# Work & Energy (LA)

1. Define work, energy and power. Give the SI units for each of the these quantities. A man whose mass is 80 kg climbs up 30 steps of the stairs in 30 s. If each step is 12.5 cm in height, calculate the power used in climbing the stairs, (g = 10 m/s2). [2011 (T-II)]

2. Define kinetic energy and potential energy. Write an expression for K.E. of a body of mass m moving with a speed v. Find the kinetic energy of a stone of 10 kg moving with a velocity of 10 m/s. [2011 (T-II)]

3. (i) Derive an expression for the kinetic energy of an object. Write the S.I. unit of kinetic
energy. [2011 (T-II)] (ii) An object of mass 10 kg is moving with a uniform velocity of 5 ms–1. Calculate the kinetic energy possessed by the object.

4. (i) Give the S.I. unit of power. [2011 (T-II)]

(ii) What is the commercial unit of energy?

(iii) An electric heater is rated 1500 W. How much energy does it use in 10 hours?

5. (i) Define the potential energy. (ii) Derive the expression for work done by an object with mass m, when it is to be raised to a height h, from the ground

(iii) An object of mass 10 kg is at a certain height above the ground. If the potential energy of the object is 400 J, find the height at which the object is with respect to the ground. (Given, g = 10 ms–2) [2011 (T-II)]

6. (i) State the law of conservation of energy.

(ii) Define mechanical energy.

(iii) Calculate the energy in kWh consumed in 10 hours by four devices of power 500 W each. [2011 (T-II)]

7. (a) State law of conservation of energy. Explain it with one example.

(b) Two girls each of weight 400 N (Four hundred N) climb up a rope through a height of 8 metres (eight metres). Girl A takes 20 (Twenty) seconds while Girl B takes fifty seconds to accomplish this task. What is the power expended by each girl?

(c) An electric heater is rated 1500 watt. How much energy does it used in 10 hours (ten
hours)? [2011 (T-II)]

8. (a) Define work done by a constant force on an object. Write an expression also for the work done.

(b) How much work will be done on an object by a force if the displacement of the object
is zero? (c) What is the kinetic energy of an object? Write an expression for the kinetic energy of an object of mass m moving with a speed v. [2011 (T-II)]

9. Prove that the work done on a moving object is always equal to the change in its kinetic energy. Calculate the work done required to stop a car of mass 1500 kg moving with a velocity of 60 km/h. [2011 (T-II)]

10. (a) Define the term ‘average power’. [2011 (T-II)]

(b) When a player hits a football it moves along the curved path and then falls to the ground. Calculate the work done by the force of gravity on the football.

(c) Two friends Ram and Shyam, each having weight of 40 kg, go for rock climbing. Ram
climbs to a height of 3 m in 10 s and Shyam covers the same height in 12 s. Is the work
done by Ram and Shyam equal? or not? Compare the power of Ram and Shyam.

11. (a) Give the mathematical relation between power, force and velocity.

(b) Can a body have energy, without having momentum? If yes, why?

(c) A car of mass 2000 kg is lifted up to a distance of 30 m by a crane in 1 minute. A second crane does the same job in a 2 minutes. What is the power applied by each crane? Do the cranes consume the same or different amount of fuel? [2011 (T-II)]

12. (a) Define work. State two factors on which the magnitude of work depends. [2011 (T-II)]

(b) A car and a truck have the same speed of 30 m/s. If their masses are in the ratio 1:3, find the ratio of kinetic energy?

13. What do you mean by work? Give an example of negative work done. What is the work to be done to increase the velocity of a car from 18 km/hr to 90 km/hr if the mass of the car is 2000 kg? [2011 (T-II)]

14. (a) How do we define power of a body. Also define 1 joule.

(b) State the transformation of energy taking place when :

(i) Green plants prepare their food.

(ii) Head of a nail is hammered hard and it becomes warm.

(c) With what velocity should a body of mass 4 kg be thrown so that it acquires 1.25 kJ of kinetic energy. [2011 (T-II)]

15. Derive an expression to calculate the energy for an object in motion and calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h. [2011 (T-II)]

16. (a) A body of mass 15 kg possesses kinetic energy of 18.75 kJ. Find the velocity. (b) An electric bulb of 100 W is used for 4 hrs a day. Calculate the energy consumed by it in a day in joules and kilowatt hour unit. [2011 (T-II)]

17. (a) Define the S.I. unit of power.

(b) Establish a relationship between S.I. unit and commercial unit of energy.

(c) A car of weight 20000 N climbs up a hill at a steady speed of 8 m/s gaining a height of 120 m in 100 s. Calculate;

(i) work done by the car.

(ii) power of engine of car. [2011 (T-II)]

18. (a) A boy pushes a book by applying a force of 40 N. Find the work done by this force as the book is displaced through 25 cm along the path. [2011 (T-II)]

(b) A body of mass ‘m’ is raised to a vertical height h through two different paths X and
Y. What will be the potential energy of the body in the two cases? Give reason for your answer.

(c) An electronic bulb of 60 W is used for 6 hours per day. Calculate the units of energy
consumed in one day by the bulb.

19. (a) Define ‘Power’. Differentiate between kilowatt and kilowatt hour.

(b) Two girls each of weight 400 N climb up a rope through a height of 8 m. Let the name
of one of the girls is A and that of other is B. Girl A takes 20 s while B takes 50 s to accomplish this task. Calculate the power expended by each girl. [2011 (T-II)]

20. (a) List the factors on which the amount of work done depends.

(b) In a tug of war one team gives way to other. How much work is done by the wining
team?

(c) Certain force acting on a 20 kg mass changes its velocity from 5m/sec to 2m/sec. Calculate the work done by the force. [2011 (T-II)]

21. (a) The K.E. of an object of mass m, moving with a velocity 5 m/s is 25 J. What will be
its K.E. when its velocity is doubled and tripled? [2011 (T-II)]

(b) An object of mass 12 kg is at a certain height above the ground. If the P.E. of the object
is 480 J, find the height at which the object is w.r.t. the ground. (Given g = 10 m/s2)

22. (a) Define power. Give its S.I. unit.

(b) Two electric bulbs of 60 W each works for one hour a day. Calculate the units of energy consumed by these bulbs?

(c) If the rate per unit is Rs. 2.50, what will be the bill of the household using these bulbs
for 10 days? [2011 (T-II)]

23. (a) What is the change seen when an object is pushed along an inclined plane to a
height?

(b) A child pulls a toy attached to a string with a force of 5 N. If displacement of the toy
along the floor is 10 m and angle formed between F and S is 30°, what is the work done on the object? (c) Give any two examples where zero work is done. [2011 (T-II)]

OTHER IMPORTANT QUESTIONS

1. State the energy transformations taking place in the following cases : (a) When brakes are applied to a speeding vehicle.

(b) When an arrow is released from a stretched bow.

(c) When someone speaks in front of a microphone.
(d) When a tuning fork is hit against a rubber pad.
(e) When the coil of a motor moves in a magnetic field.

2. State in each of the following cases, if work is done/not done and why?

(a) A girl climbing a staircase.

(b) A man standing and holding a briefcase in hand.

(c) A porter carrying a heavy load and going down the stairs.

(d) A boy preparing for examination. (e) A planet going around the Sun.

3. (i) About how many kg of boiled potatoes would you have to eat to supply energy for half hour of swimming. Assume that your body utilises only 20% of the energy stored in
potatoes. Energy content of potatoes is 3.7 × 10^6 J/kg and the energy used in swimming is 25.6 kJ/minute.

(ii) A rocket of 3 × 10^6 kg mass takes off from a launching pad and acquires a vertical velocity of 1 km/s at an altitude of 25 km.

Calculate:

(a) Potential energy
(b) Kinetic energy. [Take the value of g = 10 ms–2]

4. (i) Distinguish between work, energy and power. State the S.I. units for each of these
quantities.

(ii) A dog of mass 16 kg is running at a constant speed of 12 m/s. Calculate the kinetic energy of the dog.

5. (i) A body has a mass m and velocity v. If the mass is increased four times and velocity is decreased two times, calculate the ratio of the kinetic energies in the above cases.

(ii) Why does a truck moving at 18 km/h cause far more serious accident than a cycle moving at the same speed?

(iii) What kind of energy transformation takes place when a sparkle is lighted?

CBSE Class 9 Science