
(c) When coal burns.
(d) When a gas burns.
(e) When water falls from a height.
Ans : (a) Kinetic energy into potential energy.
(b) Potential energy into kinetic energy.
(c) Chemical energy into heat energy.
(d) Chemical energy into heat energy.
(e) Potential energy into kinetic energy.
                 
Question. What are the factors on which the work done depends?
Ans : The work done by a force depends upon :
(i) The magnitude of the force.
(ii) The magnitude of the displacement.
(iii) The angle between force and displacement.

Question. What is power? What is its SI unit?
Ans : It is defined as the rate of doing work. Its unit is watt.

Question. Which type of energy is present in a battery?
Ans : Chemical energy.
                 
Question. Name the energy present in an object due to its position or configuration.
Ans : Potential energy.
                
Question. Give one example of potential energy due to position.
Ans : Water stored in the reservoir of a dam has potential energy.

Question. (a) An arrow moves forward when released from a stretched bow. Explain the transformation of energy in the process.
(b) A boy of mass 50 kg climbs up a vertical height of 100 m. Calculate the amount of potential energy he gains.
Ans : (a) When the bow is stretched it stores potential energy. When the arrow is released the potential energy stored in the bow gets transformed into the kinetic energy of the arrow.
(b) Given m = 50 kg, h = 100 m,
               g = 10 ms–1, PE = ?
              PE = mgh = 50 × 10 × 100 = 5000 J

Question. Give an example of potential energy due to change in shape.
Ans : A stretched bow has potential energy due to change of shape.

Question. What type of energy is possessed by a flying bird and a flying aeroplane?
Ans : Both potential energy and kinetic energy.

Question. When do we say that work is done?
Ans : Work is said to be done when a force causes displacement of an object in the direction of applied force.

Question. Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Ans : Work done = Force × Displacement

Question. Define : (a) power (b) work done (c) kinetic energy.
Give SI unit of each.
Ans : (a) The rate of doing work is called power. Its SI unit is watt.
(b) Work is the product of force and displacement. Its SI unit is joule.
(c) It is the energy possessed by a body by virtue of its motion. Its SI unit is joule.

Question. Find the energy in kWh consumed in 10 hours by a machine of power 500 W.
Ans : W = P × t
            = 500 × 10 = 5000 Wh = 5 kWh.

Question. When is work said to be done against the force of gravity?
Ans : When a body is lifted, the work is done against the force of gravity.

Question. Calculate the cost of using a 2 kWh immersion rod for heating water in a house for one hour each day for 60 days if the rate is 1.50 per unit kWh.
Ans : E = Pt = 2 kW × 60 × 1 h
           = 120 kWh = 120 unit
The cost of using a 2 kWh immersion rod for heating
    water = 120 × 1.5 = Rs 180

Question. A work of 4900 J is done on load of mass 50 kg to lift it to a certain height. Calculate the height through which the load is lifted.
Ans : Work done on road to lift = mgh
                                     4900 = 50 × 9.8 h
                                            h=10m

Question. What is the SI unit of work done and power?
Ans : Joule and Watt.

Question. When is 1 joule of work said to be done?
Ans : When a force of 1 newton acting on a body displaces it 1m in its own direction.

Question. Does work done depend upon the velocity of the body.
Ans : No.

Question. State the law of conservation of energy.
Ans : It states that energy can neither be created nor destroyed. It can only change its form.