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Homework Help: Coefficient of friction PLEASE HELP!

  1. Jun 27, 2008 #1
    1. The problem statement, all variables and given/known data

    Hi im new to these forums but i have been trying for about 2 days now and still cannot find the answer to these 4 questions if someone could please help me it would be greatly appriciated! i come from a different country this is my third month in Canada, and i am doing an online course.

    these are the questions

    7. A stationary elevator and its contents have a combined mass of 3000 kg. The elevator is suspended by a single cable. (Assume three significant digits.)

    1. Draw a free-body diagram of the elevator and calculate the values of all the forces that are acting on it when at rest.
    2. If the elevator is ascending at a speed of 3.0 m/s, what are the values of the forces acting at this point?
    3. If the elevator is descending at 3.0 m/s2, what are the values of all the forces acting at this point?

    8. A force of 1.6 N is applied to a box of mass 1.8 kg. It accelerates at 0.60 m/s2. Determine the force of friction that is acting and the coefficient of kinetic friction involved.

    9. A desk of mass 45 kg is pushed across a rough surface (mK = 0.18) for a distance of 2.0 m by a constant force of 50 N. If the desk reaches a speed of 1.0 m/s by the end of the push, what was its speed at the beginning of the push? (Assume two significant digits.)
    10. An object of mass 60.0 kg rests on the surface of a planet with a mass of 7.7 × 1022 kg and radius 4.6 × 105 m.

    1. Calculate the force of gravity acting on the object.
    2. Determine the gravitational field strength "g" at the planet's surface.
    3. Calculate the force of gravity acting on the object if it is placed at a position 6.6 × 105 m above the planet's surface.

    2. Relevant equations

    3. The attempt at a solution
    With question 7 i drew the free body diagram as the elevator has 9.8m/s2 down because of gravity.

    Then i put
    f=29400 N
    thats all i got and for the other questions, i have gotten no other formulas i am very confused please help!
  2. jcsd
  3. Jun 27, 2008 #2


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    Homework Helper

    Welcome to PF!

    Hi Rade1990! Welcome to PF! :smile:

    These are all missing-force problems …

    you write out the equation for Newton's second law, and you find that one force is missing (perhaps the friction), and you calculate what that force must be to balance the equation!

    You have the free body diagrams already … so write out Newton's second law based on those diagrams, and find the missing force! :smile:
  4. Jun 27, 2008 #3
    Not so bad - you are correct in figuring the amount of downward force on the elevator was mass(Kg) x g in Newtons.
    So when you drew the free body diagram, it shows the elevator, and the force due to gravity acting downward on it. So far, it looks like we have an elevator that is going to accelerate downward (ouch!0

    You need to show another force on that elevator diagram that will keep it stationary.

    Now - in considering part (2), you still use force=mass x acceleration. Is the elevator accelerating?

    Then - in considering part (3), you still use force=mass x acceleration. Is the elevator accelerating?
    (Yes - I know I repeated myself!) Don't let the downward direction mess with you. Go for it by imagining what you feel when you are in an elevator which is doing this stuff.

    Show what you are thinking about question (8). Try force=mass x acceleration on it, even though it is already given. Also - draw a free body diagram just like for the elevator, and try to include ALL the forces on the sliding box you can think of.

    If you get through question (8), you will probably see the way into question (9) .

    Lets do question (10) later.
  5. Jun 27, 2008 #4
    thank you so much! now how do i even start number 10?
  6. Jun 27, 2008 #5
    OK - No (10) messes with you a little by moving the scene to some other completely fictional planet which will have a different gravity. The thing is all about getting at the force between two masses.

    One key thing you get from Mr. Newton is that the whole mass can be considered as if it were acting from a single point at the centre of gravity. Think of masses m1 and m2, one being the planet. The force between will be at proportional to the masses both (more mass = more force!). The force between depends on the distance too. You also need a constant called the Universal Gravitational Constant. I am surprised that it seems not given among your starting data, because I cannot see how you can do (10) without it.

    You need this link http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html"
    Anyways you will now have a quite simple formula that will get you the force that 60Kg weight has on that planet surface.
    Then again use Newton's force = mass(60Kg) x acceleration to find out the value of "g" for that planet.
    Get that relationship between masses, distance, Universal Constant G, and force burned in, because the last part of (10) is just a slight rearrangement of the conditions. If you got this far, you can do it.

    As a final fun - check out http://www.fourmilab.ch/gravitation/foobar/" [Broken]. About 2/3 down the page, you get to see a video of gravity doing its thing, but leave it until you are done with (10) :smile:
    Last edited by a moderator: May 3, 2017
  7. Jun 27, 2008 #6
    could you show me with calculations some steps cause im stuck, there seems to be no information present and with 8 and 9 theres questions i can do but these arent showing me directions so i dont know what to add or subtract, Please
    Thanks it will help me tons!
  8. Jun 28, 2008 #7
    Actually you have the whole deal at
    http://www.fourmilab.ch/gravitation/foobar/" [Broken].
    There is even a javascript calculator, but you can only use it successfully if you know and understand what to enter, and in the correct units.

    To do question (10) you need to know all about gravitational attraction of masses. That you are completely stuck means you must have missed a main concept, and I am still unsure about why you don't have the key constant included in the question - unless it was in your earlier notes.

    The equation is built up this way ..
    You have masses m1 and m2, and the force gets greater if you increase either, so it is at least proportional to (m1 x m2).
    The force gets less as the distance squared, ref: Newton!, so it is inversely proportional to (r x r), so we now have (m1*m2) / (r *r).
    Finally, we add in a constant 'G', which was found by experiment, to turn the proportional relationships into a famous equation.
    [tex]F = G(m1.m2)/r^2[/tex]

    [tex] G = 6.6742810 \times 10^{-11} {\rm \ N}\, {\rm (m/kg)^2}. [/tex]

    Now you have the way to figure the force from any lump - even a planet-sized one! That gets you part 1
    Here on Earth, we know the tug on us comes from one mass that does not change much (Earth), and another that changes all the time, (usually by waistline!). So we pre-calculate the parts of the equation G, m1, and r^2, gathering them into a single number to multiply by, which we call 'g'. It looks like Newton's ..

    [tex] F=mg [/tex]

    In your case, you already had to find the force. So use it to get back to that planetary equivalent of 'g'. If you like, just use the equation again to figure that part of it you need to take the place of 'g'

    If you get this far, fully understanding that equation, you will be able to do the last part. If you can't, then please post the working you have done so far, even if it is full of mistakes - OK?
    Last edited by a moderator: May 3, 2017
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