The Physics Police

The Physics Police

Friday, October 18, 2013

Weight in an Elevator

According to their website, Science World is a Canadian nonprofit that engages people in science and inspires future science and technology leadership. They are famous for something called ambient marketing, where they put up entertaining and often funny posters and other types of media in an urban setting. Clearly, their goal is to get people's attention on science. I like it.

However, some of their media boast dubious scientific claims. I also found one that was simply incorrect. This scale was placed inside an elevator:

You weigh less on the way down.

The purpose of this seems to be drawing attention to the Elevator Problem. This physics thought experiment considers the effect of Newton's second law on the forces felt in an elevator. When you are accelerating upward, you feel heavier. When you are accelerating downward, you feel lighter.

This particular ad confuses velocity for acceleration.

Consider a trip from the 2nd floor to the 3rd floor. When the doors close, you are rest, using the building as a reference frame. You are also at rest, hopefully, before the doors open again. On this trip, the elevator had to accelerate upwards to begin moving, then decelerate, to stop.

This happens when you drive a car, too. Going from stop light on 2nd street to the stop light at 3rd street, you hit the gas to accelerate, drive one block, then hit the break to stop. When you hit the gas, if you drive anything like I do, you can feel your head pushed back into the headrest. When you hit the breaks, your head is pushed forward a bit.

Saying that "you weigh less on the way down" is like saying your head is pressed against the headrest going towards downtown. That's silly, because the same is true going the other way. It's not which direction you're moving, it's which direction you're accelerating!

On the way down, you weight less, then more.

On the way up, you weigh more, then less.

Since this bothered me enough to blog about it, clearly their marketing campaign is a success.

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