4 questions

[FunkyAce07]

1. If you are trying to climb to rafters thats are 20 feet up which is easier? A. Climb up 20 feet of rope tied off to the rafters on one end. B. Tie a rope around your waist, run the rope up 20 feet through a single pulley on the rafters and back down 20 feet and pull yourself up. 2. Using the stationary ground as a reference point. You have a track vehicle moving down the road at 15mph. The vehicle is mechanically fine and not sliding or spinning the tracks. How fast are the top and bottom of the track moving? A. Top: 15 bottom: 15. B. Top: 30 bottom: 0. C. Top: 15 Bottom: 0. 3. If you were on earth and traveled 89 degrees with a magnetic compass until you could no longer travel at 89 degrees where would you end up? 4. If a tree fell on the moon and no one was around to hear it would it still make a sound?

[Fuzzwood]

4. Yes. Lay your ear to the moon's surface when the tree impacts on the surface.

[FunkyAce07]

Good answer id agree w. That.

[Phi for All]

3. At one of the magnetic poles, where your compass might not reliably indicate where 89 degrees is anymore.

[md65536]

 

4. Yes. Lay your ear to the moon's surface when the tree impacts on the surface.

 

I disagree as I think that would count as being "around".

[TimeSpaceLightForce]

4. If a tree fell on the moon and no one was around to hear it would it still make a sound?

 

Yes , sound waves on solid!

To hear it though (if you are around)..

before the moon tree fall,

put some lump of gel on the ground

then press your ear on it

(so it contacts with your ear drum)

or simply press your helmet on ground

if you are wearing one.

 

2. Using the stationary ground as a reference point. You have a track vehicle moving down the road at 15mph. The vehicle is mechanically fine and not sliding or spinning the tracks. How fast are the top and bottom of the track moving? A. Top: 15 bottom: 15. B. Top: 30 bottom: 0. C. Top: 15 Bottom: 0

 

 

 

{_ _ _ _ _ _ _ _ Top <---15 rel to vehicle

{_o_o_o_o_o_o} <---15 rel to ground

Ground............Bottom --->15 rel to vehicle

 

 

 

Edited November 29, 2012 by TimeSpaceLightForce

[Phi for All]

1. If you are trying to climb to rafters thats are 20 feet up which is easier? A. Climb up 20 feet of rope tied off to the rafters on one end. B. Tie a rope around your waist, run the rope up 20 feet through a single pulley on the rafters and back down 20 feet and pull yourself up.

 

While the pulley spreads the amount of work over a longer time, making it seem easier, you've tied the rope around your waist so you're lifting that as well, so I'd say A is easier.

 

 

 

4. If a tree fell on the moon and no one was around to hear it would it still make a sound?

 

There are no trees on the moon. If there were, there's a negligible atmosphere for sound to travel in, so if no one is around to use another method of "hearing" the tree fall, I would say no, it doesn't make a sound.

 

[md65536]

 

While the pulley spreads the amount of work over a longer time, making it seem easier, you've tied the rope around your waist so you're lifting that as well, so I'd say A is easier.

 

How many feet of rope have you pulled in each case?

If with each pull of the rope, you pull 1 foot of rope, how many pulls does it take in case A and B?

What is the change in potential energy per pull of the rope in each case? Assuming negligible friction etc, the difficulty could be expressed in terms of the change in potential energy of each pull.

 

 

[Phi for All]

How many feet of rope have you pulled in each case?

If with each pull of the rope, you pull 1 foot of rope, how many pulls does it take in case A and B?

What is the change in potential energy per pull of the rope in each case? Assuming negligible friction etc, the difficulty could be expressed in terms of the change in potential energy of each pull.

With A you're not pulling any rope, you're climbing it. It's weight isn't added to the work you're doing. With B you're pulling the weight of at least half the whole rope because it's tied to your waist.

[md65536]

With A you're not pulling any rope, you're climbing it. It's weight isn't added to the work you're doing. With B you're pulling the weight of at least half the whole rope because it's tied to your waist.

I see... you're saying you've done more work with B because you've also lifted the weight of the rope around your waist. But this means that doing some amount of work is equally easy no matter how much time it takes. Bench-pressing 100 lbs once is equally easy as benching 10 lbs 10 times, and pulleys don't make anything easier. I disagree.

 

But even so, you haven't considered that as you approach the rafters, you have 20 feet of rope on one side of the pulley and an amount approaching 0 plus what's tied around your waist on the other side. If the rope was (extremely) heavy enough, you'd reach a point where the weight of the rope itself could pull you up to the top. I think that practically, the weight of the rope doesn't matter.

Edited November 30, 2012 by md65536

[FunkyAce07]

Alright, here are the answers I had : 1. (A) Rope weight and friction had nothing to do with it. One said which was easier doing 100 pounds once or 10 pounds 10 times and it would be the same if that were the case. However, it is one pulley which adds no mechanical advantage it only changes direction of the pull. The reason I choose A is that you will pull with the same force either way to pass the same amount of rope through your hands. Once you reach the top of the fixed rope you will have climbed 10 feet of rope versus 20 feet of rope.
2. (B) 30 top 0 bottom. If you used the vehicle as the reference for all speeds then it would be vehicle:0 Top:15 Bottom:-15 which explains why the track would not be ripped apart by being 30 on one side and 0 on the other.

3. North Pole. 0 degrees is North, 90 degrees is East, 180 South, and 270 West. Eventually you would simply go around and around until you reached the top where you would only be able to go 90-270 degrees.
4. At first my answer was no, because it lacks and atmosphere to have sound travel through. Then after reading comments I realized it would still make a sound due to vibrations in the ground from the impact you just wouldn't know without a seismograph. I think that could be counted as sound.

Edited January 6, 2013 by FunkyAce07

[John Cuthber]

Why do you think that only 1 pulley means there's no mechanical advantage?

 

Consider the case when I'm pulling myself up and my feet are off the ground.

The pulley is frictionless so the tension in both halves of the rope is the same.

The rope in my hands pulls up on my hands and the rope round my waist also pulls up on me. So the total force pulling me up is twice the tension in the rope, but that total force must also equal my weight.

So the tension in the rope is half my weight.

So I only have to pull half of my weight to get clear of the ground.

 

If I was just climbing a rope I would need to pull with a force equal to my whole weight.

The mechanical advantage of the pulley system is 2.