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Physics project - need help

  1. Feb 11, 2006 #1
    Physics project - need help!!!!

    I am building a physics machine that is supposed to use a hairdryer (Not sure what the Newton force of the hairdryer is) but it lifts 1 kg a certain distance in a certain length of time. You can do it in portions, like .5 kg once and then another .5 again. So far, ours is lifting nothing. We have it set up so that the hairdryer blows into cups, which are attached to a wheel. The axle rolls up string, which are connected to a series of four pulleys. The axle will turn as long as there is slack, but when hte slack runs out, it comes to a stop. Me and my physics partner have worked on it all day and we're becoming angry and violent. Do any of you have a suggestion? is there another simple machine we need to employ? Like I said before, all we can use for power is the uber physics hairdryer of pwn, which isn't very powerful. Project is due on Wednesday. Here is a crude drawing of it:


    Any help would be appreciated. Thank you for taking the time!
  2. jcsd
  3. Feb 11, 2006 #2


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    Well, there are better ways of doing this but i'd love to see you just mount the hairdryer onto the flywheel instead of cups. Can you use like.. 10 hair dryer motors? haha! Now that would be sweet! And probably the only way you could mount hairdryer motors to the thing and actually get propulsion.

    What are your limitations? Can you use anything else?

    I do remember doing something kind of like this back in my high school physics class that i bet i could adapt to such a project. What we did was assemble something that could be attached to a wire and we had the hairdryer motors mounted to this little device. Turn on the hairdryer and the fans propell the assembly across this wire. You could just attach a wire to the device and bring it over a pully attached to your weights and use the assembly as a propulsion device.

    One of the problems i see with your device is that the air pressure probably disperses fairly quickly. What would be better is just instead of cups, just have little propellers/walls setup so that it just immediately absorbs all the energy possible. Think of a spoke and wheel setup... without the actual wheel part hehe. Remember to keep the flywheel light and the pullies lubricated.
    Last edited: Feb 11, 2006
  4. Feb 11, 2006 #3
    I can only suggest that you add cups and use cups with a wide opening to catch more air. In your diagram it appears that the wheel portion runs through the middle of the cups. It would gain mechanical advantage if the cups were mounted on the outer edge of the wheel.

    Your pulley assembly must be arranged to obtain the most mechanical advantage possible. And the axles of the pulleys must not be binding. Make sure that the string runs as straight as possible from one pulley to the next to avoid any lateral loss of energy due to friction.
    Good old WD40 works wonders on moving parts.
    Last edited: Feb 11, 2006
  5. Feb 11, 2006 #4


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    Yah WD 40 does wonders. Just spray a bottle all over your project right now and theres a 90% chance it'll start working :rofl: :rofl: :rofl: :rofl: :rofl:

    You do enough mechanically intensive things and you'll want to marry wd-40.
  6. Feb 12, 2006 #5


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    What is your justification for using 4 pulleys? Let's see your computations on that part. Obviously, when you have the mass you are trying to lift, plus all of the weight of the string, plus the rotational inertia off the wheel-cups, you don't have enough power to move it all. What does that tell you? It should be telling you to try to reduce the mass in your structure so that more power goes to lifting the mass that needs to be lifted. Each pulley is going to add friction to equation as is the main wheel itself.

    You may also want to experiment with putting a nozzle of some sort over the end of the hairdryer. Are you sure that most of the flow out of the dryer is going to the cups? Do you think that some of it could be being wasted?

    One last thing you may want to look at is the diameter of the spool that the string winds up around.
  7. Feb 12, 2006 #6
    Our limitations are that there can be no outside power source but that one hairdryer, the Uber physics hairdryer of pwn. No other hairdryers are allowed. I can use any simple machines I want to facilitate the movement. Using four pulleys was my partner's idea. He thought that the more pulleys we added, the easier the weight could be lifted. I didn't like the idea; I think it just made a tangled mess. but the pulleys are attached to the frame of the machine so it's not like they're being lifted or anything. The spool connected to the axle is much wider than the axle itself. This was to set it up so that the wheel wouldn't have to turn much to do a lot of work. The cups were also his idea. He said the cups on wheels idea worked for his cousin's project so he's convinced they'll work for ours. The cups are on the outer rim, BTW, not on the edge of the wheel. I think a pinwheel would be a lot better idea. Are there any other simple machines that may facilitate the lifting?
  8. Feb 12, 2006 #7
    You will need for the spool to be as small as possible like FredGarvin suggested. The wheel has to turn a lot to do a lot of work.:smile: A smaller diameter spool is definetly a key factor.

    Think of riding a sixteen speed bicycle up a steep hill. You would want the smaller sprocket (spool)on the front turning the largest sprocket on the rear wheel.

    Try to lift some lighter weights and find out just how much your apparatus is capable of lifting. This can be a starting point to determine the ratios you will need if you keep using the pully approach.

    Assuming that the pulley system is capable of giving enough mechanical advantage , they will have to be in direct alignment with each other so that the string does no cantact any of the pulleys at an angle.
  9. Feb 12, 2006 #8


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    agent - attaching more pulleys does NO good if they don't move. A pulley that moves, a block and tackle if you will, means that you will need to move the rope further but use less force.


    Second, by increasing the diameter of the axle the rope is on does what do the force? It acts like a second class lever, it increases the FORCE needed on the pinwheel. Your hairdryer has a problem with adding enough force doesn't it? Why not make it easier for that small amount of force to move the rope by increasing its mechanical advantage. You want a smaller driven gear/pulley so its ratio is better. If the circumference is less, then yes more rotations are needed but at a much lower force to accomplish the same work.

    And more pulleys means even more rotations of the wheel to lift the load. Well, unless you find a more efficient wheel and a bigger power source, make it work first (even if slow) and then try to make it work faster. Fast=power so you need to find the best compromise, if you don't have much power you can only get stuff done so fast.

    Another idea is that you could, if within the rules, use a 1st class lever (teeter-totter) with a counter-weight that nearly offsets the 1kg. Now the pinwheel and pulleys only need to lift a small fraction of the 1kg, the difference between the two. This principle is used in elevators nearly everywhere, the counterweight is something close to half the rated capacity. That way if its half full, the electric motors only need to accelerate the mass and don't need to lift anything. But this could constitute cheating, so check to ensure its allowed.
  10. Feb 13, 2006 #9
    Thanks for the tips! Our machine is working much better now. Here is an update of the design:


    Me and my partner are rejoicing now. We can lift 1 kg 0.5 meters in 60 seconds, which is sure to give us an A. We're ready for anything Mr. M and his uber physics hairdryer of pwn can throw at us come Wednesday. Thanks again for the tips! We owe much of our success to you people.

    (BTW, the weight is actually rolling up the ramp on wheels. This has not been updated in the picture.)
  11. Feb 20, 2006 #10
    I havent read through all of the posts yet. But propulsion requires a lot more force than youd think because it is EXTREMELY inefficient.

    Proof. Ever see the episode of Mythbusters were they tried to build the 1 man flying craft. On paper it looked like it should work but the craft couldn't create into propulsion to lift itself in reality.

    Ideally you want to use the hair dryer motor like you would with a ferris wheel. Sit it on the side and let the axle turn a rope and move the weights along a thin rope loop.

    and keep the pulleys smaller. higher torque. which youre going to need.

    in summation. SCRAP PROPULSION.
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