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Physics in a Water Bottle: Ketchup Commander


Get ready to amaze your friends with this one! They will watch in awe as you use magical powers (ahem, physics) to command a packet of ketchup to rise, sink, and levitate....and it obeys you.

What you Need

  • a glass of water
  • a clear plastic water bottle with a cap
  • an obedient ketchup packet (see step one for assistance on selecting the right one for your experiment)

What to Do

  1. It is very important to select a ketchup packet that will float in water. Test a couple packets by placing them one by one in the glass of water. Choose the one that floats the highest at the surface of the water.
  2. Remove any labels on the plastic water bottle.
  3. Fill the plastic water bottle with water, leaving about an inch of air at the top.
  4. Fold the ketchup packet in half lengthwise and push it into the bottle. Double check that it floats.
  5. Screw the cap tightly back on to the bottle.

Now, it's time to command the ketchup! Holding the bottle upright in one hand, squeeze the bottle gently. What happens to the ketchup packet? Now, release the pressure of your squeeze and watch what happens. By experimenting with the amount of pressure you apply to the bottle, can you make the packet levitate in the middle of the bottle?

Now that you have mastered the ability to get the ketchup to sink, rise, and hover, find yourself an audience and get dramatic about it! Command the ketchup to move 'up', 'down', and to 'stop'. Your audience won't believe their eyes!

What’s Going On?

You are able to move the ketchup packet by using the principle of buoyancy and by using pressure to alter its density. Buoyancy is the upward force that keeps things afloat. Density is an object's mass per unit of volume. Objects that have a lower density than the liquid they are placed in will float, and objects that have a higher density than the liquid they are placed in will sink. We know that a penny will sink in a glass of water because it is more dense than water (it has more mass per unit volume than water does) and that a ping pong ball will float in water because it is less dense than water. It's a similar concept with the ketchup packet.

The ketchup packet has a small air bubble trapped inside that makes it less dense than water, allowing it to float. As you squeeze the bottle, the water pushes against the ketchup packet and compresses (or squishes) its air bubble into an even smaller air bubble. When the air bubble is compressed, the density of the ketchup packet is increased. Its weight is the same, but it now takes up less volume. It is now denser than water, so it sinks to the bottom of the bottle. When you release the pressure on the bottle, the air bubble expands back to its original size and the ketchup packet rises to the surface of the water. It is now less dense than water.

Why does an object that is denser than the liquid it is placed in sink? Gravity! The force of gravity acts on both the liquid and the object placed in it. It pulls either the object or the liquid -- whichever has a higher density-- toward the bottom.

Bonus Question:

Now that you're a buoyancy expert, it's time for a bonus question (the answer may surprise you)! Knowing that gravity is required for the principle of buoyancy to work, how do you think the ketchup packet would behave if you performed this experiment on the moon?

The answer is that it would behave similarly to how it does on earth. Many people are surprised to learn that the moon exerts its own gravitational force just like the earth does. The only difference is that the moon's gravitational force is not nearly as strong as the earth's is. This is why you would weigh less on the moon than you do on the earth (your weight would be 1/6 of what it is on earth) and why astronauts can jump a lot higher on the moon. If you were to drop a pen while standing on the moon gravity would cause it to fall to the floor, but more slowly than it would on earth. Because gravity is present, you would still be able to command the ketchup on the moon!

Try This!

  • Try doing the experiment with a mustard packet and a soy sauce packet. Does it behave the same way? Which one sinks fastest?

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