Welcome to "Work Done and Energy Transfer"!
In everyday life, you might say you are "doing work" when you sit at a desk and study. But in Physics, work has a very specific meaning. It involves moving things! In this chapter, we will explore how forces cause movement and how that movement transfers energy. Don't worry if this seems a bit abstract at first—we will use plenty of everyday examples to make it clear!
1. What is "Work Done"?
In Physics, work is done whenever a force moves an object over a distance. If you push as hard as you can against a brick wall but it doesn't move, you haven't done any "work" in the physics sense!
Simple Breakdown:
For work to be done, two things must happen:
1. A force must be applied to the object.
2. The object must move (displacement) in the same direction as the force.
Quick Review:
• Work = Energy being transferred by a force.
• No movement = No work done.
Key Takeaway: Work is just another way of saying "transferring energy using a force."
2. Calculating Work Done
We can measure exactly how much work is being done using a simple formula. You will need to remember and be able to use this in your exam.
The Formula:
\( W = F s \)
Where:
• \(W\) is work done, measured in joules (J)
• \(F\) is force, measured in newtons (N)
• \(s\) is distance (moved along the line of action of the force), measured in metres (m)
Units Matter!
One important thing to remember is that one joule of work is done when a force of one newton causes a displacement of one metre.
This means: 1 joule = 1 newton-metre (1 J = 1 Nm).
Memory Aid: The Triangle Trick
If you find rearranging equations tricky, use the triangle! Put \(W\) at the top, and \(F\) and \(s\) at the bottom.
• To find \(W\): Cover \(W\), you see \(F \times s\).
• To find \(F\): Cover \(F\), you see \(W / s\).
• To find \(s\): Cover \(s\), you see \(W / F\).
Key Takeaway: Use \( W = F s \) to find work. Always check that your distance is in metres and your force is in newtons!
3. Work Done and Energy Transfer
When you do work on an object, you are transferring energy to it. This is why the units for work done (Joules) are the same as the units for energy!
Where does the energy go?
Depending on the situation, the energy you transfer might end up in different "stores":
• Lifting an object: You do work against gravity. The energy is transferred to the object's gravitational potential energy store.
• Pushing a car: You do work to make it move. The energy is transferred to the car's kinetic energy store.
Did you know?
When you rub your hands together, you are doing work against friction. This "work" transfers energy to the thermal energy store of your hands, which is why they feel warm!
Common Mistake to Avoid:
Students often forget that work done against frictional forces always causes a rise in temperature of the object. Think of a car braking suddenly—the brake pads and tyres get very hot because work is being done against friction to stop the car.
Key Takeaway: Work done = Energy transferred. If there is friction, some of that energy will always turn into heat.
4. Step-by-Step: Solving a Work Problem
Let's try a practice scenario: A person pushes a crate with a force of 50 N over a distance of 10 metres. Calculate the work done.
Step 1: Write down what you know.
Force (\(F\)) = 50 N
Distance (\(s\)) = 10 m
Step 2: Choose the correct formula.
\( W = F s \)
Step 3: Plug in the numbers.
\( W = 50 \times 10 \)
Step 4: Calculate and add units.
\( W = 500 \text{ J} \)
Quick Review Box:
• Work Done: \( W = F s \)
• Units: Joules (J)
• Force: Newtons (N)
• Distance: Metres (m)
• Friction: Doing work against friction creates heat.
Key Takeaway: Always follow the steps: Identify values -> Formula -> Substitute -> Final Answer with Units.