5.3.5Forces
Required Practical: Investigating Force and Extension — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 9 parts Lesson 17 of 44
Section 1 of 9Aim and key idea
Extension
Extension stretched length − original length.
Aim
- Aim find how the extension of a spring depends on the stretching force and where the relationship stops being simple.
Variables:
- Independent variable is the force (load) on the spring.
- Dependent variable is the extension of the spring.
- Controlled variables are the spring itself and the points between which length is measured.
Section 2 of 9Apparatus
1
A helical spring held by a clamp stand, boss and clamp.
2
Identical slotted masses (for example 100 g) on a hanger.
- The force is the weight F = mg, so each 100 g mass gives about 1 N.
3
A metre rule clamped vertically beside the spring.
4
A pointer, set square or marker to read the position without parallax error.
Section 3 of 9Method
1
Hang the spring from the clamp and measure its original length l0 between two clear points.
2
Add a load of 1 N and wait for the spring to stop moving.
3
Record the new length l and calculate the extension e = l − l0.
4
Add further masses one at a time, recording load and length each time up to about 8–10 masses, without overstretching.
5
Remove the masses one by one, recording lengths, to check that the spring returns to its original length.
6
Repeat and average if time allows.
Section 4 of 9Results and graph
Results and graph
Record load F (N), length l (cm) and extension e (cm) in a table.
Plot force on the vertical axis and extension on the horizontal axis.
For small loads the graph is a straight line through the origin
- For small loads the graph is a straight line through the origin extension is directly proportional to force.
Beyond the limit of proportionality the graph curves
- Beyond the limit of proportionality the graph curves the spring stretches more for each extra newton.
If the spring does not return to its original length, it has passed the elastic limit and is permanently deformed.
Section 5 of 9Processing the results
1
Draw a line of best fit through the straight section and the origin, with points scattered evenly on both sides.
2
Calculate the gradient with a large triangle.
- With force in N and extension in m, the gradient is the spring constant k in N/m.
3Conclusion
- Conclusion extension is directly proportional to force up to the limit of proportionality, which is Hooke's law.
4
Elastic bands give a curved graph even for small loads, so they do not obey Hooke's law.
Section 6 of 9Errors, improvements and safety
1Parallax when reading the rule
- Parallax when reading the rule place the rule close to the spring and use a pointer and set square.
2Oscillating spring
- Oscillating spring wait until it is still before reading.
3Measuring from different points
- Measuring from different points use the same two reference points each time.
4Non-identical masses
- Non-identical masses use the same set and add load in small steps.
5
A graph that misses the origin suggests the original length was not measured with the spring hanging freely.
6Safety
- Safety wear eye protection because a spring could snap back, and keep feet clear of falling masses.
Section 7 of 9More from the specification
1Hooke's Law and Elastic Extension
- The extension of an elastic object is directly proportional to the applied tensile force provided the limit of proportionality is not exceeded: F = kx,
- where k is the spring constant in N m−1.
2Resultant Forces and Acceleration Dynamics
- When the forces acting on an object are unbalanced, a non-zero resultant force causes a change in motion, accelerating the object in the direction of the net force according to F = ma.
3Friction as an Opposing Resistive Force
- Frictional resistance acts between contacting surfaces to oppose relative velocity, converting kinetic energy into internal thermal energy and bringing moving bodies to rest.
4Centripetal Force in Circular Motion
- When an object travels in a circular path at constant speed, its direction of motion constantly changes, requiring a resultant inward centripetal force to maintain the circular trajectory.
5Velocity
- Rate of change of displacement.
6Acceleration
- Rate of change of velocity.
7Friction
- Force parallel to the surface opposing relative motion or its tendency.
8Balanced
- Having equal and opposite moments.
9Constant speed
- Speed that does not change.
10Constant velocity
- Zero acceleration.
11Circular motion
- Motion along a circular path with a force towards the centre.
12Resultant force
- Another name for net force.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Extension | Increase in length over the original length. |
| Limit of proportionality | Point beyond which force and extension are not proportional. |
| Elastic limit | Greatest load a spring can recover from. |
| Original length | Length of the spring with no load. |
| Load | Force applied to the spring. |
Section 9 of 9Exam technique
1
Using the total spring length as the extension — subtract the original length.
2
Reading the rule at an angle — this causes parallax error.
3
Adding masses far beyond the limit — the spring becomes permanently stretched.
4
Mixing up mass and force — the force is the weight of the masses, F = mg.
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