Forces and Materials

In this lesson, students build on their knowledge of forces by exploring the different forces that act on structures. They focus on five forces—tension, compression, bending, shear, and torsion—and investigate how each of these forces acts on different materials and shapes.

• Forces are acting on us all the time, even if we’re not aware of them.
• A structure is anything that is made up of parts and can support and withstand all of the forces that act on it.
  
  

• Why is gravity considered a force?
• When does gravity act on us?
• Why are the forces of gravity pulling down and the ground pushing up considered balanced?
• How can you apply a force to something?
• Can you use your body to illustrate how compression acts on a structure?
• How could you illustrate tension on your body? How could you illustrate bending with your body? How could you illustrate torsion with your body?
• How would you use a piece of paper to illustrate shear force?
  
  

Misconception: If an object is at rest, there are no forces acting on it.

Fact: There are forces everywhere. When an object is at rest, all of the forces acting on it are balanced.

Misconception: A force is necessary to keep an object moving.

Fact: An unbalanced force changes an object’s motion. An object that is already in motion will continue moving until acted on by an unbalanced force, such as friction.

Misconception: Only moving objects have energy.

Fact: Nonmoving objects have stored potential energy.

Bending : when forces cause tension to happen on one side of an object or material and compression to happen on the other side

Compression : when forces push the ends of an object toward each other

Material : any kind of matter that makes up objects

Shear: when forces push one part of a structure one way and another part the opposite way

Structure: anything that is made up of parts and can support and withstand all of the forces that act on it

Tension : when forces pull the ends of an object in opposite directions

Torsion : when forces cause an object to twist

Walking Across a Tightrope

Nik Wallenda loves walking on a tightrope. He first walked across a tightrope when he was 2 years old. He had to hold onto his parents’ hands. He began walking across tightropes as a job when he was 13.

He was the first person to walk a tightrope across Niagara Falls in 2012. One year later, he walked across a gorge near the Grand Canyon. He walks on a 5.1-centimeter (2-inch-thick) steel cable.

Different forces act on Nik as he walks across the tightrope. The force of gravity always pulls down on him. In response, the steel cable pushes back up. It pushes with an equal but opposite force. When this happens, the forces acting on Nik are balanced.

Tension on a Tightrope

Before Nik begins any tightrope walk, he thinks about where to attach his cable. The cable needs to attach to something on either end to hold it up in the air.

The cable needs to be pulled as tightly as it can be. This is to increase the tension on the cable. Tension happens when forces pull the ends of an object in opposite directions. Tension makes objects longer.

It is easier for Nik to walk across when the cable has a lot of tension. This is because it doesn’t sag very much. Sometimes the cable cannot be pulled as tightly across as Nik would like. When this happens, the cable will sag a lot in the middle. Nik trains so that he knows how to move on both kinds of cable.

For the hands-on activity of this lesson, students test different materials and shapes to see how they respond to different types of forces. Students select different materials to use as they experiment with the effects of different forces on those materials in different shapes.