Onyx8 wrote:The answer is zero due to the people on the other side of the escalator descending to the lower floor at the same time...

Ah, see, you've assumed the existence of a descending escalator. But, even if it existed, it would be a separate device, not coupled to the ascending escalator.

LjSpike wrote:

[Reveal] Spoiler:

Thomas Eshuis wrote:

crank wrote:You'll have to make a lot of assumptions, like how many people at one time, avg weight of the people, and some loss to the friction and efficiency of the motor. Then it's just weight lifted a meter in time t. What is minimum, get someone to the top in less than 10 years?

What a terrible question.

I'd agree its a rather terrible question.
Supposedly exam boards are judging our ability to make reasonable assumptions and estimations.
I could only even begin to answer the question when doing research on the average speed of escalators, their average angle. Weight of a person, and so on.

There is however, a worse question in the book. I'll quote both part a & b, but its just part b that is the nasty one:

11. During a spacewalk, an astronaut, of mass 60kg, becomes detached from the spacecraft and is floating out into space. She has the presence of mind to throw a spanner, of mass 1kg, at a velocity of 4m/s
a. In which direction should she throw the spanner?
b. Calculate her velocity after she has thrown the spanner.

Now, you could argue that we are meant to make a reasonable assumption as to her velocity floating away from the spacecraft, as they haven't given us that, however I'm fairly sure there isn't a normal speed to be floating away from a spacecraft at. If your floating away from your spaceship and your only hope is a spanner, then "Houston, we really do have a problem". She could be floating away at 0.1m/s, 4m/s or 50m/s. Her velocity afterward could be anything at all!

The astronaut already has a wicked velocity, if she is in orbit around our planet.

The simple answer to part A is she should throw it in the direction of her drift away from the ship. What they really wanted to ask in part B was what is her delta-V after throwing the wrench.

Except that simple answer in part A wouldn't quite work, either. If she threw that wrench in the direction of her drift, and gave herself enough delta-v, she would reverse her drift away from the ship, but she would also raise her orbit and fly right past the ship in a higher orbit.

Actually, since the question doesn't specify what the astronauts goal is in throwing the wrench, there's no way to answer the question dispassionately. If she wants to die, the answer would be different. If she wants revenge on someone else, it would be different again.


Setting that aside, and assuming the goal was to increase the likelihood of survival, wouldn't it make sense to throw the wrench in a direction which would result in her slowing down, and therefore dropping into a slightly lower orbit?

And then again, since the problem was vague about what "into space" meant, relative to the craft she would want to return to, the "solution" is again, impossible.

if you're out in space, in orbit, with relative velocities and separation distances quite small, wouldn'tt a basic, non-orbiting estimation be more than adequate, meaing throw wrench in the direction she is moving wrt the ship? It's a good assumption that she wasn't hurled off the ship, she just accidentally detached somehow. It might not be best to use the CG of the ship, but the center of the angular extent from her viewpoint.

Well, she has one wrench. Miss a handgrip by two centimeters and it may as well be ten meters. Pretty dicey situation.


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igorfrankensteen wrote:
Setting that aside, and assuming the goal was to increase the likelihood of survival, wouldn't it make sense to throw the wrench in a direction which would result in her slowing down, and therefore dropping into a slightly lower orbit?

And then again, since the problem was vague about what "into space" meant, relative to the craft she would want to return to, the "solution" is again, impossible.

There you go! I hadn't considered implied factors. I think that's what these problems are for... get the student to consider all the variables. Like you say, it isn't solvable without more data. I was completely disregarding an orbit. I only imagined a spacecraft and an astronaut. You've reminded me she has an altitude, if they're in orbit. Acceleration due to gravity needs to be a major consideration. Her trajectory away from and back to the spacecraft would be parabolic, not a straight line.

If Sandra Bullock can do it. Then hypothetical astronaut can too.

scott1328 wrote:If Sandra Bullock can do it. Then hypothetical astronaut can too.

Yes, but Sandra Bullock is a hot babe. She's inherently extremely attractive. Hence, she could simply hang around, and Earth would come to HER.

We haven't been told in this problem, whether this particular astronaut is cute enough to survive or not.