Key concepts
Physics
Engineering
Ratio
Stability
Forces

Introduction
If you’ve ever built towers with blocks, you’ve probably knocked some of them over—either on purpose or by accident. Sometimes your tower gets too tall and wobbly, and you just can’t keep it standing! In this activity you’ll learn about the trade-offs you need to make when building a tower that is tall and stable.

Background
What determines if a block tower will stay standing or fall over? Think about a single rectangular block placed horizontally on the ground. The block has a wide base relative to its height so it is very stable. If you push on its side with your finger, it will probably slide sideways but not tip over. Now take the same block and stand it vertically on one end. The base is now narrower relative to its height. If you push on the block near the top, it will probably fall over instead of sliding. What if you add more blocks? Two rectangular blocks stacked on top of each other horizontally will be much more stable than if you stack the same two blocks end to end vertically. The tower's stability depends on the ratio of its width to its height. This poses a challenge: Given a limited number of blocks, how can you build a tower that is as tall as possible—but also stable? (That is, it won’t tip over easily when you push it or blow on it.) In this project you will try it, and find out!

Note this same concept applies to real buildings. Very short structures, such as single-story houses, are generally very stable. Tall structures that get narrower as they reach the top, like pyramids, are also very stable. Tall, skinny structures such as skyscrapers can be much more susceptible to natural disasters including high wind and earthquakes, which can cause them to topple. It takes a lot of extra engineering to make sure they stay safe!

Materials

• Toy cardboard or wooden blocks, or an assortment of empty cardboard boxes—cereal, tissue, shoe boxes, etcetera (Do not use plastic snap-together blocks.)
• Pencil and paper
• Fan (optional)

Preparation

• Make sure you have enough open floor space to build a tall tower and that it will not break anything if it falls over.
• Gather all your blocks before you start. Count how many blocks you have and sort them roughly by size (small, medium and large).
• Make sketches of a few different block tower designs. Try to make the towers as tall as possible—but also plan for them to be stable so they won’t fall over easily.

Procedure

• Try building one of your tower designs. If the tower gets higher than you can reach, stand on a chair or ask an adult for help to add more blocks. Can you finish building the tower before it falls over?
• If you manage to finish building the tower, try gently tapping it with your finger in a few places. Does the tower fall over?
• If your tower fell over, think about how you can redesign it to be more stable. Do you need to sacrifice some height to accomplish this?
• If your tower did not fall over, push the limits! Can you make it even taller while keeping it stable?
• Keep iterating (redesigning and rebuilding) your tower until it is stable but you cannot make it any taller without making it unstable.
• Extra: Aim a fan at your tower to simulate wind. Does your tower stay standing?

Observations and results
Building the tallest possible tower may at first seem very simple—just stack all of your blocks vertically end to end, one on top of the other. That should give you the maximum possible height, right? Technically, yes, but it will also result in a tower that is extremely unstable because it’s too tall and skinny. It might fall over on its own before you’re even done building! Conversely, you can build an extremely stable tower by making it very short and wide—but then it won’t be very tall! The best solution probably lies somewhere in between, with a tower that is wider at its base than at its top. You probably had to go through multiple iterations of building your tower until you found the design that worked best. This is a normal feature of the engineering design process.

More to explore
Building the Tallest Tower, from Science Buddies
Earthquake-Proof Engineering for Skyscrapers, from Scientific American
Popsicle Stick Trusses: What Shape Is Strongest?, from Scientific American
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies