interesting physics

michael · 2004-01-13, 14:50

I saw a few physics problems already in this forum so here's another one.

What happens to a marble laying on the floor of a bus when the bus slows down? (no resistance from the floor)

What happens to a balloon filled with helium when the bus slows down? (attached with a rope to the driver's seat)

And why...

-michael

cheesehead · 2004-01-14, 17:37

Generally (with the usual set of sensible assumptions and idealizations), inside a slowing (decelerating) bus, unattached stuff (marble, balloon, air, passengers ...) will tend to move to the front of the bus. Where two or more different kinds of unattached stuff are in contact, the denser will tend to crowd out the less-dense and preferentially continue to move to the front.

Therefore, the marble, in contact with the air, will tend to roll toward the front while displacing air. The air, in contact with the helium-filled balloon (which is less dense than air), will tend to move toward the front while displacing the balloon, which will tend to float toward the back of the bus insofar as it can under restraint by the rope. Passengers will tend to lean toward the front insofar as not restrained by holding onto rigidly attached portions of the bus or pushing back with leg/arm muscles.

I remember the first time my sister and I took home helium-filled balloons from the circus. We were delighted that when our car turned, the ballons moved toward the inside of the turn rather than toward the outside of the turn as our bodies did.

2004-01-13, 14:50	#1
michael Dec 2003 Belgium 5·13 Posts	interesting physics I saw a few physics problems already in this forum so here's another one. What happens to a marble laying on the floor of a bus when the bus slows down? (no resistance from the floor) What happens to a balloon filled with helium when the bus slows down? (attached with a rope to the driver's seat) And why... -michael

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2004-01-14, 17:37	#2
cheesehead "Richard B. Woods" Aug 2002 Wisconsin USA 2²×3×641 Posts	Generally (with the usual set of sensible assumptions and idealizations), inside a slowing (decelerating) bus, unattached stuff (marble, balloon, air, passengers ...) will tend to move to the front of the bus. Where two or more different kinds of unattached stuff are in contact, the denser will tend to crowd out the less-dense and preferentially continue to move to the front. Therefore, the marble, in contact with the air, will tend to roll toward the front while displacing air. The air, in contact with the helium-filled balloon (which is less dense than air), will tend to move toward the front while displacing the balloon, which will tend to float toward the back of the bus insofar as it can under restraint by the rope. Passengers will tend to lean toward the front insofar as not restrained by holding onto rigidly attached portions of the bus or pushing back with leg/arm muscles. I remember the first time my sister and I took home helium-filled balloons from the circus. We were delighted that when our car turned, the ballons moved toward the inside of the turn rather than toward the outside of the turn as our bodies did.