Kinetic energy
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Lesson notes
What is Kinetic Energy?
- **Kinetic energy** is the energy an object has because it is **moving**.
- The word 'kinetic' comes from the Greek word×kinesis×, meaning 'motion'.
- Kinetic energy is one of the two main types of energy, the other being **potential energy** (stored energy).
- Any moving object—a rolling ball, a flowing river, a flying bird—has kinetic energy.
What Does Kinetic Energy Depend On?
- Kinetic energy depends on two factors: the object's **mass** and its **speed**.
- The greater the mass, the more kinetic energy the object has (if speed is the same).
- The greater the speed, the more kinetic energy the object has (if mass is the same).
- Speed has a **larger effect** than mass: doubling the speed **quadruples** the kinetic energy.
The Kinetic Energy Formula
- The mathematical formula for kinetic energy (KE) is: **KE = ½ × mass × speed²**.
- In symbols: **KE = ½ m v²**, where m is mass (in kg) and v is speed (in m/s).
- The SI unit of energy is the **joule (J)**. One joule is the kinetic energy of a 2 kg object moving at 1 m/s.
- This formula applies when the object's speed is much less than the speed of light.
Kinetic Energy and Work
- **Work** is the energy transferred to an object when a force moves it over a distance.
- To increase an object's kinetic energy, you must do **work** on it (e.g., pushing a swing).
- The amount of work done equals the change in kinetic energy (work-energy theorem).
- When an object slows down, it does work on something else (e.g., brakes heat up).
Energy Transformations Involving Kinetic Energy
- Kinetic energy can be **transformed** into other forms of energy, and vice versa.
- Example: A cyclist pedaling converts **chemical energy** (from food) into **kinetic energy**.
- Coasting up a hill converts kinetic energy into **gravitational potential energy**.
- Braking converts kinetic energy into **thermal energy** (heat) due to friction.
- A generator on a bicycle converts kinetic energy into **electrical energy**.
Kinetic Energy is Relative
- Kinetic energy depends on the **frame of reference** of the observer.
- A person sitting on a train has zero kinetic energy relative to the train, but has kinetic energy relative to the ground.
- There is no single 'true' kinetic energy—it always depends on how you measure speed.
Real-World Examples
- A **car** moving at 60 km/h has more kinetic energy than the same car moving at 30 km/h (four times as much).
- A **bowling ball** rolling slowly has more kinetic energy than a **tennis ball** rolling at the same speed because it has more mass.
- **Spacecraft** gain huge kinetic energy during launch, which is converted to heat during re-entry.
- **Wind** (moving air) has kinetic energy that can be captured by wind turbines to generate electricity.
Key Takeaways
- Kinetic energy = energy of motion.
- Depends on mass and speed: KE = ½ m v².
- Speed affects KE more than mass (squared relationship).
- Energy is never lost, only transformed from one form to another.
Kinetic energy is related to particle motion: particles in a gas have more kinetic energy than in a liquid or solid.
Slides
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Practice questions
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1.What is kinetic energy?
Easy- AEnergy stored in an object due to its position
- BEnergy of an object due to its motion
- CEnergy released during chemical reactions
- DEnergy transferred by heat
2.Which factor does kinetic energy depend on?
Easy- AMass and speed
- BMass and height
- CSpeed and temperature
- DHeight and temperature
3.A car and a bicycle are moving at the same speed. Which has more kinetic energy?
Medium- AThe car
- BThe bicycle
- CBoth have the same kinetic energy
- DIt depends on the direction of motion
4.If the speed of an object doubles, its kinetic energy becomes:
Medium- ATwice as much
- BFour times as much
- CHalf as much
- DThe same
5.A 2 kg ball is moving at 3 m/s. What is its kinetic energy?
Hard- A3 J
- B6 J
- C9 J
- D18 J
6.The SI unit of kinetic energy is the:
Easy- ANewton
- BJoule
- CWatt
- DPascal
7.A moving object has kinetic energy. When it comes to a stop, the kinetic energy is:
Medium- ADestroyed
- BConverted to other forms of energy
- CStored as potential energy
- DIncreased
8.Two objects have the same kinetic energy. Object A has twice the mass of object B. How do their speeds compare?
Hard- ASpeed of A is half that of B
- BSpeed of A is √2 times that of B
- CSpeed of A is 1/√2 times that of B
- DSpeed of A is twice that of B
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