Forces & Elasticity (AQA GCSE Physics): Revision Notes
Required practical - Force and extension
What you're investigating
You need to find out how the extension of a spring changes when you add different weights to it. This helps you understand the relationship between force and how much a spring stretches.
This practical investigation is fundamental to understanding elastic materials and forms the basis for studying spring constants and Hooke's Law in physics.
Equipment you need
- A spring
- A ruler or measuring tape
- A pointer (to mark measurements)
- Different weights or masses
- A retort stand with clamps and bosses
- Eye protection (safety glasses)
Safety Equipment is Essential Eye protection must be worn throughout this experiment as springs store elastic energy and can cause injury if they snap.
How to do the experiment
Setting up:
- Clamp the spring vertically to the retort stand
- Attach a pointer to the bottom of the spring
- Use the ruler to measure the original length of the spring before adding weights
- Position the ruler so you can read measurements easily
Taking measurements:
- Add weights one at a time to the spring
- After each weight, measure the new position of the pointer
- Calculate the extension using:
- Collect enough measurements to plot a good graph
Safety tip: Always wear eye protection. Springs store elastic energy and could snap back and hurt your eyes if they break.
Avoiding measurement errors: Read the ruler at eye level to avoid parallax errors. This means your eye should be level with the measurement mark for accurate readings.
Important facts about weight and mass
Critical Distinction - Weight vs Mass:
- Weight is a force measured in newtons (N)
- Mass is measured in grammes (g) or kilogrammes (kg)
- A mass of 100g creates a weight force of about 1N
- Remember: (approximately)
What your results should show
When you plot your results on a graph:
- Put force (in N) on the x-axis
- Put extension (in cm or m) on the y-axis
- You should get a straight line through the origin
This straight line shows there's a linear relationship between force and extension.
What this means
The spring extends by the same amount each time you add the same weight.
Worked Example: Spring Extension Pattern
For a typical spring, you might observe:
- Add 1N → spring extends 2cm
- Add another 1N → spring extends another 2cm
- Add another 1N → spring extends another 2cm
This consistent pattern continues until you reach the limit of proportionality - the point where the spring stops following this pattern.
This relationship demonstrates that extension is directly proportional to the applied force within the elastic range.
Mathematical skills needed
Essential Mathematical Requirements:
- Convert measurements from mm to m before calculating energy values
- State results in joules (J) for energy calculations
- Convert mass values from g to N for force calculations using
- Plot accurate graphs with properly labelled axes and units
Your conclusion
There is a linear relationship between the extension of a spring and the weight you apply to it. The extension is directly proportional to the weight, as long as you don't exceed the elastic limit of the spring.
This relationship only works within the spring's elastic range - if you add too much weight, the spring will be permanently damaged and won't return to its original length.
Key Points to Remember:
- Weight is a force (N), mass is measured in grammes
- The graph should be a straight line through the origin
- More force means more extension (until the elastic limit)
- Always wear eye protection when working with springs
- Extension is directly proportional to applied force within elastic limits