Work and Energy
Concept-Based Learning
Concept 1: Work
In physics, work is defined as the product of the force applied to an object and the distance the object moves in the direction of the force.
Where θ is the angle between the force vector and the direction of motion.
Example: If you push a box with a force of 10 N for a distance of 5 m in the same direction as the force, the work done is:
W = 10 N × 5 m × cos(0°) = 50 J
Key points about work:
- Work is a scalar quantity (has magnitude but no direction)
- The SI unit of work is the joule (J)
- No work is done if there is no displacement
- Work can be positive or negative depending on the direction of force relative to displacement
Concept 2: Kinetic Energy
Kinetic energy is the energy possessed by an object due to its motion. Any object that is moving has kinetic energy.
Example: A 2 kg object moving at 3 m/s has kinetic energy:
KE = ½ × 2 kg × (3 m/s)² = ½ × 2 × 9 = 9 J
Key points about kinetic energy:
- Kinetic energy depends on both mass and velocity
- It's a scalar quantity
- The SI unit is joule (J)
- Kinetic energy is always positive or zero
- It represents the work needed to accelerate an object from rest to its current velocity
Concept 3: Potential Energy
Potential energy is the energy stored in an object due to its position or configuration.
The most common type is gravitational potential energy, which depends on an object's height above a reference point:
Example: A 5 kg object lifted to a height of 2 m has potential energy:
PE = 5 kg × 9.8 m/s² × 2 m = 98 J
Other forms of potential energy include:
- Elastic potential energy: Stored in stretched or compressed springs
- Chemical potential energy: Stored in chemical bonds
- Nuclear potential energy: Stored in atomic nuclei
Concept 4: Conservation of Energy
The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another.
Example: A roller coaster at the top of a hill has maximum potential energy and minimum kinetic energy. As it goes down, potential energy converts to kinetic energy.
At the top: PE = max, KE = min
At the bottom: PE = min, KE = max
Total energy remains constant (ignoring friction)
Key points about energy conservation:
- Energy transformations often involve some energy loss as heat due to friction
- In closed systems, total energy remains constant
- This principle allows us to solve many physics problems without knowing all the details of the process
Concept 5: Power
Power is the rate at which work is done or energy is transferred. It measures how quickly work is completed.
Since work is energy transfer, power can also be expressed as:
Example: If a machine does 100 J of work in 5 seconds:
P = 100 J / 5 s = 20 W
Key points about power:
- The SI unit of power is the watt (W), where 1 W = 1 J/s
- Power is a scalar quantity
- Higher power means work is done faster
- Common power units include horsepower (hp) where 1 hp ≈ 746 W
Learning Complete
Key Concepts Covered:
- Work: Force applied over a distance
- Kinetic Energy: Energy of motion
- Potential Energy: Stored energy due to position
- Conservation of Energy: Energy cannot be created or destroyed
- Power: Rate of doing work
