1.2 Squashing and stretching
Summary: Why don’t you fall through the chair you are sitting on? The chair changes shape when you sit on it. This produces the force that pushes you up. Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe how Forces deform (change the shape of) objects
- explain how solid surfaces provide a support force.
Think back
- What are the four ways you can apply forces to a solid?
- How do you change the length of a spring?
- What happens to a mattress when you lie on it?
Key idea Forces can deform (change the shape of), compress (squash), or stretch objects.
Key words deform, compress, stretch, support force, reaction force, extension, tension, elastic limit
Why don’t you fall through the chair you are sitting on? The chair changes shape when you sit on it. This produces the force that pushes you up.
How do forces affect solid objects?
When a golf club hits a golf ball, like in Figure 1, the ball changes shape, or deforms. Forces can compress (squash), stretch, twist (turn), or bend objects. When you apply forces to an object, you can deform it. Even solid objects can change shape.
Figure 1Even a solid golf ball is deformed when it is hit hard enough.
A Suggest what happens to the strings of a tennis racket when you hit a tennis ball.
How can the floor push you up?
The floor pushes up on you when you stand on it. It seems strange to talk about the floor exerting a force on you because you cannot see anything happening.
Figure 2 shows a model of a solid. It can help explain what is happening. In this model, the particles are held together with bonds that are like springs. You will use a different version of this model when you talk about solids in chemistry.
Figure 2Two or more forces on a solid compress or stretch it. Full text alternative to the content
Solid materials, like a chair or the floor, are only compressed a very small amount when you apply a force to them. The support force from a chair or the floor is called the reaction force or the normal force.
B Compare the compression of the floor when you stand on it with the compression of your bed when you lie on it.
What happens when objects stretch?
Figure 3 shows a bungee cord. Bungee cords, springs, and even lift cables (wires) all stretch when you apply a force to them. The amount that they stretch is called the extension.
Figure 3The shape of a bungee cord changes when you stretch it.
A bungee cord stretches as the jumper falls. When the bungee cord has stretched as far as it will go, it pulls them back up. This force is called tension.
Springs are special. If you double the stretching force acting on a spring, the extension will double as well.
At some point, the spring will not go back to its original length when you remove the force. This point is called the spring’s elastic limit.
Not all objects behave like a spring when you stretch them. If you double the force on an elastic band, the extension may not double.
C Compare tension and compression in terms of the directions that the forces act in.
Maths skills You have a spring that is 4 cm long. When you exert a force of 3 N on it, it stretches to a length of 6 cm. What is the extension?
Stretch zone
- Copy and complete the sentences. When objects deform, their colour/shape changes. In a model of a solid, we say solids are made of balls/particles connected by spring-like bonds. The bonds pull/push back on you when you compress them. This produces a force called the reaction/tension force. If you double the force acting on a spring, the extension doubles/halves.
- Describe how your chair pushes you up.
Stretch zone
- Describe the reaction force when a spring is extended (stretched).