Welcome to the World of Work!
In everyday life, "work" might mean doing your homework or going to an office. But in Physics, work done has a very specific and exciting meaning. It is all about how energy is moved from one place to another. By the end of these notes, you’ll understand how energy is stored, how it changes, and how to calculate exactly how much "work" is being done in different situations. Let's dive in!
1. What Exactly is "Work Done"?
In Physics, work is done whenever a force makes an object move over a distance. If you push as hard as you can against a brick wall but it doesn't move, you might feel tired, but in Physics terms, you have done zero work! No movement means no work.
The Rule of Thumb: To do work, you need a force and you need movement in the same direction as that force.
The Formula for Work Done
You can calculate work done using this simple equation:
\(Work \text{ } done \text{ } (J) = Force \text{ } (N) \times Distance \text{ } (m)\)
Key Units to Remember:
• Work Done is measured in Joules (J).
• Force is measured in Newtons (N).
• Distance must be in metres (m) (along the line of action of the force).
Memory Aid: Think of W-F-D — "Work Finds Distance."
Quick Review: Units
Did you know? 1 Joule is exactly the same as 1 Newton-metre (Nm). If you see "Nm" in a question, don't panic! It's just another way of saying Joules.
Takeaway: Work done is just another way of saying "energy transferred." When you do work on an object, you are giving it energy!
2. The Law of Conservation of Energy
This is one of the most important rules in all of science. It states that energy cannot be created or destroyed. It can only be transferred from one store to another.
Closed Systems:
In a closed system (a fancy way of saying a group of objects where nothing can get in or out), the total energy always stays the same. Even if energy moves from a battery to a motor to heat, if you add it all up at the end, it’s exactly what you started with.
Analogy: Imagine you have £20 in your pocket. You move £5 to your wallet and £15 to your bag. You have "transferred" the money, but the total amount you have is still £20. Energy works the same way!
3. Energy Stores and Changes
When a system changes, energy moves between different "stores." The syllabus identifies several common situations you need to be able to describe:
- An object projected upwards: As a ball is thrown up, Kinetic Energy (movement) is transferred into Gravitational Potential Energy (height).
- A moving object hitting an obstacle: The Kinetic Energy of the object is transferred into Thermal Energy (heat) and Sound Energy as it hits the obstacle.
- An object accelerated by a constant force: For example, a car engine doing work. Chemical Energy (fuel) is transferred into Kinetic Energy (speed).
- A vehicle slowing down: When a car brakes, Kinetic Energy is transferred into Thermal Energy in the brakes due to friction.
- Boiling water in a kettle: Electrical Energy from the mains is transferred into Thermal Energy in the water.
Takeaway: Whenever you see something happen, ask yourself: "Where was the energy at the start, and where did it go?"
4. Calculating Energy in Different Stores
Don't worry if these formulas look scary at first! With a bit of practice, they become much easier. Here are the three main ones you need to be able to use:
A. Kinetic Energy (\(E_k\))
This is the energy of a moving object. If it’s moving, it has \(E_k\).
\(E_k = \frac{1}{2} \times mass \text{ } (kg) \times (speed \text{ } (m/s))^2\)
Common Mistake: Students often forget to square the speed! Always do the speed squared first.
B. Gravitational Potential Energy (\(E_p\))
This is the energy an object has because of its height.
\(E_p = mass \text{ } (kg) \times gravitational \text{ } field \text{ } strength \text{ } (N/kg) \times height \text{ } (m)\)
Note: On Earth, the gravitational field strength (\(g\)) is usually taken as \(10 \text{ } N/kg\).
C. Elastic Potential Energy (\(E_e\))
This is the energy stored in a stretched or squashed spring.
\(E_e = \frac{1}{2} \times spring \text{ } constant \text{ } (N/m) \times (extension \text{ } (m))^2\)
Quick Review Box:
Check your units! Mass must be in kg (not grams), and distance/height/extension must be in metres (not cm or mm). If the exam gives you 50cm, change it to 0.5m immediately!
5. Work Done and Current Flow
Work isn't just done by pushing things with your hands; it's also done when electricity flows! When a current flows in a circuit, work is being done by the power source on the charge carriers.
You may be asked to calculate energy changes in electrical appliances. Remember that energy is measured in Joules (J), but for home electricity bills, we sometimes use kilowatt-hours (kWh).
• 1 kWh is the energy used by a 1kW appliance running for 1 hour.
Step-by-Step for Calculations:
1. Write down the values you know from the question.
2. Convert any units (e.g., cm to m, or grams to kg).
3. Pick the right formula.
4. Plug in the numbers and calculate!
5. Don't forget to add the unit (usually Joules) at the end.
6. Common Pitfalls (How to keep the examiner happy!)
1. The "Distance" Trap: The distance in \(W = F \times d\) must be in the direction of the force. If you carry a heavy box while walking horizontally, you aren't doing "work" against gravity because the lifting force is upwards, but the movement is sideways!
2. Squaring Numbers: In the Kinetic Energy formula, only the speed is squared, not the mass or the half!
3. Energy "Loss": We often say energy is "lost." In Physics, we should say it is dissipated (spread out) to the surroundings, usually as heat. It’s not gone; it’s just not useful anymore.
Takeaway: Be precise with your language. Use "transferred" instead of "moved" and "dissipated" instead of "lost."
Summary: The Big Ideas
- Work Done = Energy Transferred. They are both measured in Joules (J).
- Force \(\times\) Distance = Work Done.
- Energy cannot be created or destroyed, only shifted between stores (Conservation of Energy).
- A closed system has no net change in total energy.
- Always check your units (kg, m, s) before starting a calculation!