Forces and elasticity — AQA GCSE Physics
Test yourself on Forces and elasticity with AQA GCSE practice questions.
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Forces and elasticity explained
This equation links the force applied to a spring, the spring constant in newtons per metre and the extension in metres.
Read the full explanation
The spring constant is a property of that particular spring: a large value means a stiff spring that extends very little. Rearranged, k = F/e and e = F/k. A 3.0 N load on a spring of 25 N/m gives e = 3.0/25 = 0.12 m. Almost every lost mark here is a unit. Extensions are usually measured in centimetres or millimetres and must be converted to metres first, and a mass in grams has to be turned into a weight in newtons before it can be used as the force. The same relationship covers compression, where e is how much shorter the spring has become, which is why car suspension springs and bathroom scales obey it too.
Your focus
- Calculate the extension of a spring from a given force and spring constant.
- Rearrange the spring equation to find a spring constant from a measured force and extension.
- Apply the same relationship to a compressed spring by treating e as the compression.
Forces and elasticity exam tips
Marking Points
- one mark for the equation force = spring constant × extension, or a correct rearrangement
- one mark for converting the extension to metres, and a mass to a weight in newtons where needed
- one mark for the correct substitution
- one mark for the answer with its unit, N/m for a spring constant or m for an extension
Examiner Tips
- 💡Underline whether the question has given you a length or an extension before substituting.
- 💡If the question gives a mass, convert it to a weight first; the value of g is printed on the paper.
Common Mistakes
- substituting the total length of the spring instead of its extension
- leaving the extension in centimetres, so the spring constant comes out a hundred times too small
- using a mass in kilograms as the force instead of its weight in newtons
- assuming the equation fails for compression, when e is simply the compression