πŸ“˜ Chapter 11 – Work and Energy

πŸ“˜ Chapter 11 – Work and Energy

🧠 Detailed & Easy Notes

πŸ”Ή 1. Work

  • Work is said to be done when a force is applied and the object moves in the direction of the force.
    πŸ“Œ Work = Force Γ— Displacement Γ— cosΞΈ
    Where ΞΈ is the angle between force and displacement.

πŸ“Œ Units:

  • SI Unit: Joule (J)
  • 1 Joule = 1 Newton Γ— 1 metre

πŸ”Ή 2. Positive, Negative, and Zero Work

Type of WorkExampleDirection
PositiveLifting object upForce and displacement same direction
NegativeFrictionOpposite direction
ZeroHolding object stillNo displacement

πŸ”Ή 3. Energy

  • Capacity to do work
    πŸ“Œ SI unit: Joule
    πŸ“Œ 1 kJ = 1000 J

πŸ”Ή 4. Kinetic Energy (KE)

  • Energy due to motion
    πŸ“Œ KE = Β½ mvΒ²

πŸ”Ή 5. Potential Energy (PE)

  • Energy due to position or configuration
    πŸ“Œ PE = mgh
    m = mass, g = gravity, h = height

πŸ”Ή 6. Work-Energy Theorem

  • Work done on an object = Change in its kinetic energy
    πŸ“Œ W = Ξ”KE

πŸ”Ή 7. Power

  • Rate of doing work
    πŸ“Œ Power = Work / Time
    πŸ“Œ SI unit: Watt (W)
    πŸ“Œ 1 kW = 1000 W

πŸ”Ή 8. Commercial Unit of Energy

  • Kilowatt-hour (kWh)
    πŸ“Œ 1 kWh = 3.6 Γ— 10⁢ J

πŸ”Ή 9. Law of Conservation of Energy

  • Energy can neither be created nor destroyed. It can only be transformed from one form to another.

βœ… MCQs

  1. Work is done when:
    a) Force is applied
    b) Object moves
    c) Force & displacement are same direction
    d) All of these
  2. Unit of work is:
    a) Watt
    b) Newton
    c) Joule
    d) kg
  3. When is work zero?
    a) Object moves
    b) Force is applied
    c) No displacement
    d) None
  4. Work done is negative when:
    a) Force and displacement same
    b) Force and displacement opposite
    c) Force is zero
    d) Displacement is zero
  5. Formula of KE is:
    a) mvΒ²
    b) Β½mv
    c) Β½mvΒ²
    d) mgh
  6. Potential energy depends on:
    a) Mass
    b) Height
    c) Gravity
    d) All of these
  7. Unit of energy is:
    a) Newton
    b) Joule
    c) kg
    d) Watt
  8. Power =
    a) Work Γ— Time
    b) Work / Time
    c) Force / Time
    d) Time / Work
  9. 1 kWh =
    a) 1000 J
    b) 3600 J
    c) 3.6 Γ— 10⁢ J
    d) 100 J
  10. Energy of a body at rest:
    a) KE
    b) PE
    c) Heat
    d) Power
  11. Work-energy theorem relates:
    a) Force and velocity
    b) Power and time
    c) Work and KE
    d) Force and PE
  12. Law of conservation of energy:
    a) Energy destroyed
    b) Energy increased
    c) Energy transformed
    d) None
  13. 1 watt =
    a) 1 J/s
    b) 10 J
    c) 100 J
    d) 1 N/m
  14. Unit of power:
    a) Watt
    b) Joule
    c) Newton
    d) kWh
  15. What is commercial unit of energy?
    a) Joule
    b) kWh
    c) Newton
    d) Watt

βœ… Assertion & Reason

A: Work done is zero if object doesn’t move.
R: Displacement is necessary for work.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Kinetic energy depends on speed.
R: KE = Β½mvΒ²
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Lifting object increases PE.
R: PE = mgh
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Energy can be destroyed.
R: It can vanish in space.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true

A: Watt is unit of energy.
R: Energy = power Γ— time
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true


βœ… Case-Based Study

πŸ“˜ Passage:
A boy lifts a bag of 5 kg to a height of 2 metres. Later, he throws it and it moves with a speed of 4 m/s. Assume g = 10 m/sΒ².

  1. Work done in lifting bag = ?
    a) 10 J
    b) 50 J
    c) 100 J
    d) 20 J
  2. PE of bag = ?
    a) 50 J
    b) 100 J
    c) 20 J
    d) 10 J
  3. KE of bag after throw = ?
    a) 20 J
    b) 40 J
    c) 60 J
    d) 80 J
  4. What form of energy bag had at top?
    a) Only KE
    b) Only PE
    c) Both KE and PE
    d) Zero
  5. What does it prove?
    a) Energy destroyed
    b) Energy transformed
    c) No energy
    d) Force lost

βœ… Reason-Based Questions

  1. Why is work zero if there’s no displacement?
    Because work = force Γ— displacement.
  2. Why is work negative against friction?
    Because direction is opposite.
  3. Why is kinetic energy scalar?
    Because it has only magnitude.
  4. Why does speed affect KE more?
    Because KE ∝ speed².
  5. Why is energy needed to do work?
    Because energy is ability to do work.
  6. Why do higher objects have more PE?
    Because PE = mgh, more h = more PE.
  7. Why is unit of energy Joule?
    Because 1 Joule = 1 N Γ— 1 m.
  8. Why does a moving car have KE?
    Because it has mass and velocity.
  9. Why do we use kWh for electricity?
    Because joule is very small unit.
  10. Why is work done zero while holding bag?
    Because displacement is zero.
  11. Why is PE max at top of swing?
    Because height is maximum.
  12. Why is KE max at lowest swing point?
    Because speed is maximum.
  13. Why is power called rate of work?
    Because power = work/time.
  14. Why energy conserved during fall?
    PE converts to KE.
  15. Why is unit of power watt?
    Because 1 watt = 1 joule/second.

βœ… Final Answer Key

MCQs:
1–d, 2–c, 3–c, 4–b, 5–c, 6–d, 7–b, 8–b, 9–c, 10–b, 11–c, 12–c, 13–a, 14–a, 15–b

Assertion & Reason:
16–a, 17–a, 18–a, 19–d, 20–d

Case-Based:
21–c, 22–b, 23–b, 24–c, 25–b