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Please help me with this physics problem.

  1. Apr 29, 2005 #1
    Hi, I need a little help on a problem I did. I've most of the solutions but I think they're all wrong, can someone help me point out what I did wrong?

    Here is the problem:

    The Sojourner rover vehicle was used to explore the surface of Mars in 1997.
    Here is some info to aid solving the below problems:

    Mars data: Radius = 0.53 x Earth's R Mass = 0.11 x Earth's mass
    Sojourner data: Mass = 11.5 kg Wheel diameter = 0.13 m Stored energy available: 5.4 x 10^5 J Power required for driving under avg. conditions: 10W
    Land speed: 6.7 x 10^-3 m/s

    a. Determine the acceleration due to gravity at the surface of Mars in terms of g, the acceleration due to gravity at the surface of Earth.

    For a, I just used the same method that calculates the Earth's acceleration of gravity. I used:

    g = G (m/r^2), with m and r being Mars' of course.
    so it would be g = 6.67x10^-11 (6.567x10^23 / 1.1434x10^13), which gave me 3.05 m/s^2.

    B. Calculate Sojourners weight on Mars.
    I just used the W=mg concept. Sojourner is 11.5 kg and Mars' g is 3.05, so multiple and it = 35.13 N.

    C. Assume that the vehicle is rolling down a ramp inclined at 20 degress to the horizontal. The ramp must be lightweight but strong enough to support the vehicle. Calculate minimum NORMAL force that must be supplied by the ramp.

    What I did was I used the normal force equation.

    so I got Normal F=mgcostheta = 11.5(3.05)(cos 20) = 32.96 N.

    D. What is the net force on the vehicle as it travels across the Martian surface at constant velocity?

    I kinda got stuck on this one so I made a guess. Since it is constant v, there is no acceleration. So according to the net force equation, F=ma, it becomes F=m(0), so there is no net force.

    E. Determine the max distance that the vehicle can travel on a horizontal Martian surface using its stored energy.

    I got completely stuck on this one, didn't do anyting for it.

    Any help will be appreciated, thank you!
  2. jcsd
  3. Apr 29, 2005 #2


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    I did not check your arithmetic, but your approach is correct for A-D. For E you have to use the information you were given in the probleam about power needed under "average" conditions. This means there is some force opposing the motion at all times. It could be due to a layer of sand or rolling friction. It does not matter what the source is. You just know that steady power has to be applied to keep the rover moving. Power is the rate of using energy, and your energy supply is limited.
  4. Apr 30, 2005 #3
    For A, express mars' gravitation in terms of the Earths. aka

    [tex] g_{mars} = \left(\frac{g_{mars}}{g_{earth}}\right) g_{earth} [/tex] where the quantity in brackest is numerical and the rest are just units.
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