Chapter 10 – Work and Energy - Short Notes

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🌱 1. Work

✅ Key Definition

Work is said to be done when a force applied on an object causes displacement in the direction of the force.

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🌱 Explanation

• If force is applied but there is no displacement, no work is done.
• Work is done only when:
  1. A force acts on the object.
  2. The object moves (displacement occurs).
  3. The displacement is in the direction of force (or has a component in that direction).
• Formula of Work:

  Work = Force \times Displacement

W = F \times s 

• SI Unit of Work: Joule (J)
• 1 Joule = Work done when 1 N force moves an object by 1 m in the direction of force.

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📚 Important Cases of Work

1. Positive Work – When displacement is in the direction of force.
   Example: Pushing a trolley forward.

2. Negative Work – When displacement is opposite to the direction of force.
   Example: Friction acting on a moving object.

3. Zero Work – When:

   • No displacement (pushing a wall)
   • Force is perpendicular to displacement (carrying a bag horizontally)

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🧠 Memory Trick

👉 “No Move = No Work” (Remember this for exams!)

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🌱 2. Energy

✅ Key Definition

Energy is the capacity to do work.

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🌱 Explanation

• An object having energy can do work.
• Energy and work have the same unit: Joule (J).
• Different forms of energy:
  • Kinetic Energy
  • Potential Energy
  • Heat Energy
  • Light Energy
  • Chemical Energy

(For Class IX, focus mainly on Kinetic and Potential Energy.)

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🌱 3. Kinetic Energy

✅ Key Definition

The energy possessed by a body due to its motion is called kinetic energy.

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🌱 Explanation

• If an object is moving, it has kinetic energy.
• Faster the object → More kinetic energy.
• Heavier the object → More kinetic energy.
• Formula:

  KE = \frac{1}{2} mv^2

Where:

• m = mass
• v = velocity

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🧠 Important Point

Kinetic energy depends on the square of velocity.
If velocity doubles → KE becomes four times.

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🌱 4. Potential Energy

✅ Key Definition

The energy possessed by a body due to its position or configuration is called potential energy.

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🌱 Explanation

• Example: Water stored in a dam.
• A stretched rubber band.
• A book kept at a height.

For an object at height:

PE = mgh

Where:

• m = mass
• g = acceleration due to gravity
• h = height

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📊 Difference Between Kinetic and Potential Energy

Basis	Kinetic Energy	Potential Energy
Definition	Energy due to motion	Energy due to position
Formula	½ mv²	mgh
Depends on	Mass & velocity	Mass & height
Example	Moving car	Water in dam

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🌱 5. Mechanical Energy

✅ Key Definition

The sum of kinetic energy and potential energy of a body is called mechanical energy.

Mechanical Energy = KE + PE

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🌱 Explanation

• Example: A falling object.
• At top → Only PE
• During fall → Both KE and PE
• Just before hitting ground → Only KE

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🌱 6. Law of Conservation of Energy

✅ Key Definition

Energy can neither be created nor destroyed; it can only be transformed from one form to another.

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🌱 Explanation

• Total energy remains constant.
• Example: A freely falling object.
  • At top → Maximum PE
  • In middle → PE decreases, KE increases
  • At bottom → Maximum KE
• Total mechanical energy remains constant (if air resistance is neglected).

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🖊 Diagram Description

Draw a ball falling from a height:

1. Draw three positions of the ball:
   • Top position (label: PE maximum, KE zero)
   • Middle position (label: PE decreasing, KE increasing)
   • Bottom position (label: KE maximum, PE zero)
2. Show arrows indicating downward motion.
3. Write “Total Energy = Constant” on the side.

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🧠 Memory Trick

“Energy Changes Form, But Total Remains Same”

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🌱 7. Rate of Doing Work (Power)

✅ Key Definition

Power is the rate of doing work.

Power = \frac{Work}{Time}

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🌱 Explanation

• More power means work is done faster.
• SI Unit: Watt (W)
• 1 Watt = 1 Joule per second
• Larger unit: Kilowatt (kW)

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📚 Commercial Unit of Energy

• 1 kilowatt-hour (kWh)
• 1 kWh = 3.6 × 10⁶ J
• Used in electricity bills.

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🧠 Important Point

1 unit of electricity = 1 kWh

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🌟 Quick Revision Summary

• 🔥 Work = Force × Displacement
• 🔥 SI Unit of Work & Energy = Joule
• 🔥 No displacement → No work
• 🔥 KE = ½ mv²
• 🔥 PE = mgh
• 🔥 Mechanical Energy = KE + PE
• 🔥 Energy cannot be created or destroyed
• 🔥 Power = Work ÷ Time
• 🔥 SI Unit of Power = Watt
• 🔥 1 kWh = 1 unit of electricity

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