Core Practical: Investigating Force & Extension (Edexcel IGCSE Physics (Modular))
Revision Note
Core practical 2: investigating force & extension
Experiment 1: investigating force and extension for springs and rubber bands
The aim of this experiment is to investigate the relationship between force and extension for a spring and a rubber band:
Variables
Independent variable = Force, F
Dependent variable = Extension, e
Equipment list
Equipment | Purpose |
Clamp and stand | To apply an upward force to the spring and rubber band |
Ruler | To measure original length and extension |
Spring and rubber band | To measure the extension of |
5 × 100 g masses | To apply a downward force to the spring and rubber band |
100 g mass hanger | To hold additional masses |
Pointer (also called a fiducial marker) | To accurately read the extension from the ruler |
G-clamp | To secure the clamp stand to the bench so that the equipment does not fall over |
Resolution of measuring equipment:
Ruler = 1 mm
Method
Example set-up of the equipment used to investigate force and extension for a spring
Align the marker to a value on the ruler with no mass added, and record this initial length of the spring / rubber band
Add the 100 g mass hanger onto the spring / rubber band
Record the mass (in kg) and position (in cm) from the ruler now that the spring / rubber band has extended
Add another 100 g to the mass hanger
Record the new mass and position from the ruler now that the spring / rubber band has extended further
Repeat this process until all masses have been added
Remove the masses and repeat the entire process again, until it has been carried out a total of three times, and an average length (for each mass attached) is calculated
Example results table
Experiment 2: investigating force and extension for metal wires
The aim of this experiment is to investigate the relationship between force and extension for a metal wire
Variables
Independent variable = Force, F
Dependent variable = Extension, e
Equipment list
Equipment | Purpose |
Ruler | To measure original length and extension |
Spring, rubber band and metal wire | To measure the extension of |
5 × 100 g masses | To apply a downward force to the wire |
100 g mass hanger | To hold additional masses |
Tape (to use as a marker on the wire) | To accurately read the extension from the ruler |
Bench pulley | To allow the masses to hang vertically from the metal wire |
G-clamp & wooden blocks | To apply an opposing force on the end of the wire |
Resolution of measuring equipment:
Ruler = 1 mm
Method
Example set-up of equipment to investigate the force and extension for a metal wire
Set up the apparatus so the wire is taut with no masses added
Measure the original length of the wire using a metre ruler and mark a reference point with tape preferably near the beginning of the scale eg. at 1 cm
Record the initial length of the wire to the marker
Add a 100 g mass onto the mass hanger
Read and record the new reading of the tape marker from the meter ruler now that the metal wire has extended
Repeat this process until all masses have been added
Remove the masses and repeat the entire process again, until it has been carried out a total of three times, and an average length (for each mass attached) is calculated
Example results table
Analysis of results
The force, F added to the spring / rubber band / metal wire is the weight of the mass
The weight is calculated using the equation:
W = m × g
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, to calculate the force, F
The force can be calculated by multiplying the mass (in kg) by 10 N/kg
The extension e of the spring / rubber band is calculated using the equation:
e = average length – original length
The final length is the length of the spring / rubber band recorded from the ruler after the masses were added
The extension e of the metal wire is calculated using the equation:
e = new marker reading − reference point reading
The original length is the length of the spring / rubber band / metal wire when there were no masses attached
Plot a graph of the force against extension for the spring / rubber band / metal wire
Draw a line or curve of best fit
If the graph has a linear region (is a straight line), then the force is proportional to the extension
Evaluating the experiments
Systematic errors
Make sure the measurements on the ruler are taken at eye level to avoid parallax error
Random errors
The accuracy of such an experiment is improved with the use of a pointer (a fiducial marker)
Wait a few seconds for the spring / rubber band / metal wire to fully extend when a mass is added, before taking the reading for its new length
Make sure to check whether the spring has not gone past its limit of proportionality otherwise, it has been stretched too far
Safety considerations
Wear goggles during this experiment in case the spring, rubber band or wire 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 so hard that it breaks or pulls the apparatus over
Exam Tip
Remember - for the spring and rubber band, 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.
For the metal wire, each extension is measured by finding the difference between the new marker point and the original reference point.
A common mistake is to calculate the increase in length by each time instead of the total extension. If each of your extensions is roughly the same, then you might have made this mistake!
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