Friction

Friction is the force that resists the motion of an object that moves relative to the other. Scientists speculate that friction is the outcome of electromagnetic attraction among the charged particles of surfaces that touch each other.
Using floating rice, we can perform a simple friction experiment.

What you need:
• Raw grains of rice
• A funnel
• A plastic bottle
• A stick, pen, chopstick, or pencil

How you do it:
1. Get a plastic bottle to the lip with the raw rice grains. You can opt to use a funnel to fill it up more quickly.
2. Press on the rice as you fill the bottle so that the grains are packed tightly.
3. Get the dull end of the chopstick (or pencil, pen, or stick) into the grains of rice in the bottle. Press the chopstick into the grains until it reaches the bottom of the bottle.
4. Lift the bottle to see if the chopstick comes out. If it doesn’t, pull out the chopstick and repeat the procedure multiple times. Each time you attempt to try to push the chopstick in, it becomes more difficult.
5. Ultimately, you won’t be able to pull out the chopstick anymore. You will be able to lift the grain-filled bottle with the chopstick still embedded in it.

Analysis:
As you start to fill the bottle with raw rice, there are air pockets between grains inside the bottle. When you start to stab the grains with the chopstick, the grains of raw rice press against each other to provide space for it. The grains then become closer to each other, each time you push the chopstick in. With each repetitive stabbing of the chopstick, the grains rub together, decreasing and eliminating any air pockets. When this happens, the friction makes the chopstick adhere to the grains of rice.
The friction between the grains and the chopstick gains strength that holds the pencil right where it is. That’s why you were able to lift the rice-filled bottle by the chopstick.

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