Maxwell's Wheel

What Does A Scale Really Measure?

Why does the scale reading change when nothing was added or removed from it? The answer is that scales don’t measure weight (Earth’s gravitational pull on an object).

Instead, scales measure how hard they press on an object (the normal force exerted by the scale). For stationary objects on flat ground, this force has the same value as the object’s weight—the two must balance for the object to stay in place.

In contrast, the normal force the scale exerts is not equal to an object’s weight if the object is accelerating (speeding up, slowing down, or changing direction). The normal force must increase or decrease to cause this acceleration, so the scale reading changes when the wheel speeds up, slows down, or changes direction.

This is also why you feel heavier in a rising elevator and lighter in a descending one. Though your weight never changes, the floor pushes you with a normal force greater than your weight to accelerate you upward and a normal force less than your weight to accelerate you downward.