Strength and Stability

This pathway is designed for students in the 3^rd - 5^th grade.

For more information on this topic, see: www.exploratorium.edu/structures

Curriculum standard addressed:

What makes a structure stand up? Why doesn't it fall down? What makes one structure stronger or flimsier than another one? What about the structure of our bodies? How come WE don't just fall down every time we try to walk? This pathway looks at some of the ways in which some objects stay put, or stay in one piece!

Can't find an exhibit or have a question? Ask an explainer

Catenary Arch

Read and Do:

- Does the completed arch seem strong or stable to you?

[The completed arch does NOT seem very strong or stable, but there is a hidden strength in it, as the following experiment should make clear]

More Experiments:

- Carefully lay a belt or a shoestring over the top of the arch. Now, gently pull evenly on both ends of the string or belt (try to apply the same amount of force on both ends), making sure the direction of your pull is along the length of the arch.

- What happens?

[The arch does not fall down!]

- What happens if you don't apply your force evenly?

[The arch might well buckle and fall apart]

- Can you build a vertical tower using blocks #1 through #10? How did you do it?

(Hint: Try #1 on the bottom, #2 on top with groove on other side, then #3 with groove side reversed again, etc)

[It's possible to do this, but not necessarily easy!]

GRAPHIC- diagram of forces?

Voussoir Arch Bridge

(Hint: Place the wooden supports underneath first, then build the bridge on top. Now pull out the blocks, and cross the bridge!)

Read and Do:

•  
• What is holding you up? Why don't the blocks fall down?
  

[The blocks that make up the bridge are "wedged in," supported by each other and by the "anchors" on both ends.]

More Experiments:

•  
• When is the bridge easier to topple- with or without someone standing on it?
  
• What effect does a sideways motion have on the bridge while you are standing on it?
  

[It's easier to topple when there is no body on it, because the weight of a person's body makes a "tighter fit" between blocks and between the blocks and the anchors. The friction force holding everything together is increased.]

[A sideways motion loosens up the blocks because their sideways motion is not counteracted by an upward force from the anchors]

- Where does the greater stability of the bridge come from?

[see above]

Bridge Beams

The effects of a force on something at a distance from it’s pivot or support point depend on the amount of force and on where the force is applied.

Read and Do:

•  
• Which beam supports your weight better? (You can draw it or describe it)
  

[The beam with the longer edge vertical, not horizontal]

More Experiments:

•  
• Which part of each beam seems to give the most support, the ends or the middle?
  

[Definitely the ends- you notice much less "sag"]

- Draw what happens when someone stands in the middle of a beam.

- Draw what happens when someone stands near the end of a beam.

Bone Stress

Read and Do:

Look at the shape of the bone.

- Where do you think lies it's greatest strength?

[It's thicker parts]

More Experiments:

•  
• Where are arm and leg bones most likely to break? Why?
  

[In their thinner parts]

- Why might they be thicker at the end?

[They receive most of their stress at their connections with other bones]

Did you know?

Bones grow along stress lines- this means that they grow as we need them, and they grow as strong as we need them.

- Have you ever broken a bone? Briefly describe the impact.

Take it From the Top

Try to make the top block stick out beyond the length of the bottom block. GRAPHIC

Read and Do:

- Is it easier with the small blocks or the large blocks?

[Depends on the person, but I think the smaller ones are easier to adjust]

More Experiments:

Try standing with your backside against a wall, then, without bending your knees, bend down toward your toes without removing yourself from the wall.

• What happens?
  

[Usually, one starts to tip over! This is because they are not able to keep their center of gravity over their feet, their "foundation."]

• Can you touch your toes (or anywhere near them)?
  

[Nope, not if they're really keeping their butt on the wall]

- Put the blocks on the floor and see what the tallest possible structure you can make with the thick ones, and then the thin ones. How many block-lengths tall is your structure? What do you think contributes the most to it falling down?

[GRAPHIC - different configurations of blocks showing different centers of gravity]

Balance the Stick

Notice that you can slide the weight up and down.

Read and Do:

- When is the stick easier to balance- with the weight high or the weight low?

[Though you might think its easier to balance with the weight lower, it's actually easier with the weight higher. This is because the weight is further from the pivot point, and so it falls more slowly. This quality of "angular momentum" is similar to why a figure skater slows down as they extend their body outward from the axis of their spin.]

Center of Gravity

"Friction" is how a car starts to move, and how a car stops moving...

Starting with an edge from each hand under each end of the cane, slowly bring your two hands together. Watch closely the points where your hands touch the cane.

Read and Do:

•  
• Do your hands move smoothly together, or with stops and starts?
  

[Stops and starts. This is because the weight over each hand is constantly changing as both move toward the center. More weight (mass * gravity) on one side means more friction needs to be overcome, and so the stop. When there is more weight on the other side, it starts again.]

- When your hands come together, are you balancing the stick?

[Hopefully!]

More Experiments:

-What do you think makes one hand stop while the other keeps moving?

[The greater force of friction on that side, due to the greater weight of that side of the stick]

- Try it with two people (using one hand each).

What happens?

[Should still work]