Have you ever wondered why sidewalks are made with so many cracks or lines through them?  Discuss your ideas with a friend.  Did you agree about why these cracks are present? Did either of you have a different idea about why a sidewalk has cracks?

## I'm stuck and can't get out!

Look closely at these experiments that explain why sidewalks have cracks in them.

In the first experiment we will look at what happens to a solid metal ball when it is heated.

1. Watch the Quicktime movie to see that the ball fits easily through the ring.
2. Next let’s heat the ball.

1. Watch the Quicktime movie to see if the ball still fits through the ring.

Analysis:

1. Why do you think the ball won’t fit?
 Drag your mouse to highlight the box below to check your answer! When the ball was heated, the particles in it began to move faster, making it expand.  The expanded ball no longer fits through the ring.  It won’t fit until the ball cools and the particles in it slow down.  When the particles cool and slow, the ball will contract and will again fit through the ring.

## Bending metal

In the second experiment we heat a metal strip. Watch the Quicktime movie closely.

Analysis:

1. Why do you think the metal strip bent when the strip was heated?
 Drag your mouse to highlight the box below to check your answer! When the strip is heated, the heated side of the strip expands more than the cooler side, making the strip curve.

Now can you answer the question about why there are sidewalk cracks?  The cracks are actually expansion joints.  They allow the sidewalk to expand and contract as it warms and cools.

What happens to concrete if expansion joints are not properly placed? When the concrete is heated, it expands and cracks randomly. Look closely at the two photos below to compare the need for expansion in materials.

Extension:

Look for other examples of expansion joints in buildings, highways, or other structures.  Make a multimedia presentation complete with digital photos to report your findings.

Review science safety rules here.

Get the plug-ins: , and . (The Quicktime plug-in is needed to play sounds and movies correctly.)

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