View Full Version : Pysics Question

07-05-06, 02:47
A man is eating an apple while standing in an elevator at the 25th floor of a skyscraper. The doors close and the elevator begins to travel down. The cable holding the elevator suddenly breaks and the elevator picks up speed as it heads to the ground floor. The man is startled and lets go of the apple. What happens to the apple when he lets go of it? What happens to the man? Why can't he jump up from the floor at the last split second to save himself?

07-05-06, 02:50
Consider air friction is negligible...when the elevator break, normal force is 0, so all forces applied to the apple, man and the elevator are the force of gravity...which means they all share the same acceleration...thus maintain the same position...

As for the second part of the question...I'll throw in a guess...the man and the elevator share the same acceleration...thus the man cannot really push himself against the floor of the elevator when he is experiencing freefall...

07-05-06, 02:53
Yes, good explanation, Lost_Raider. I agree. The man, the apple and the elevator all fall at a constant rate, all at once.

07-05-06, 02:53
Yes, they are all in free fall, so even if the man lets go of the apple after having entered free fall, the apple will stay at the same position relative to the man and the elevator. They all had the same initial velocity, anyway.

07-05-06, 02:54
free body diagram anyone...?

07-05-06, 02:55
Why can't he jump up from the floor at the last split second to save himself?

I would think the force the man would apply to the elevator floor would just add to the force pulling the elevator down, instead of getting the man to jump. Anyway, 25 stories... the man would die and the apple would be eaten by me.

07-05-06, 02:55
Very good. The apple and the man - and the elevator would be weightless and float until the earth got in the way and spoiled the party - at about 120 mph. You feel this sometimes in an airliner during turbulence.

07-05-06, 02:57
Whatever...I'm studying all those stuff just for mathmatical logic...I'll never understand my university...

07-05-06, 03:00
Since the apple has a small surface are you can leave out air resistance as it would not have mcuh effect on it, but if the object was the size of a football then the opposing force "air resistance" would have an effect on the whole situation...

07-05-06, 03:01
Yeah, then it will reach a constant speed or what not...

07-05-06, 09:30
He would have to jump at the same speed as what the elevator is travelling to save himself

07-05-06, 09:36
How high could you jump with your head already touching the ceiling?.

Thats how it would feel freefalling in the elevator!, the force of gravity would not allow you to make that last second jump.:p

07-05-06, 09:42
Here's my guess. Don't take any of this as fact please because it's probably wrong. :p

I think it would be the same as an astronaut using another astronaut as a kickpad to move about in the spacecraft's cabin - the elevator acts like the kickpad astronaut and the ground is the walls of the cabin. The astronaut [man] wouldn't get very far by pushing against the other astronaut [elevator floor] because the force of his jump would be half absorbed. So if the man tried to jump at the last second, and ignoring the fact he'd have to be better than an Olympic high-jumper to cancel out the force of his fall by jumping, he'd only end up creating a slight distance between himself and the elevator floor - he couldn't change the speed of his fall.

07-05-06, 10:19
Very good.

Is lost Raider correct?:confused: