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Physics Chapter – 3: Work and energy

12 August, 2024

Physics Chapter - 3: Work and energy

Introduction: Work and energy

Work

Work is defined as the product of the force applied on an object and displacement caused due to the applied force in the direction of the force. Work is a scalar quantity. It has no direction of its own but a magnitude.

Example of work done are:

  • Moving a chair from one location to another
  • Lifting a book from the shelf and placing it on a table
  • Pushing a pebble lying on the ground.

In all these situations we are applying a force on an object which is then changing the state of rest or motion of the object. So, we can conclude that work is done if and only if:

  • A force is applied to an object.
  • If the object is displaced from one point to another point.

Energy

  • Energy is defined as the ability to do work. Its unit is the same as that of work.SI unit of energy or work = Joule (Nm)


Energy has different forms: Light, heat, chemical, electrical or mechanical.
Mechanical energy is the sum of:
(i) Kinetic energy (K.E)
(ii) Potential energy (P.E) 

Kinetic Energy

Objects in motion possess energy and can do work. This energy is called Kinetic Energy.
Kinetic Energy = K.E= 1/2 mv2 (taking initial velocity u=0)
When two identical bodies are in motion, the body with a higher velocity has more K.E.

Work-energy theorem


The work-energy theorem states that the net work done by a moving body can be calculated by finding the change in KE. 

net = KE final − KE initial

Factors affecting kinetic energy-

  1. Mass
  2. Velocity
  3. Momentum

Potential Energy


Energy can get stored in an object when work is done on it. For example, stretching a rubber string. The energy that is possessed by a body by virtue of its configuration or change in position is known as Potential Energy.

The potential energy of an object at a height. When an object is raised to a certain height, work is done against gravity to change its position. This energy is stored as Potential Energy.

               W = F.s
F = ma
 In the case of increasing the height, F = mg
Therefore , W (P.E) = mgh   ⇒ ΔPE=mg(h final−h initial)

Law of conservation of energy states that energy can neither be created nor destroyed, but can be transferred from one form to another.

The total energy before and after the transformation remains constant. For example: consider a ball falling freely from a height.

At height h, it has only PE = mgh.By the time it is about to hit the ground, it has a velocity and therefore has K.E = ½ mv2 Therefore, energy gets transferred from PE to KE, while the total energy remains the same.

Total energy = KE + PE

Power


The rate of doing work or the rate of transfer of energy is called power. It is denoted by P

P = W/T
SI unit is Watt (Js−1).
Average power =  Total energy consumed/Total time taken
The commercial unit of power is kWh i.e. energy used in 1 hour at 1000 Joules/second.
1kWh=3.6×106J.

Write the answer of the following questions.

  1. Which of the following is a scalar quantity?
    A) Force
    B) Velocity
    C) Work
    D) Acceleration
  2. The unit of work done is:
    A) Newton
    B) Joule
    C) Watt
    D) Kilogram
  3. When a force moves an object in the direction of the force, it:
    A) Increases the potential energy of the object
    B) Decreases the kinetic energy of the object
    C) Decreases the potential energy of the object
    D) Increases the kinetic energy of the object
  4. If the force and displacement are in the same direction, then work done is:
    A) Positive
    B) Negative
    C) Zero
    D) None of the above
  5. The work done by a gravitational force on a body is:
    A) Positive
    B) Negative
    C) Zero
    D) None of the above
  6. Which of the following has zero work done when a force is applied?
    A) Lifting a book
    B) Pushing a wall
    C) Holding a bag
    D) Kicking a ball
  7. If a force of 20 N moves an object a distance of 5 meters, the work done is:
    A) 25 J
    B) 100 J
    C) 5 J
    D) 4 J
  8. If the angle between force and displacement is 90°, then work done is:
    A) Maximum
    B) Minimum
    C) Zero
    D) Negative
  9.  Work done is a measure of:
    A) Time
    B) Force
    C) Energy transfer
    D) Distance
  10. The energy possessed by a body due to its motion is called:
    A) Kinetic energy
    B) Potential energy
    C) Gravitational energy
    D) Mechanical energy
  11. Which of the following is a form of potential energy?
    A) Sound energy
    B) Thermal energy
    C) Chemical energy
    D) Kinetic energy
  12. The gravitational potential energy of an object depends on its:
    A) Mass only
    B) Height above the ground only
    C) Mass and height above the ground
    D) Velocity
  13.  The SI unit of power is:
    A) Joule
    B) Watt
    C) Newton
    D) Kilogram
  14. Which of the following is a unit of work?
    A) Nm
    B) N/s
    C) N/kg
    D) N/m
  15. When work is done against friction, energy is converted into:
    A) Kinetic energy
    B) Potential energy
    C) Heat energy
    D) Mechanical energy
  16. The conservation of energy principle states that:
    A) Energy can be created but not destroyed
    B) Energy can neither be created nor destroyed
    C) Energy is always conserved in an isolated system
    D) Energy is only conserved in mechanical systems
  17. A machine with a mechanical advantage greater than 1:
    A) Multiplies force
    B) Reduces force
    C) Increases speed
    D) Decreases speed
  18. A pulley system is an example of a machine that:
    A) Increases force
    B) Increases speed
    C) Increases both force and speed
    D) Decreases force
  19. The efficiency of a machine is always:
    A) Less than 100%
    B) Equal to 100%
    C) Greater than 100%
    D) Cannot be determined
  20. If a force of 50 N is applied to lift a load of 100 kg vertically upward, the work done in lifting the load through a height of 2 meters is:
    A) 50 J
    B) 100 J
    C) 200 J
    D) 500 J
  21. When a simple pendulum is at its mean position, it primarily possesses:
    (a) Kinetic energy
    (b) Potential energy
    (c) Kinetic + Potential energy
    (d) Sound energy
  22. As a body falls from a height h to h/2 its energy distribution shifts towards:
    (a) Only kinetic energy
    (b) Half kinetic and half potential energy
    (c) Only potential energy
    (d) More kinetic and less potential energy
  23. Given two objects with masses 1 × 10^-3 kg and 4 × 10^-3 kg having equal momentum, what is the ratio of their kinetic energies?
    (a) 2:1
    (b) 4:1
    (c) 1:4
    (d) 1:2
  24. The work done on an object is independent of:
    (a) Angle between force and displacement
    (b) Force applied
    (c) Initial velocity of the object
    (d) Displacement
  25. An iron sphere of mass 10 kg and an aluminium sphere of mass 3.5 kg, with the same diameter, are simultaneously dropped from a tower. When they are 10 m above the ground, they experience the same:
    (a) Potential energy
    (b) Momenta
    (c) Acceleration
    (d) Kinetic energy
  26. In a public address system, a microphone converts:
    (a) Sound energy into mechanical energy
    (b) Sound energy into electrical energy
    (c) Electrical energy into sound energy
    (d) Microwave energy into sound energy
  27. Which energy change involves frictional force?
    i. Potential energy to sound energy
    ii. Chemical energy to heat energy
    iii. Kinetic energy to heat energy
    iv. Chemical energy to heat energy

    Select the correct option:

    (a) Both (i) and (ii)
    (b) Only (iv)
    (c) Both (ii) and (iii)
    (d) Only (iii)
1C
2B
3D
4A
5B
6C
7B
8C
9C
10A
11C
12C
13B
14A
15C
16B
17A
18D
19A
20C
21A
22D
23B
24C
25C
26B
27D

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