Our #TryThisTuesday this week, is a challenge for you. The task is to fill a dry bottle with rice and lift it up using only a pencil.
Have a go or challenge your friends, once you think you’ve cracked it (or given up) scroll down to see how we did it!
Take the lid off the bottle and push the pencil half way into the rice. Take the pencil out again and push it back in, repeat this about 10 times. Eventually, when you pull the pencil to take it out, the bottle will lift up with it!
This occurs due to the force of friction acting on the pencil and holding it in place. When you first pour the rice into the bottle, it will arrange itself with lots of gaps but every time you insert the pencil you push the rice down making it more compact or dense. Some grains may even break or change shape under impact with your pencil. The more you do this, the greater the surface area of rice that comes into contact with the pencil. This gives a greater force of friction. Friction is a force of resistance between two objects when they move past each other. The force is so strong at this point that it doesn’t allow the pencil to slip past the rice and so the rice (and the bottle) moves with the pencil as you lift it.
In the Real World…
This works in a similar way to quicksand. If you were to step onto quicksand, you would compact the particles, making them move closer together and lock around your foot, pulling you in. The friction makes it difficult for you to pull your foot out. Don’t worry too much though – quicksand is much denser than a human being so you wouldn’t be able to completely sink in it. As we learnt from our ketchup packet submarine and the oil and water experiment – less dense substances float above denser substances so you would stay above the surface of quicksand!
For the week’s science demonstration, you will need a metal mug or screw, a pencil and string.
- Tie one end of your string onto the handle of the mug and the other to your bolt.
- Hold onto the screw and pick up your pencil with your other hand.
- Lift up the string with the pencil and hold it about half way along the string, on the same level as the screw, allowing the cup hang down.
What do you think will happen from this position if you let go off the screw?
You may think that the cup will simply fall to the floor due to the pull of gravity and the string will pull the screw along, leaving you holding a pencil mid-air.
In reality, nothing (hopefully) hits the floor. You are right in thinking, gravity wants to pull the cup down, but it also wants to pull the screw down too. As the cup begins to drop it pulls the string, pulling the screw in towards the pencil, as the screw is being pulled from two directions it ends up swinging towards them. As it has a bit of weight behind it, it builds up enough momentum to go around the pencil a few times, wrapping the string around it.
So now the string is wrapped around the pencil and the cup still hasn’t dropped. If you try to pull the screw now, you’ll see why. It’s difficult to move the string. This is due to the force of friction. Friction is a force that occurs between two objects, it is the resistance that occurs when they move over each other. As the string is wrapped around the pencil a few times, there is a larger area of string touching the pencil, so a greater force of friction. This keeps the string in place to stop it sliding off, allowing the cup to hit the floor.
Try this out with your family and friends, see if you they can guess it correctly!
You’ve probably got exams coming up, maybe you’re supposed to be revising now, chances are you’re surrounded by textbooks. If so here is a quick little experiment you can try.
All you need is two large books with lots of pages, around 200 or so.
Start by interleaving the pages one on top of the other to sandwich the books together, like so:
This doesn’t require any kind of glue or tape but the two books should now be securely stuck together. Challenge your friends to try to pull the books apart – no matter how strong they are, they won’t be able to do it!
So if there’s no glue, why is this? It’s all because of friction. Friction is a force that occurs when one object moves over another – it is the resistance that is felt. When you try to pull the books apart there is friction acting on each page opposing the movement. If you consider there are over 200 pages, this force is multiplied and so becomes super strong!
When you pull the books the pulling motion squishes the pages in the middle with a greater force, this in turn makes the force of friction greater as it acts to oppose this force. So the harder you pull, the more difficult it is to separate the books!