Required practical - Force and extension Revision Notes for AQA GCSE Physics Combined Science

Required practical - Force and extension

What is this practical about?

This practical investigation helps you understand the relationship between force and extension when you stretch a spring. You'll discover how springs behave when different weights are attached to them.

infoNote

This is one of the most important practical investigations in physics because it demonstrates a fundamental principle that applies to many elastic materials, not just springs.

Aim of the investigation

The main goal is to find out how the extension of a spring changes when you add different weights to it. This helps prove that there's a linear relationship between force and extension.

Equipment you need

A spring
A ruler
A pointer (to help read measurements accurately)
Different weights or masses
A retort stand with bosses and clamps
Eye protection (safety goggles)

infoNote

The pointer is crucial for accurate readings - it helps eliminate parallax errors by giving you a clear reference point against the ruler.

Method - step by step

Setting up:

Clamp the spring vertically to the retort stand
Attach a pointer to the bottom of the spring
Position a ruler next to the spring
Measure and record the original length of the unstretched spring

Taking measurements: 5. Add the first weight to the spring 6. Wait for the spring to stop moving 7. Read the new position on the ruler at eye level 8. Calculate the extension using: $\text{Extension} = \text{new length} - \text{original length}$ 9. Repeat by adding more weights, one at a time 10. Record all your results in a table

chatImportant

Accuracy Tips:

Always wait for the spring to stop oscillating before taking readings
Take multiple readings and calculate an average to improve accuracy
Ensure the spring hangs vertically to avoid systematic errors

Important safety points

chatImportant

Essential Safety Requirements:

Always wear eye protection - springs store elastic energy and could snap back and damage your eyes
Read measurements at eye level to avoid parallax errors (where the reading looks different from different angles)
Check that weights are securely attached before releasing them

Understanding weight and mass

This concept is really important for your exam and often causes confusion:

chatImportant

Key Distinctions:

Weight is a force, measured in Newtons (N)
Mass is not a force, measured in grammes (g) or kilogrammes (kg)
Weight depends on gravitational field strength, while mass is constant everywhere

infoNote

Useful Conversion to Remember: 100g mass = approximately 1N weight on Earth (where g ≈ 10 m/s²)

This means: 50g → 0.5N, 200g → 2N, 500g → 5N

Plotting your results

Creating an accurate graph is essential for demonstrating the relationship:

Plot a graph with force (weight) on the x-axis and extension on the y-axis
You should get a straight line through the origin
This straight line proves there's a linear relationship between force and extension
You can use this graph to find the weight of unknown objects by measuring how much they stretch the spring

lightbulbExample

Example Graph Analysis:

Step 1: Plot your data points carefully

X-axis: Force/Weight (N)
Y-axis: Extension (cm or mm)

Step 2: Draw the best fit line

Should pass through (0,0) if no systematic errors
Ignore any obvious outliers

Step 3: Calculate the gradient

Gradient = spring constant (k)
Steeper gradient = stiffer spring

What your results show

Your investigation demonstrates several important physics principles:

There's a linear relationship between the weight applied and the extension of the spring
The extension is directly proportional to the weight (as long as you don't stretch the spring too far)
This relationship only works within the limit of proportionality - if you stretch the spring too much, it won't return to its original length

bookmarkSummary

Key Findings:

Linear relationship: Extension ∝ Applied force
Straight-line graph: Through the origin (if no systematic errors)
Hooke's Law verified: F = kx (where k is the spring constant)
Elastic limit: Beyond a certain point, the spring becomes permanently deformed

Key calculations

Understanding the mathematical relationships is crucial:

lightbulbExample

Essential Formulas:

Extension calculation: $\text{Extension} = \text{new length} - \text{original length}$

Spring constant from graph: $k = \frac{\text{Force}}{\text{Extension}} = \text{gradient of F-x graph}$

Hooke's Law: $F = kx$ where F = force (N), k = spring constant (N/m), x = extension (m)

bookmarkSummary

Key Points to Remember:

Weight is a force measured in Newtons, not mass measured in grammes
Extension = new length - original length
The relationship is linear - you get a straight line graph
Always wear eye protection when working with springs
Read measurements at eye level to avoid errors
100g mass weighs approximately 1N on Earth
The gradient of your graph gives the spring constant

Required practical - Force and extension (AQA GCSE Physics Combined Science): Revision Notes