Forces
We are learning how to:
Understanding stretch and compression
• Explain the relationship between and applied force and the change of shape of an object. • Investigate the forces involved in stretching and compressing materials. • Identify applications for compressible and stretchable materials.
Imagine a mattress made of solid wood; clothes with no stretch; balls that don’t bounce. The world would be a very different place if materials and objects could not change shape when a force is applied to them. Comparing materials
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All materials can be squashed (for example, by a compression force) or stretched (for example, by a tension force) to some extent. Some materials change shape by tiny, unnoticeable amounts – even with extremely large forces. Some materials may change shape with a small force but then break. When materials return to their original shape after the force is removed, it is called elastic behaviour. 1. Name some materials or objects that can be noticeably compressed or stretched and show elastic behaviour.
FIGURE 2.1.3a: The angler benefits from elastic materials.
2. Name materials that show non-elastic behaviour when they are compressed or stretched.
Size of force and amount of deformation
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If you compress or stretch a material too far, it may not be able to return to its original shape – it remains deformed or it may break. In these situations the compressing or stretching force is beyond the elastic limit of the material. Materials that break with a relatively small force (only slightly beyond their elastic limit) are said to be brittle. 3. Name some brittle materials. 4. Look at the data in Table 2.1.3. This data can be displayed using a graph. List the features the graph should include. 5. Plot a line graph to display the data in Table 2.1.3. Describe what your graph shows about how the force applied affects the spring.
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TABLE 2.1.3: Data showing effect of force on a spring.
Force applied (N)
Length the spring has been compressed by (cm)
0
0
10
3.1
20
6.2
30
9.3
40
12.4
50
15.5
60
16.1
AQA KS3 Science Student Book Part 2: Forces – Contact forces and Pressure
SB2_Ch1.indd 12
06/02/2017 10:31