Work

1. Overview

Work is the measure of energy transfer that occurs when an object is moved over a distance by an external force. Understanding work is essential because it links the concept of forces to energy changes, allowing us to quantify how much energy is needed to perform physical tasks.

Key Definitions

• Work Done: The product of the force applied to an object and the distance moved in the direction of that force.
• Joule (J): The SI unit of work and energy. One joule of work is done when a force of one newton moves an object one meter.
• Energy Transferred: The amount of energy that changes from one form to another (or moves from one object to another) during a process.

Core Content

Work as Energy Transfer

In physics, "doing work" is the same as "transferring energy." If you do 50 J of work to lift a box, you have transferred 50 J of energy from your chemical store to the box’s gravitational potential energy store.

• Mechanical Work: Energy transferred by a force moving an object (e.g., pushing a car).
• Electrical Work: Energy transferred by a current (e.g., a battery powering a motor).

Calculating Work Done

To calculate the mechanical work done, we use the force applied and the distance moved. It is important that the distance used is specifically the distance moved in the direction of the force.

Worked Example: A crate is pushed with a constant force of 150 N across a warehouse floor for a distance of 10 m. Calculate the work done.

• $W = F \times d$
• $W = 150 \text{ N} \times 10 \text{ m}$
• $W = 1500 \text{ J}$ (or $1.5 \text{ kJ}$)

Extended Content (Extended Only)

There are no additional Supplement objectives for this specific sub-topic.

Key Equations

The Work Equation: $$W = F \times d$$

• $W$ = Work done (Joules, J)
• $F$ = Force (Newtons, N)
• $d$ = Distance moved in the direction of the force (metres, m)

Note: Since Work Done = Energy Transferred, you may also see this written as $\Delta E = F \times d$.

Common Mistakes to Avoid

• ❌ Wrong: Thinking that holding a heavy weight stationary is "doing work."
  • ✓ Right: Even if it feels tiring, if there is no distance moved, the work done is zero.
• ❌ Wrong: Forgetting to convert units (e.g., using cm instead of m).
  • ✓ Right: Always convert distance to metres and force to Newtons before calculating.
• ❌ Wrong: Squaring the distance or velocity as if you were calculating Kinetic Energy.
  • ✓ Right: The formula is simply $F \times d$. Do not square any numbers in the work equation.
• ❌ Wrong: Confusing Power with Work.
  • ✓ Right: Work is the total energy transferred ($J$); Power is the rate ($J/s$) at which that work is done.
• ❌ Wrong: Focusing only on the force applied without considering the distance.
  • ✓ Right: A large force over zero distance results in zero work; always look for the product of both.

Exam Tips

1. Check the Units: Examiners often give the distance in centimeters (cm) or the force in kilonewtons (kN). Convert these to meters (m) and Newtons (N) immediately to avoid losing easy marks.
2. Direction Matters: If an object moves at a right angle to the force (for example, carrying a box horizontally while the lifting force is vertical), no work is being done by that specific force on the object.
3. The "Work-Energy" Link: If a question asks for "energy transferred" and gives you force and distance, use the work formula! Students often get stuck looking for an energy formula when the work formula is the one required.