Why does a helium balloon in the car go backwards when you step on the brakes?

Dear Car Talk

Dear Car Talk | May 01, 1996

Dear Tom and Ray:

The other day my kids had a helium-filled balloon in the car. Whenever I
stepped on the brakes, all the passengers were pushed forward, but the
helium filled balloon went backwards. Why is this? -- Jeff

TOM: Great question, Jeff. I guess this was one of those laws of physics
that Newton never got around to writing.

RAY: The reason the helium-filled balloon goes backwards when you step on
the brakes is because everything else in the car -- including the air --
goes forward.

TOM: According to our buddy Isaac Newton, when your car is in motion and
you step on the brakes, your passengers and the air around them "remain in
motion" -- that is, they go forward. And when the air "goes forward," it
pushes the helium (which is lighter than air) out of the way, or toward the
back of the car.

RAY: It's the same reason we say "hot air rises." Hot air doesn't actually
"rise." Cold air sinks! Cold air is heavier, so it drops down toward the
floor and displaces the warmer, lighter air, which gets pushed up to the

TOM: The more interesting question, Jeff, is what happens when you suck in
the helium, and then -- in a helium-induced Mickey Mouse voice -- ask an
unsuspecting toll-taker to "take me to your leader, earthling."

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