Dec. 02, 2011

Stab a potato!

by Lynn Brunelle

In this Tabletop Science video, see how you can use the power of air pressure to stab a potato with an ordinary drinking straw.


Video by lynnbrunelle

Activity 1: Stab a potato

What you need

  • A raw potato
  • A plastic drinking straw

What You Do

    1. Hold the potato in one hand.
    2. Try pushing the straw into it. Does it work? Does the straw collapse?
    3. Now place your finger over the end of the straw and then jam it quickly through the potato.
    4. The straw goes right through!

What’s going on?

Why does this happen? The edges of the plastic drinking straw are very thin and because they’re thin, they make a good knife. Pushing the straw slowly into the potato doesn’t work because the sides of the straw are too flexible and they collapse. But when you place your finger over the end, you trap air molecules inside the straw, and those molecules compress under the pressure created when you jab the straw into the potato -- they keep the sides of the straw strong so the ends can cut through the potato.

Try it!

Try it with other fruits and vegetables. But do NOT try it on you -- it will cut into you, too.

Activity 2: The Mighty Plungers!

What happens when you take two old fashioned plungers and stick them together? Pulling them apart is a lot harder than you think.

What You Need

  • 2 old fashioned red rubber-wood-stick plungers
  • Water
  • Two strong people

What You Do

  1. Wet the rims of each plunger.
  2. Hold one plunger in your hands with the plunger part pointed away from you. Face a friend who his/her plunger facing you.
  3. Push them together hard enough that air escapes.
  4. Now try and pull them apart.
  5. Can you do it?

What’s Going On?

Air pressure is keeping the plungers together.

When you push the two plungers together, air is forced out of the cavity that the insides of the plunger bells make when pushed together. This reduces the air pressure inside the cavity.

The air pressure in the atmosphere surrounding the outside of the plungers -- 14.7 pounds per square inch (psi) -- is greater than the low pressure area you created between the two plungers. So the atmospheric pressure pushes in on the plungers keeping them together.

Activity 3: The Unwilling Wad of Paper!

Here’s something you’ll never be able to do no matter how hard you try. Air pressure will win EVERY time.

What You Need

  • Empty pop bottle
  • Small wadded up paper towel
  • A Deep breath

What You Do

  1. Place an empty plastic pop bottle with no cap on its side on a table.
  2. Wad up a small piece of paper towel to make a ball about ½ the size of the bottle’s opening.
  3. Place the ball in the mouth of the bottle.
  4. Now try to blow the paper into the bottle by using your breath.
  5. Try as hard as you can.
  6. Can you do it?

What’s Going On?

Why is this so impossible? The bottle isn’t empty. It’s filled with air molecules and those molecules are pushing out. By blowing, you're trying to force the paper -- along with more air -- into the bottle. There's nowhere for the air already inside the bottle to go, except back out the mouth of the bottle, taking the paper ball with it. It’s air pressure that makes this trick impossible.

Activity 4: The Old Air Pressure Upside-down, Don’t Spill a Drop Glass of Water Trick

Air pressure is everywhere. We have about a mile of air molecules above us at any given time pushing down, in, and up on us all the time. It’s the atmosphere. And it never stops pushing. The pushing is air pressure. You can see it in action in the following experiment, where you can flip a glass full of water upside down without spilling a drop.

What You Need

  • A drinking glass
  • Water
  • Sink
  • A playing card (The Ace of Spades or the Joker are always good choices for a magic trick.)

What You Do

  • Fill your glass up to the very top with tap water. The water should come right up to the edge of the mouth of the glass.
  • Hold the glass over a sink and place the playing card over the top. Hold the card in place firmly and turn the whole thing upside down.
  • Carefully remove your hand, leaving the playing card in place. (Don’t jiggle the card!)
  • The card stays put and the water stays inside the glass even though it’s upside down!

What’s Going On?

Turn a glass of water upside down and gravity pulls the water down every time. So, how does the playing card on the glass defy gravity? For one thing, there needs to be water to create a seal. Water molecules are built in such a way that they cling to one another. But mostly it’s all about air pressure. The air is pushing on us all the time. When you put a card over the water, it’s balanced. Pressure is pushing in and up and down. Flip it over and there is a balance. The pressure of the atmosphere is greater than the weight of the water so the card and the water stay put. But watch it! Bump the card and the water comes pouring out.

About Lynn Brunelle

The views expressed are those of the author and are not necessarily those of Science Friday.

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