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# Behaviour of Solids

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## Behaviour of Solids

(**Note: Plastic** behaviour is sometimes called **inelastic** behaviour.)

In the 1600s, a scientist called Robert Hooke discovered a law for **elastic** materials.

An **elastic** material is one that will return to its original shape when the force applied to it is taken away.

A **plastic **(or **inelastic**) material is one that stays deformed after you have taken the force away.

If you apply too big a force a material will lose its elasticity.

Hooke discovered that the amount a spring stretches is proportional to the amount of force applied to it. This means if you double the force its extension will double, if you triple the force the extension will triple and so on.

**Click on the weights below to see what happens:**

The **elastic limit** can be seen on the graph. This is where the graph stops being a straight line. If you stretch the spring beyond this point it will not return to its original shape.

**You can write Hooke's law as an equation:**

**F = kx**

where:

- F is the applied force (in newtons, N),
- x is the extension (in metres, m) and
- k is the spring constant (in N/m).

The spring constant measures how stiff the spring is. The larger the spring constant the stiffer the spring. You may be able to see this by looking at the graphs below:

**Elastic behaviour** is very important in car safety, as car seatbelts are made from elastic materials. However, after a crash they must be replaced as they will go past their elastic limit.

**Why have seat belts that are elastic?**

**Why not just have very rigid seatbelts that would keep you firmly in place?**

The reason for this, is that it would be very dangerous and cause large injuries. This is because it would slow your body down too quickly. The quicker a collision, the bigger the force that is produced.

This can be seen very plainly by comparing the effect of kicking a football, which squashes as you kick it giving a big collision time, followed by kicking a brick. The brick doesn't squash, giving a very quick collision time and a very painful foot.

**Press play to view this happening:**

This is why airbags and crumple zones can reduce injuries (these are both parts of a car designed to squash rather than be rigid).

So to reduce injuries in a collision, always slow down in as long a time as possible. This is why you bend your legs when landing after a jump and why parachutists roll when they hit the ground.