AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Explain why, to change the shape of an object (by stretching, bending or…
Full specification wording
Forces and elasticity: explain why, to change the shape of an object (by stretching, bending or compressing), more than one force has to be applied – this is limited to stationary objects only
A single force would make a stationary object accelerate or move, not just change shape.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 19
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A single force would make a stationary object accelerate or move, not just change shape. To deform an object without moving it, the forces must balance overall, so at least two forces are needed. For a spring, one hand pulls one way and the other hand pulls the opposite way; the two forces are equal in size and opposite in direction, so the resultant force is zero and the spring stays in place while stretching. The same applies to compression, where two inward pushes balance, and to bending, where forces at different points balance. Because the object remains stationary, the forces must be balanced, and balanced forces require more than one force.
What earns the marks
- A single force on a stationary object gives a non-zero resultant force, so the object would move or accelerate rather than only change shape.
- Two or more forces are needed so that the resultant force is zero and the object stays stationary while deforming.
- In stretching, the two forces are equal in size and opposite in direction, for example two hands pulling a spring apart.
- In compression, two inward pushes balance; in bending, forces at different points balance to keep the object stationary.
- The explanation should link balanced forces to the object remaining stationary, not just state that two forces are applied.
Common mistakes and how to put them right
Claiming one force can change shape while the object stays still.
Put it right: One force would produce a resultant force and move the object.
Saying the two forces cancel so there is no force at all.
Put it right: The forces are balanced, so the resultant is zero, but each force still acts.
Forgetting to state that the forces are equal in size and opposite in direction.
Put it right: Both conditions are needed for balance.
Exam tips
- Use the phrase 'resultant force is zero' when explaining why the object stays stationary.
- State both conditions for balance: equal size and opposite direction.
- Apply the idea to a named example, such as two hands stretching a spring, to make the explanation concrete.
Try the lesson
The start of the Forces and elasticity: explain why, to change the shape of an object (by stretching, bending or compressing), more than one force has to be applied – this is limited to stationary objects only lesson, made from checked teaching notes for this specification point.
Have a go first
A rubber band is initially stationary and is then pulled at one end only. Why does the band not change shape?
Listen
A single force would make a stationary object accelerate or move, not just change shape. To deform an object without moving it, the forces must balance overall, so at least two forces are needed. For a spring, one hand pulls one way and the other hand pulls the opposite way; the two forces are equal in size and opposite in direction, so the resultant force is zero and the spring stays in place while stretching. The same applies to compression, where two inward pushes balance, and to bending, where forces at different points balance. Because the object remains stationary, the forces must be balanced, and balanced forces require more than one force. A strong answer earns marks for these points. A single force on a stationary object gives a non-zero resultant force, so the object would move or accelerate rather than only change shape. Two or more forces are needed so that the resultant force is zero and the object stays stationary while deforming. In stretching, the two forces are equal in size and opposite in direction, for example two hands pulling a spring apart. In compression, two inward pushes balance; in bending, forces at different points balance to keep the object stationary. A common slip: Claiming one force can change shape while the object stays still. Instead, one force would produce a resultant force and move the object. Exam tip: Use the phrase 'resultant force is zero' when explaining why the object stays stationary.
See it
Key words
Say what each word means before you turn the card.
Balanced forces
Try answering aloud or in your head before turning the card.
Deformation of stationary objects
Try answering aloud or in your head before turning the card.
See every definition
- Balanced forces
- Forces that are equal in size and opposite in direction, so the resultant force is zero and the object stays stationary.
- Deformation of stationary objects
- Stretching, bending or compressing needs more than one force so that the resultant force is zero and the object stays stationary while deforming.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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