Investigating Force & Extension (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Investigating Force & Extension
Aim of the experiment
The aim of this experiment is to investigate the relationship between force and extension for a spring
Variables
Independent variable = Force, F
Dependent variable = Extension, e
Control variables:
Spring constant, k
Equipment
Equipment list
Equipment | Purpose |
---|---|
Clamp & stand | To apply an upward force on the spring |
Ruler | To measure the extension of the spring |
Spring | To measure the extension of |
5 × 100 g masses | To apply a downward force on the spring |
100 g mass hanger | To hold additional masses |
Pointer | To accurately read the extension from the ruler |
Resolution of measuring equipment:
Ruler = 1 mm
Method
Equipment for investigating the extension of a spring
Align the marker to a value on the ruler with no mass added to the spring, and record this initial length of the spring
Add the 100 g mass hanger onto the spring
Record the mass (in kg) and position (in cm) from the ruler now that the spring has extended
Add another 100 g to the mass hanger
Record the new mass and position from the ruler now that the spring has extended further
Repeat this process until all masses have been added
The masses are then removed and the entire process is repeated again until it has been carried out a total of three times, and an average length is calculated
Example results table
Analysis of results
The force, F added to the spring is the weight of the mass
The weight is calculated using the equation:
Where:
W = weight in newtons (N)
m = mass in kilograms (kg)
g = gravitational field strength on Earth in newtons per kg (N/kg)
Therefore, multiply each mass by gravitational field strength, g = 9.8 N/kg, to calculate the force, F
The extension of the spring is calculated using the equation:
Extension = final length – original length
The final length is the length of the spring recorded from the ruler when the masses were added
The original length is the length of the spring when there were no masses
Plot a graph of the force against extension
Draw a line of best fit
If the graph has a linear region (is a straight line), then the force is proportional to the extension in this region
An example of a force-extension graph
Evaluating the experiment
Consider any systematic errors or random errors
Systematic errors:
Make sure the measurements on the ruler are taken at eye level to avoid parallax error
Random errors:
The precision of the experiment is improved with the use of a pointer at the bottom of the spring
Wait a few seconds for the mass to become stationary after it is added, before taking the readings for its length
Check that the spring has not gone past its limit of proportionality otherwise, it has been stretched too far and will no longer obey this relationship
Make sure the measurements are taken from the same point on the bottom of the spring every time
Safety considerations
Wear goggles during this experiment in case the spring snaps
Stand up while carrying out the experiment making sure no feet are directly under the masses
Place a mat or a soft material below the masses to prevent any damage in case they fall
Use a G clamp to secure the clamp stand to the desk so that the clamp and masses do not fall over
As well as this, place each mass carefully on the hanger and do not pull the spring too hard that it breaks or pulls the apparatus over
Examiner Tip
Remember - the extension measures how much the object has stretched by and can be found by subtracting the original length from each of the subsequent lengths.
A common mistake is to calculate the increase in length instead of the total extension – if each of your extensions is roughly the same then you might have made this mistake!
The proportional relationship between force and extension is known as Hooke's law. You do not need to remember the name of the law for your exam, but you do need to remember the relationship.
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