Explain the forces that stretch, bend or compress stationary objects, why shape changes require more than one force, and the difference between elastic and inelastic deformation.
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GCSE Physics Revision
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GCSE Physics revision
Forces and elasticity
Forces and elasticity
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AQA student objectives
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Apply the scientific explanation of forces and elasticity to a relevant example.
Analyse a new situation involving forces and elasticity and explain the scientific reasoning.
Revision summary
Key knowledge
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Applying Forces to Deform Objects
- When a force is applied to an object, it can cause the object to compress, stretch, or bend.
- At least two forces must act on an object for it to change shape without moving — for example, a spring pulled downwards is also held upwards by its support.
- Objects like balls or phones are less elastic than springs, so their deformation is harder to notice, but the same principles apply.
Elastic vs Inelastic Deformation
- Elastic deformation occurs when an object returns to its original shape after the forces are removed, like a stretched elastic band springing back.
- Inelastic deformation (also called plastic deformation) occurs when an object does not return to its original shape and remains permanently deformed after the forces are removed.
- Knowing whether deformation is elastic or inelastic is key to understanding how materials behave under force.
Understanding Extension
- Extension is the increase in length of a spring when it is stretched beyond its natural length.
- The natural length of a spring is measured before any external force is applied, though the spring's own weight causes a very small extension that is usually ignored.
- When a mass is hung from a spring, its weight pulls the spring downwards, increasing its length and producing a measurable extension.
Spring Constant (k)
- The spring constant, denoted by k , measures the stiffness of a material and is expressed in newtons per metre (N/m).
- A higher spring constant means the material is stiffer, as more force is required to stretch it by the same amount.
- The spring constant is specific to each object, so different springs will extend by different amounts under the same applied force.
Hooke's Law
- Hooke's Law states that the force applied to a spring is directly proportional to its extension, written as F = ke, where F is force (N), k is the spring constant (N/m), and e is the extension (m).
- This relationship only holds true within the elastic limit, meaning the deformation during this phase is elastic and the object will return to its original shape.
- Force and extension are described as directly proportional because doubling the force doubles the extension.
Force-Extension Graphs
- A force-extension graph plots force (N) on the y-axis against extension (m) on the x axis.
- In the region where Hooke's Law applies, the graph shows a straight line passing through the origin, confirming direct proportionality.
- The gradient of the straight-line section of the graph is equal to the spring constant, k .
The Elastic Limit (Limit of Proportionality)
- The elastic limit, also called the limit of proportionality, is the point beyond which Hooke's Law no longer applies.
- Beyond the elastic limit, the force-extension graph begins to curve, indicating that extension increases more rapidly for each additional unit of force.
- Past the elastic limit, the deformation becomes inelastic, meaning the object will not return to its original shape when the force is removed.