Why do the bristles on an electric toothbrush feel firm when it is turned on and they vibrate but when it’s turned off, you can feel that they’re really soft.

Question

Also, does this mean that if we vibrated something like cooked spaghetti it would feel firm?

In: Physics

Answer 1

Here’s my hot take, but I’m curious if anyone has more complete thoughts on this. The motor is creating motion through force. This force adds to the resistance you feel when you try to deform it. As you deform it, the motor is continuously moving the bristles back and forth. When it hits your finger you will feel that, making it feel stiffer. To bend it, you will also have to now resist the additional force of the motor trying to bring it back and force into position, increasing the stiffness you feel.

Answer 2

Why is it when you put your hand out the window air feels harder? Because your hand is moving relative fast to the wind. The same thing would happen if you blew fast moving air on your hand. It’d “feel” hard.

The feeling of “hard” is just how much is something pushing on your hand, how much give does it have, etc.

With the toothbrush, it’s moving so fast in a way that if you tried to touch it, it would push your hand away. That’s why it feels hard. When it’s off, it’s not moving, so it doesn’t push.

Technically, everything is pushing. Normal force. Even things that are staying still and you’re grabbing is “pushing” again your hand, causing the feeling of hardness/softness.