9th Standard Chapter 10 Work And Energy
Exercises
-
Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.
-
Suma is swimming in a pond.
Answer: Work is done as she applies force and moves in the direction of force. -
A donkey is carrying a load on its back.
Answer: No work is done in the scientific sense since the force (upward) and displacement (forward) are perpendicular. -
A wind-mill is lifting water from a well.
Answer: Work is done as the force by the windmill lifts water upward, displacing it in the direction of force. -
A green plant is carrying out photosynthesis.
Answer: No work is done in the physics sense as there is no displacement due to applied force. -
An engine is pulling a train.
Answer: Work is done as the engine applies a force on the train, causing displacement in the direction of force. -
Food grains are getting dried in the sun.
Answer: No work is done as there is no displacement due to an applied force. -
A sailboat is moving due to wind energy.
Answer: Work is done as the force by wind moves the boat in the direction of the force.
-
Suma is swimming in a pond.
2. 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 work done by the force of gravity on the object?
Answer: Work done by gravity = weight × vertical displacement = m × g × h,
Here, the vertical displacement = 0 (same initial and final height),
Thus, work done = 0.
3. A battery lights a bulb. Describe the energy changes involved in the process.
Answer: Chemical energy in the battery → Electrical energy → Heat and light energy in the bulb.
4. Certain force acting on a 20 kg mass changes its velocity from 5 m s–1 to 2 m s–1. Calculate the work done by the force.
Answer:
Using:
W = ½ m (v2 – u2)
= ½ × 20 (22 – 52)
= 10 (4 – 25) = 10 (–21) = –210 J
The work done by the force is –210 J (negative as velocity decreases).
5. A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
Answer: Work done = weight × vertical displacement = 0 (as displacement is horizontal).
Thus, work done by gravity = 0.
6. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Answer: No, it does not violate the law. The potential energy is converted to kinetic energy during the fall, so total energy remains conserved.
7. What are the various energy transformations that occur when you are riding a bicycle?
Answer: Muscular (chemical) energy → Mechanical energy → Heat energy (due to friction) + Sound energy (if any).
8. Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
Answer: Yes, energy is spent, but no work is done on the rock as there is no displacement. The energy is transformed into heat in your body.
9. A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Answer:
1 unit = 1 kWh = 3.6 × 106 J
250 × 3.6 × 106 = 9 × 108 J
The energy consumed is 9 × 108 J.
10. An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
Answer:
Potential Energy: PE = mgh = 40 × 9.8 × 5 = 1960 J
At halfway (2.5 m down), lost potential energy = 40 × 9.8 × 2.5 = 980 J,
Kinetic energy at halfway down = 980 J.
11. What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Answer: Zero, because the force of gravity acts perpendicular to the displacement of the satellite during circular motion.
12. Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.
Answer: Yes, if the object is already in motion, it can continue moving (Newton’s First Law) even in the absence of a force, hence displacement is possible without force.
13. A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.
Answer: No work is done in the scientific sense as there is no displacement, even though the person gets tired.
14. An electric heater is rated 1500 W. How much energy does it use in 10 hours?
Answer:
Energy = Power × Time = 1500 W × 10 h = 1500 × 10 × 3600 = 54,000,000 J = 54 MJ.
15. Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?
- At extreme position: maximum potential energy, zero kinetic energy.
- At mean position: maximum kinetic energy, minimum potential energy.
- Energy continuously converts between potential and kinetic.
- Eventually, it comes to rest due to air resistance and friction, converting energy to heat.
- No violation of conservation as energy is converted to heat, not lost.
16. An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest?
Answer:
Work done = Change in kinetic energy
= 0 – ½mv2 = –½mv2
(The work required is –½mv2.)
17. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Answer:
60 km/h = 16.67 m/s
Work = –½ × 1500 × (16.67)2 = –208,500 J
18. In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.
- Force in the direction of displacement: Positive work.
- Force opposite to displacement: Negative work.
- Force perpendicular to displacement: Zero work.
19. Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?
Answer: Yes, if the net force is zero (forces cancel each other), acceleration will be zero even if forces are acting.
20. Find the energy in joules consumed in 10 hours by four devices of power 500 W each.
Answer:
Total power = 4 × 500 W = 2000 W
Energy = Power × Time = 2000 × 10 × 3600 = 72,000,000 J = 72 MJ.
21. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
Answer: It is converted into heat and sound energy and partly used in deforming the object and ground on impact.
Comments
Post a Comment