Quick Kitchen Science – Pressure and Area

This activity takes just a few minutes, but can be enjoyed by kids of all ages.  Older kids can take this party trick on the road to their next sleepover!

LEARN SCIENCE VOCABULARY:

Area – the amount of space in a 2-dimensional surface. In this demo, we are comparing area of circles (which is calculated by the formula A=∏r² if you remember back to Jr. High math class).

Force – any push or pull on an object.  In this demo, force is equal to the weight of the child.

Pressure – the amount of force when an object pushes on a given area.  (Pressure = Force divided by Area)  You can easily demonstrate this by pressing (gently, of course!) against your child’s hand, first with your thumb (small area of pressure) and then with your whole hand (larger area of pressure).

MATERIALS NEEDED:

  • 10 plastic cups (Dixie cups will also work, but you will need more since they are smaller)
  • Cardboard circle (I used the one that comes with a cheap-o frozen pizza)
  • Small to medium-size child

HOW TO DO IT:

  1. Ask your child if they think a plastic cup can support their weight if they stand on it.  Set the cup upside on the floor and let them try it.  Observe that the FORCE (their body weight) they applied to the cup caused it to break.

    20170401_210226.jpg
    The large pressure crushes one cup when he steps on it.
  2. Now talk about AREA.  “The cup has a small AREA, but if we can increase the AREA of the cups, do you think it can stand up to the FORCE of your body weight?”
  3. Count out 9 plastic cups and set them upside down on the floor in a square.

    20170401_210315.jpg
    Setting up 9 cups will increase the surface area!
  4. Take the cardboard circle and place it on top of the cups.  Have the child observe how the AREA has changed.  The FORCE they apply will remain the same (since their weight hasn’t changed).  They should make a hypothesis about what will happen when they try to stand on it again.
  5. Help the child carefully stand on top of the cardboard circle (it helps if they step towards the middle).  They should be able to stand on it without breaking the cups!

    20170401_210356.jpg
    Ta Da! The larger surface area reduces the pressure on each cup and holds up his weight.

THE SCIENCE BEHIND IT:

Because the area of a single cup is small, the pressure exerted when the child stands on the cup is very large.  When we put the cardboard circle over the 9 cups, the child’s weight is distributed over a greater area, so the pressure applied to each cup is reduced.

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