Questions on Work and Energy, IGCSE, CBSE, ICSE.

WORK AND ENERGY

Two marksPerson holds a suitcase for 30 minutes and gets tired. Has he done some work or not? Justify your answer.Describe an activity to show that the work done by a force can be either positive or negativeThe kinetic energy of an object of mass `m’ moving with a velocity of 5ms-1 is 25J. Calculate its kinetic energy when its velocity is doubled.
List two conditions which need to be satisfied for the work to be done on an object ?A box of about 10kg is placed at a point A on a horizontal surface. It is moved to a point B which is at a distance of 2m from A. If the line joining A and B is horizontal find the work done by the gravitational force on the box. Justify your answer. (g  10 m/s2)
Define energy. Name and define its SI unit.What is the SI unit of kinetic energy ? Find kinetic energy of a body of mass 15 kg moving with a velocity of 4 m/s. When is work said to be done against the force of gravity ? State and define SI unit of work.How is work done by a force measured ? A porter lifts a luggage of 20 kg from the ground and puts it on his head 1.7 m above the ground. Find the work done by the porter on the luggage. (g10 m/s2)
Define the term kinetic energy. Derive an expression for the kinetic energy of an object of mass „m‟ moving with a uniform velocity v
Name the commercial unit of energy. Establish a relationship between this unit and SI unit of energy. A battery lights a torch bulb. State the energy transformations involved in this process.Define potential energy and derive an expression for it.List two conditions which need to be satisfied for the work to be done on an objectState and define commercial unit of energy. Convert it into Joules„Work and Energy are related to each other‟. Explain the given statement
(a) When a compressed spring is dropped in an acid it is dissolved. What happens to the PE of the spring ? Explain. (b) What is the commercial unit of energy ? Define it.A body of mass 2 kg is flying horizontally with a velocity of 72 km/h at a height 10 m. Compute the total energy.(a) Define kinetic energy (b) A horse of mass 200 kg and a dog of mass 20 kg are running at the same speed. Which of the two possesses more kinetic energy(a) Define one Joule of work. (b) What will be the work done in the following situations ? State whether it will be positive, negative or zero, giving reason in each case Three marks

(a) Define one joule of energy. (b) Two cars X and Y of masses m1 and m2 accelerate from rest in linear direction. The mass of Y is twice of X. The acceleration due to car X is 1 ms2. After a time of t seconds it is noticed that the Kinetic energy of X is twice of Kinetic energy of Y. Calculate the acceleration produced by the car Y. The kinetic energy of an object moving with a velocity of 5 m/s is 25J. Find the mass of the object. What will be its kinetic energy when its velocity is made (i) two times (ii) three times ? Justify your answers.

Two bodies of same mass start from rest and move with velocities of V and 2V respectively. Find the ratio of their kinetic energies. (a) State the law of conservation of energy. (b) Taking the example of a simple pendulum, explain the variations in the forms of energy and the inter conversions involved during its oscillatory motion (i) A porter holds a luggage over his head for 40 minutes and gets tired. Has he done work ? Justify your answer. (ii) What is the angle between the direction of motion of an object and the direction of the applied force, if the work done on the object is said to be negative ? (iii) Define 1 joule of work Two boys Ram and Shyam have mass equal to 40 kg each. They start climbing a rope separately and both reach a height of 6 m. Ram takes 20 seconds and Shyam takes 25 seconds to complete the task. Who has more power? Explain your answer. (Take g =10 m/s2) Observe the diagrams I and II carefully. An object of mass m is lifted from A to B to height h along path 1 and path 2. What would be the work done on the object in both the cases ? Give reasons for your answers  (a) Define energy. What is its S.I. unit (b) How is the commercial unit of electrical energy related to its S.I. unit ?   (a) A bus starts from rest accelerates against friction due to road from t  0 sec to t  10 sec. Then it is allowed to decelerate from t  11 sec to t  30 sec. Comment about the nature of work done in these different time intervals. (b) What is the work done by the force of gravity on a satellite while moving around the earth ? Justify your answer. A body of mass 5 kg initially at rest is subjected to a force of 20N. Find the kinetic energy acquired by the body at the end of 10s When do we say that the work is done by a force ? A porter lifts a luggage of 15 kg from the ground and puts it on the head 1.7 m above the ground. Find the work done by the porter on the luggage. ( g10 m/s2 ) A body of mass 5 kg is thrown vertically upwards with a speed of 10 m/s. What is its kinetic energy when it is thrown ? Find its potential energy when it reaches at the highest point. Also find the maximum height attained by the body. (g10 m/s2) When do we say that the work done is negative ? Certain force acting on a 20kg mass changes its velocity from 5 m/s to 2 m/s. Find the work done by the force Define commercial unit of energy. Derive relationship between this unit of energy and SI unit of energy. An electrical device of 500 W is used daily in a household for 10 hours. Calculate the energy consumed in the month of April.

 When is the work done by a force said to be negative? Give one situation in which one of the forces acting on the object is doing positive work and the other is doing negative work.Define one Joule of work. Calculate the work done in lifting a box weighting 150 kg through a vertical height of 7 meters (take g=10 ms2)
Define 1 Watt of power. A lamp consumes 1000 J of electrical energy in 10s. Calculate its power.How much work should be done on an object of 120 kg to increase its speed from 25 m/s to 40 m/s ?
A child of mass 35kg is sitting on a trolley of mass 5 kg. The trolley is given a push by applying a force so that begins to move with a speed of 4m/s. The trolley comes to rest after covering a distance of 16 m. Find (i) the work done on the trolley (ii) the work done by the trolley before coming the nest.(a) An object of mass ‘m’ is moving with a velocity ‘v’ on a level ground. State the amount of work done by a person who wants to stop the object. Justify your answer. (b) A satellite of mass ‘m’ is moving round the earth with a speed ‘v’. State the work done by the force of gravity on the satellite. Justify your answer.Define work. How is work measured ? When is work done by a force said to be negative ?A force is applied on a car of mass 1500 kg, so that its speed increases from 54 km/h to 72 km/h. Find the workdone by the force on the carDefine 1 watt of power. A lamp consumes 1000 J of electrical energy in 10s. Calculate its powerHow much work should be done on an object of 120 kg to increase its speed from 25 m/s to 40 m/s ?
 Five marks

(a) What is the commercial unit of energy ? Give the relationship between commercial unit of energy and joules. (b) In an office, a tube light of 40W, a fan of 75 W and a cooler of 150 W are installed. If the appliances are used for 8 hours per day, find the energy consumed per day in commercial unit State the law of Conservation of energy. Illustrate the law of Conservation of energy by discussing the energy change which occurs when a pendulum bob is drawn to one side and allowed to oscillate. Why does the bob eventually come to rest ? What happens to its energy eventually ? Explain whether it is a violation of Law of Conservation of energy (a) Define Kinetic energy and derive an expression for it. (b) How many joules of energy make one KWh. (c) A bullet of mass 50 gm is fired from a gun. It leaves the barrel with a velocity of 100 m/s in 0.01 seconds. Calculate the power expended by the gun in firing the bullet. (a) State and prove law of conservation of energy for a freely falling body. (b) What is the power expended by the engine of the car mass 1.5 tonnes which increases the velocity of the car from 36 km/hr to 72 km/hr in 20 seconds. (a) Define power. Two boys A and B do the same work in 5 minutes and 6 minutes respectively. Which of the two has more power and why ? (b) Derive an expression for the kinetic energy of a moving object

State the law of conservation of energy. Illustrate this law by discussing the free fall of an object.

 (i) An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the total workdone against the force of gravity and by the force of gravity on the object ? Explain with proper mathematical expressions.(ii) Certain force acting on a 20kg mass changes its velocity from 5ms-1 to 2ms-1 calculate the work done by the force.
(i) Define kinetic energy. Derive an expression for the kinetic energy of an object.(ii) The power of a motor pump is 5kW. How much water per minute the pump can raise to height of 20m? Take g=10ms-2
  (i) A light and a heavy object have the same momentum. What is the ratio of their kinetic energies ? Which one has a larger kinetic energy ?(ii) A ball is dropped from a height of 10m. If the energy of the ball reduces by 40% after striking the ground, how much high can the ball bounce back ? (g  10ms2)(i) Define power. Write its SI unit. (ii) A body of mass 15 kg possesses kinetic energy of 18.75 kJ. Find the velocity. (iii) 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 unitList one main difference between potential and kinetic energy with one example of each. Derive an expression for the kinetic energy of an object of mass ‘m’ moving horizontally with uniform velocity.
 (i) State the law of conservation of energy.(ii) Show that the energy of a freely falling body is conservedDefine Kinetic energy. Derive an expression for the Kinetic energy possessed by an object of ‘m’ moving with a velocity ‘v’ . A light and a heavy object has the same momentum Find the ratio their kinetic energies. Which one has a larger Kinetic energy ?State the law of conservation of energy. Show that the energy of a (i) freely falling object is conserved (ii) vibrating pendulum is conserved.What is meant by power of a machine ? Name and define its SI unit. How is kilowatt different from kilowatt hour ? From a 20m high fall nearly 25 metric tonnes of water fall per second. Calculate the equivalent power if all this energy is utilized. (g  10 m/s2)
(a) Define potential energy. Derive an expression for the gravitational potential energy of a body of mass „m‟ raised to a height „h‟ above the ground. (b) State the energy transformation in the following : (i) heat engine (ii) electric motor
(a) State and explain with example the law of conservation of energy. (b) An electric lamp of 100w is used daily for 5 hours. Calculate the number of units consumed in a month of 30 days. (a) A Person is holding a bucket of water by applying a vertical force of 100N. He first moves a horizontal distance of 5 m and then climbs up through stairs a vertical distance of 10 m. calculate the total work done by the person on the bucket.(b) Establish the relationship between kinetic energy and momentum of a body(a) Define average power. Calculate the total energy consumed in the month of November in a household in which four devices of power 500 W each are used daily for 10 hours. (b) A freely falling object eventually stops on reaching the ground. What happens to its energy ?(a) What is meant by mechanical energy ? State its two forms. State the law of conservation of energy. Give an example in which we observe a continues change of one form of energy into another and vice a versa. (b) Calculate the amount of work required to stop a car of 1000 kg moving with a speed of 72 km/h.(a) State the law of conservation of mechanical energy. (b) Explain the conservation of energy in the case of a freely falling object. (c) Find the velocity of a body of mass 2 kg having kinetic energy of 100 J.(a) Name the physical quantity defined by rate of doing work. Define its SI unit. (b) Why is concept of average power useful ? How is it determined ? (c) A boy of mass 45 kg runs up a staircase of 45 steps in 9 s. If the height of each step is 15 cm, find his power. (g10 m/s2)
(a) State the law of conservation of energy.(b) Explain the law with two suitable example.(c) A bulb lights up when connected to a battery. State the energy change which takes place in the battery.(a) ―An arrow and the stretched string on the bow is said to possess energy‖. Comment.
(b) A bag of wheat weighs 50kg. Calculate the height to which it should be raised so that its potential energy is 5000J(i) A light and a heavy object have the same momentum. What is the ratio of their kinetic energies ? Which one has a larger kinetic energy ?(ii) A ball is dropped from a height of 10m. If the energy of the ball reduces by 40% after striking the ground, how much high can the ball bounce back ? (g  10ms2)(i) State the law of conservation of energy.(ii) Show that the energy of a freely falling body is conserved.State the law of conservation of energy. A body of mass 2 kg is projected upward with a velocity 36 km/h. Show that initial KE is same as final P.E.Define power. Give its SI unit. A boy of mass 40 kg runs up a height of 100 stairs, each measuring 20 cm in one minute and 20 seconds. Calculate the power of the boy. (g10 m/s2)