Tag Archives: forces

Physics, forces and flipping

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Ever wondered what makes the perfect pancake flip? Let’s look at the physics behind getting those pancakes flying through the air.

If we start with our pancake at rest in the pan, the pancake will not move unless a force is put upon it. This is Newton’s first law of motion, an object at rest will remain at rest unless acted on by an outside force. In order to launch our pancake in the air we therefore apply a force to it. We can use an energy equation to work out the velocity (speed with a direction) needed to launch the pancake into the air as follows:

m – Mass of the pancake

g – Gravitational field of the earth 9.81 metres per second

h – Height of pancake flip

 v – Launch velocity

We now need Newton’s second law of motion to find out how fast we need to flip the pancake. Newton’s second law of motion states that a force, acting on an object, will change its velocity by changing either its speed and/or its direction. In the case of our pancake the flipping force will increase the velocity and send the pancake in an upwards direction. The energy equation can be rearrange to give the pancake launch velocity:

If we want to flip the pancake 1 metre in the air we need a launch velocity of 4.4 metres per second. If our launch velocity is over 6 metres per second however, our pancake will get stuck to the ceiling! We also have to be fast to catch the pancake as it falls back down or we could be left with pancake on the floor! For a flip of 1 metre we only have 0.9 of a second to catch our pancake. We can calculate the air time (t) of our pancake using the following equation:So when you are flipping your pancakes today think of all the wonderful physics behind that perfect flip!

#TryThisTuesday: Rice Bottle

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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!

The Solution

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!

#TryThisTuesday: Cup Drop

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For the week’s science demonstration, you will need a metal mug or screw, a pencil and string.

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  1. Tie one end of your string onto the handle of the mug and the other to your bolt.
  2. Hold onto the screw and pick up your pencil with your other hand.
  3. 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!

#TryThisTuesday: Book Pulling

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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.

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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!

World Egg Day: Eggsellent Experiments

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Happy world egg day! Here are some cracking eggsperiment that you can at home on this very important day:

Egg in a Bottle

For this experiment you will need a hard boiled egg, an empty plastic bottle, a scrap of paper and a lighter.

Light the paper and drop it into the bottle. After a second place the egg on top of the bottle and observe the results.

The lit paper heats up the air in the bottle, causing it to expand slightly and for some air to escape. The egg creates a seal so more air cannot enter. As the air cools inside the bottle it decreases the pressure and forces the egg into the bottle.

Floating Egg

All you need to try this one is an egg, a glass, water and salt.

Fill you glass half full with tap water and carefully place the egg inside. It should sink. Add some salt until the egg floats. The salt increases the density of the water, when you add enough the egg becomes less dense than the water so floats to the top.

Next dribble spoonfuls of tap water down the side of the glass until it is full. The egg should appear to float in the middle of the glass, it is actually floating on top of the salt water with a layer of fresh water above it.

Hard boiled Spin

Lay a hard boiled egg flat on its side and spin it. Put your finger on it to stop and then let go, nothing remarkably happens there. Try the same with a raw egg and when you let go it will start spinning again on its own accord.

This is all due to momentum. When you spin the eggs you spin their insides too. In the hard boiled egg, the insides are fixed to the shell so it behaves as you would expect. In the raw egg the insides continue to spin after you’ve stopped the shell. When you let go, the momentum of the spinning yolk carries the shell and the whole egg starts spinning again.

 

#TryThisTuesday: Floating Paperclip

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Can you make a paperclip float in water? Your standard metal paperclip isn’t very buoyant and as you would expect, tends to sink in water. But with just a piece of tissue paper and a pencil you can make a paperclip float…

  1. Place a small piece of tissue paper in the water so it floats
  2. Carefully put your paperclip on the tissue paper without you touching the paper
  3. Slowly use the pencil to push the paper down so it sinks – try not to touch the paperclip.
  4. You should be left with a floating paperclip – but is it really floating?


The science

Technically, the paperclip isn’t actually floating. It is held on the surface of the water by surface tension. Water molecules tend to attract one another and this forms a ‘skin’ on top where the water particles hold tightly together. This surface tension is strong enough to hold the paperclip. But if you poke the paperclip or shake the bowl, you break the surface tension and the paperclip will sink.

Pond skaters also use water tension to walk on water. They have evolved legs that distribute their weight evenly and so are adapted to life on the water’s surface.

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