A man sitting on a bus holding a red balloon on a string.When you’re on a bus, corners can push you sideways. But what does it do to a balloon? Grab a balloon and get on board to learn a thing or two about inertia.

You will need

  • A bus to ride on
  • Helium balloon with a string attached
  • Parent’s permission to catch a bus!

What to do

  1. Hold onto the string of the balloon.
  2. Get on a bus. Remember to ask the bus driver if it’s okay to have a balloon when you hop on.
  3. A man on a bus holding a red balloon on a stringMake sure you keep hold of the string so the balloon doesn’t get in the driver’s way … or in the faces of other passengers!
  4. What happens to you when the bus accelerates?
  5. Man sitting on a bus with a red balloon on a string. The balloon is leaning on an angle.What happens to the balloon when the bus accelerates?
  6. What happens to you when the bus corners?
  7. What happens to the balloon when the bus corners?


What’s happening?

You’re probably familiar with the feeling of a car, bus, or train speeding up. As the vehicle starts moving, an invisible force seems to push you back into your seat.

But if you have a helium balloon, it reacts in precisely the opposite way. It moves forward instead, against the acceleration!

One way to understand this strange observation involves Einstein’s ‘equivalence principle’. When he was working on his theory of relativity, Einstein concluded that if you closed your eyes there was no way to tell the difference between the tug of gravity and the tug you feel when you’re changing speed.

So when you’re on an accelerating bus, it’s like there is a small planet behind the car pulling you towards it. While it’s pulling you back into the seat, the air is also pulled back. Just as a helium balloon floats up to where the air is thinner in the sky, it floats ‘up’ to the front of the bus, where the air has thinned slightly.

Whenever the bus turns a corner, we can say it’s accelerating in a new direction. Keeping the equivalence principle in mind, this is why you get pushed to the side of your seat, and why the balloon floats in the opposite direction – acceleration is like gravity’s strange twin.

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3 responses

  1. Leci Avatar

    That was a good one!!!

  2. Imogen, 9 Avatar
    Imogen, 9

    This is fun to try!

  3. Bonnie Avatar

    That was awesome!

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