ShareMatter - All Cracked Up!
Have
you ever wondered why sidewalks are made with so many cracks or lines
through them? Discuss your ideas with a friend. In what ways to
you agree that 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 a couple of experiments that will help 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.
- Watch the Quicktime movie to see that the ball fits easily through the ring.
- Next, let’s heat the ball.
Watch the Quicktime movie to see if the ball still fits through the ring.
Analysis:
- 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 would not longer fit 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:
- 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 it is time to return to the sidewalks mentioned at the beginning
of this page!
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.
