Jumping up and Down on a Train Going 60 Mph?
Most recent answer: 07/16/2012
- steve sanderson
Carlisle
LeeH
(published on 07/16/2012)
Follow-Up #1: How do objects move in accelerating frames?
- Lisa (age 34)
Jessup, md usa
Hey Lisa,
Interesting question. In general, in a decelerating frame objects act as if there's a force forwards on them. For this reason, when you slam on the brakes in your car, you are thrown forward and bang your head on the dashboard.
However, it's important to remember that everything in this frame gets thrown forward also. For example, flies and air molecules and helium balloons are thrown forward also. So what happens to a helium balloon in a decelerating train?
While you are thrown forwards, a helium balloon is actually thrown backwards when you slam on the brakes! This happens (even though there is a forward force on the balloon from the decelerating frame!) because the air around the balloon is heavier than the balloon itself, so when the air is thrown forward, it pushes the balloon out of the way (i.e. backwards). You can find cool videos of this on Youtube.
Aside: here's a slick way to prove what I just argued. Einstein's theory of equivalence says that a reference frame with acceleration can be modeled exactly by an inertial frame with a force of gravity in the opposite direction. We know that helium balloons move in the opposite direction as gravity (since they are lighter than air). So, if we slam on the brakes (accelerate backwards), we feel an "effective" gravitational pull towards the front, and so the helium balloon moves against this towards the back. Awesome!
So, what happens to a fly? Well, a fly is a lot heavier than an air molecule, so it flies forward just like a human.
Hope that long answer satisfies your curiosity!
David Schmid
(published on 08/22/2013)