Zion studied the spring-and-ball model.
When you push on an object, the object pushes back. This is the essence of “normal force,” which our young men studied in this week’s STEM lab.
A person standing on the ground is being pulled toward the earth by gravity, but the floor isn’t just lying there — it’s actually pushing back. A chair exerts the same force when someone is sitting on it, or a wall when someone is pushing on it.
In golf, we see normal force when the ball is at rest on the tee or the ground. Normal force is what keeps the ball from tumbling to the center of the earth! When a driver strikes a ball, the applied impulse (instantaneous force) momentarily adds to the normal force, which sends the ball into flight. When the ball falls back down to the ground and comes to rest, the earth’s gravity is again balanced by the normal force from the ground. The ball has reached equilibrium.
Why does normal force exist? Because solid objects store elastic energy, and solids follow a spring-like model, even when they appear to be as hard as a wood floor.
Our teachers, Tiffani and Veeshan, illustrated the concept with springs and sponges. The model above shows how the molecules in a solid interact with each other. The balls are the molecules, and the springs are the intermolecular bonds between them. When force is applied to the model, the bonds compress and distort and push back. When the force is removed, they return to their previous shape.
The stiffness of the bonds between the molecules determines the amount of normal force. A sponge has less rigid bonds than a table and is therefore easier to compress.
To illustrate this concept, Tiffani and Veeshan had the students order five springs from most to least stiff, and then think of an object with a similar stiffness to each spring. The stiffest spring might be like a rock, for example, while the middle spring might be like a firm mattress.