What You Need:
- Your hands and legs
- A sheet of paper
To Do and Observe:
1. Have a seat and put your right hand on your right leg. Press down. Feel the force that you're putting on your leg by your hand.
2. Keep pressing down with your right hand. Now, put one finger from your left hand on your left leg. Press down with that one finger with the same amount of force as your whole right hand is doing. Does it feel different?
3. Now that we have looked at how we feel a pressure depends on an area as well as the force, let's look at how a sheet of paper reacts to different stresses.
4. Take the sheet of paper and hold the two corners that are on the same side of the paper. Pull each corner in opposite directions. Can you tear the paper?
5. Now make a small tear in the middle of the side of the paper. Then, try to pull the paper in half. Can you do it now?
What's Going On:
Stress isn't only what you feel when you're studying for a big test. Objects experience stress, too, a different kind of stress! Objects can't feel anxious when they have a deadline, but they do feel pressure, which is a form of stress. Pressure is simply defined as a force spread out over an area.
In the first part of this activity you used one hand and one finger to apply the same amount of force to your legs, and you felt a difference in pressure. The forces were the same, but the area with which you applied the force over was different. Your whole hand has a much greater area than one fingertip! Here's the equation: Force / Area = Pressure (Stress) As the area gets bigger, the pressure gets smaller. But if the force gets bigger, the pressure also gets bigger. If you were to press harder on your legs with your hand and finger, the pressure should increase on both legs.
In the second part of the activity, you explored how a sheet of paper is affected by different stresses. The first attempt to tear the paper was probably more difficult than the second. In the second attempt, you made a stress concentration in the paper when you made the tear. This means that you made an area in the paper where the area was much smaller and therefore the pressure or stress in the paper was much higher, and made it easier to tear!
So now you can see that pressure and stress aren't just experienced by people, but also by objects. You can cause something to feel much higher stresses by decreasing the area which distributes the forces.
Have the participants try to rip the paper in half without making the small tear in middle. The position of their hands should be closer to the center of the paper as they try to pull the paper in half. You can also have the participants try ripping at different positions to see which is the “easiest”.