
π Chapter 11 β Work and Energy
π§ Detailed & Easy Notes
πΉ 1. Work
- Work is said to be done when a force is applied and the object moves in the direction of the force.
π Work = Force Γ Displacement Γ cosΞΈ
Where ΞΈ is the angle between force and displacement.
π Units:
- SI Unit: Joule (J)
- 1 Joule = 1 Newton Γ 1 metre
πΉ 2. Positive, Negative, and Zero Work
Type of Work | Example | Direction |
---|---|---|
Positive | Lifting object up | Force and displacement same direction |
Negative | Friction | Opposite direction |
Zero | Holding object still | No displacement |
πΉ 3. Energy
- Capacity to do work
π SI unit: Joule
π 1 kJ = 1000 J
πΉ 4. Kinetic Energy (KE)
- Energy due to motion
π KE = Β½ mvΒ²
πΉ 5. Potential Energy (PE)
- Energy due to position or configuration
π PE = mgh
m = mass, g = gravity, h = height
πΉ 6. Work-Energy Theorem
- Work done on an object = Change in its kinetic energy
π W = ΞKE
πΉ 7. Power
- Rate of doing work
π Power = Work / Time
π SI unit: Watt (W)
π 1 kW = 1000 W
πΉ 8. Commercial Unit of Energy
- Kilowatt-hour (kWh)
π 1 kWh = 3.6 Γ 10βΆ J
πΉ 9. Law of Conservation of Energy
- Energy can neither be created nor destroyed. It can only be transformed from one form to another.
β MCQs
- Work is done when:
a) Force is applied
b) Object moves
c) Force & displacement are same direction
d) All of these - Unit of work is:
a) Watt
b) Newton
c) Joule
d) kg - When is work zero?
a) Object moves
b) Force is applied
c) No displacement
d) None - Work done is negative when:
a) Force and displacement same
b) Force and displacement opposite
c) Force is zero
d) Displacement is zero - Formula of KE is:
a) mvΒ²
b) Β½mv
c) Β½mvΒ²
d) mgh - Potential energy depends on:
a) Mass
b) Height
c) Gravity
d) All of these - Unit of energy is:
a) Newton
b) Joule
c) kg
d) Watt - Power =
a) Work Γ Time
b) Work / Time
c) Force / Time
d) Time / Work - 1 kWh =
a) 1000 J
b) 3600 J
c) 3.6 Γ 10βΆ J
d) 100 J - Energy of a body at rest:
a) KE
b) PE
c) Heat
d) Power - Work-energy theorem relates:
a) Force and velocity
b) Power and time
c) Work and KE
d) Force and PE - Law of conservation of energy:
a) Energy destroyed
b) Energy increased
c) Energy transformed
d) None - 1 watt =
a) 1 J/s
b) 10 J
c) 100 J
d) 1 N/m - Unit of power:
a) Watt
b) Joule
c) Newton
d) kWh - What is commercial unit of energy?
a) Joule
b) kWh
c) Newton
d) Watt
β Assertion & Reason
A: Work done is zero if object doesnβt move.
R: Displacement is necessary for work.
a) A and R true, R explains A
b) A and R true, R doesnβt 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 doesnβt 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 doesnβt 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 doesnβt 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 doesnβt explain A
c) A true, R false
d) A false, R true
β Case-Based Study
π 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.
- Work done in lifting bag = ?
a) 10 J
b) 50 J
c) 100 J
d) 20 J - PE of bag = ?
a) 50 J
b) 100 J
c) 20 J
d) 10 J - KE of bag after throw = ?
a) 20 J
b) 40 J
c) 60 J
d) 80 J - What form of energy bag had at top?
a) Only KE
b) Only PE
c) Both KE and PE
d) Zero - What does it prove?
a) Energy destroyed
b) Energy transformed
c) No energy
d) Force lost
β Reason-Based Questions
- Why is work zero if thereβs no displacement?
Because work = force Γ displacement. - Why is work negative against friction?
Because direction is opposite. - Why is kinetic energy scalar?
Because it has only magnitude. - Why does speed affect KE more?
Because KE β speedΒ². - Why is energy needed to do work?
Because energy is ability to do work. - Why do higher objects have more PE?
Because PE = mgh, more h = more PE. - Why is unit of energy Joule?
Because 1 Joule = 1 N Γ 1 m. - Why does a moving car have KE?
Because it has mass and velocity. - Why do we use kWh for electricity?
Because joule is very small unit. - Why is work done zero while holding bag?
Because displacement is zero. - Why is PE max at top of swing?
Because height is maximum. - Why is KE max at lowest swing point?
Because speed is maximum. - Why is power called rate of work?
Because power = work/time. - Why energy conserved during fall?
PE converts to KE. - Why is unit of power watt?
Because 1 watt = 1 joule/second.
β Final Answer Key
MCQs:
1βd, 2βc, 3βc, 4βb, 5βc, 6βd, 7βb, 8βb, 9βc, 10βb, 11βc, 12βc, 13βa, 14βa, 15βb
Assertion & Reason:
16βa, 17βa, 18βa, 19βd, 20βd
Case-Based:
21βc, 22βb, 23βb, 24βc, 25βb