Floating Rice Friction
Friction is a force we hear about everyday and often don’t notice, but we can’t underestimate its power. In fact, we’re going to lift a bottle of rice with just the power of friction in today’s demo.
What You Need
- A empty plastic water bottle
- Uncooked rice
- A pencil
What to Do
Fill the empty bottle to the top with rice (tap the bottle against the counter to help the rice settle). Slowly push the pencil (point down) into the bottle, and slowly pull it up again. Repeat this motion; it will become increasingly difficult to push the pencil down. Eventually, you won’t be able to pull out the pencil anymore; you’ll be able to pick up the bottle with the pencil instead!
What's Going On?
When the rice is inside the bottle, there are grains next to one another, but there is a little bit of space — an air pocket — in between each grain and its neighbor. As you press the pencil into the bottle, the grains of rice are pushed together to make room for the pencil. As you continue to push the pencil in, the grains merge closer and closer together until they are rubbing against neighboring grains of rice.
This is when friction comes into play. Like when you rub your palms together, rice grains rubbing against each other feel a force resisting their motion: friction. Once the grains are packed so closely together that the friction force becomes overwhelming, they will push against the pencil with a strong enough force to render the pencil stuck, allowing you to pick up the whole bottle with the pencil.
What happens when you have more or less rice in the bottle? Does this experiment work if you use something besides rice — like jelly beans? Why or why not?
Where else do you see the influence of friction in your daily life?
- Aliya Merali