Core Practical: Investigating Force & Extension (Edexcel IGCSE Physics)

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

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

1. Align the marker to a value on the ruler with no mass added, and record this initial length of the spring / rubber band
2. Add the 100 g mass hanger onto the spring / rubber band
3. Record the mass (in kg) and position (in cm) from the ruler now that the spring / rubber band has extended
4. Add another 100 g to the mass hanger
5. Record the new mass and position from the ruler now that the spring / rubber band has extended further
6. Repeat this process until all masses have been added
7. 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

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

1. Set up the apparatus so the wire is taut with no masses added
2. 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
3. Record the initial length of the wire to the marker
4. Add a 100 g mass onto the mass hanger
5. Read and record the new reading of the tape marker from the meter ruler now that the metal wire has extended
6. Repeat this process until all masses have been added
7. 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

1. Plot a graph of the force against extension for the spring / rubber band / metal wire
2. Draw a line or curve of best fit
3. 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