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Homework Help: Joule Mobile Mass Transporter Project

  1. Dec 28, 2008 #1
    Objective: To build a device that will transport a 1 kilogram mass a distance of 10 meters using the energy released as the mass falls a distance 10 centimeters.

    1. The only source of energy that may be used to move the vehicle is a 1 kg mass falling 10 cm.
    2. All parts of the vehicle, including the 1 kg mass, must stay together for the entire trip
    3. After the vehicle begins to move, it must be self-guided.

    I thought of using a pulley connected to an axle (with string pulling the 1 kg mass down on the pulley), but I don't know 1) how to connect a pulley to a cart-like-thing, 2) what materials to use to be lightweight, and 3) how to release the string without it rewinding like a yo-yo.

    I thought of using CD's for the 4 wheels. I don't know what would be an effective axle, though. Also I thought of using an empty spool for the reel part of the pulley. Now it's a matter of attaching the reel to some sort of construction to hold UP the mass.

    If anyone could help, I'd appreciate it! Thank you!
  2. jcsd
  3. Dec 29, 2008 #2
    How strict is your teacher? Do you think something cheap would work for him, elsewise, let's look at our other options.
  4. Dec 29, 2008 #3
    I think it's safe to say that [tex]\frac{l}{d}=\frac{r}{R}[/tex], where [tex]l=0.100[/tex]m, [tex]d=10.00[/tex]m, and [tex]r,R[/tex] are appropriate radii of the axle and the wheels such that the equality is true. This is neglecting friction, so you'll want to make [tex]R >> r[/tex] to account for the energy lost due to friction.
  5. Jan 1, 2009 #4
    My teacher isn't that strict, anything is ok as long as it follows those rules.

    I built my Joule Mobile, but it doesn't work!! The mass falls down REALLY slowly and with the pulley, only goes about 3 meters. Is it the friction? I'm not sure why it is not working properly.. I would think that theoretically it makes perfect sense.
  6. Jan 2, 2009 #5
    If the mass is falling the total length and the axle-wheel ratio follows my work, then the only reason the cart isn't traveling its full distance is friction on the wheels or slip within the pulleys.

    For the minimum ratio between the wheel and axle, we solve the inequality. Make the wheels of radius, [tex]R=1.00[/tex]m. That puts the radius of the axle to be, [tex]r=0.01[/tex]m (1 centimeter). If this is what you're doing, then we can solve for friction using energy formulas:

    [tex]U_i+K_i = U_f+K_f+W\rightarrow mgh=W=0.980655[/tex]J. From this, we can analyze the frictional forces acting upon the system to solve for some of the necessary information:

    Considering [tex]0.980655=W=Fd=10F_f=10C_rN=10(1+m)gC_r\rightarrow C_r(1+m)<0.01[/tex]. From this, using standard rolling friction values for 'smooth' surfaces (non-rubber), we'll find [tex](1+m)<1.5[/tex]kg, where [tex]m[/tex] is the mass of the cart. Make your cart small.
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