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Can i get help with my F=ma problems?

  1. Dec 11, 2011 #1
    Four questions that i dont know how to do at all.

    1. A student standing on a scale in an elevator at rest sees that it reads 650 N.
    As the elevator rises, the scale reads 700 N.
    Find the acceleration of the elevator.

    2. A sign in an elevator states that the maximum occupancy is 20 persons. Suppose safety engineers assume the mass of an average person is 70 kg. The elevator itself has a mass of 540 kg. The supporting cable can tolerate a maximum force of 32471 N. What is the greatest acceleration the elevator's motor can produce without snapping the cable?

    3. A student pushes a 12.9 kg lawnmower. The handle makes a 28 degree angle with the ground.
    If the friction is 68 N, find how much force the student must apply to the lawnmower in order to accelerate it at 3.7 m/s^2.

    4. A 30 kg box is pushed across the level ground with a horizontal force of 172 N. As a result it accelerates at 5 m/s^2. How much friction must be present?

    please explain every step because im totaly lost and my teacher just threw us right into it without explain at all. Very much apperciated.
     
  2. jcsd
  3. Dec 11, 2011 #2
    Start by drawing a diagram of the forces ON the person.

    Next give of list of your given information including units of measure and give a variable for what you are tying to find

    Then write the equation or equations you think you have to use to solve this
     
  4. Dec 11, 2011 #3

    e.bar.goum

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    All of these problems are essentially the same. You've got newtons second law, F = ma, two known values and one unknown in each question. Substitute in the known values (and in the case of question 3 do some trigonometry) and solve for F or a as required.
     
  5. Dec 11, 2011 #4
    Come on - do it the long way which is easier. Do the diagram. List the info with the variables and then the formula in that nice neat format they give you on here. Once you see it, you can work it.
     
  6. Dec 11, 2011 #5
    I've been trying to get it for the past few hours. ITs online homework so it tells you instantly if it is wrong or right. Just like on the last question. the question gives the force mass and acceleration, so using f=ma i just dont see how i can get teh friction out of it.
     
  7. Dec 11, 2011 #6

    e.bar.goum

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    Well, in the instance of the last question, there should be a discrepancy if you evaluate F=ma.

    Draw a free body diagram!
     
  8. Dec 11, 2011 #7
    Discrepancy? How could i tell?
     
  9. Dec 11, 2011 #8

    e.bar.goum

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    Well, in this case, you have values for F, m and a, does F = ma in this instance?
     
  10. Dec 11, 2011 #9
    I guess not. 170≠150... So if i divided 170 by 150 would that be the answer? 1.14 more or less?
     
  11. Dec 11, 2011 #10

    e.bar.goum

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    No, dividing would be precisely the wrong thing to do. If you're ever having to guess, try thinking of it dimensionally, Newtons/Newtons is dimensionless, but friction is a force.

    So, how much force are you missing?
     
  12. Dec 11, 2011 #11
    12 newtons. ( i put 170 up there on accident)
     
  13. Dec 11, 2011 #12

    e.bar.goum

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    150+12 ≠ 172, But I suspect you're on the right track. So, where do you think the extra force on the box went?
     
  14. Dec 11, 2011 #13
    crap. i meant 22 newtons. Im so tired.
     
  15. Dec 11, 2011 #14

    e.bar.goum

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    Yep! So, where do you think the missing 22 newtons went?
     
  16. Dec 11, 2011 #15
    To the force of the friction? Is 22 the answer?
     
  17. Dec 11, 2011 #16
    Yes it works! thanks so much. Now three more to go haha
     
  18. Dec 11, 2011 #17

    e.bar.goum

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    Yes! It's friction!

    And as to whether 22 is the answer, you did the math! Be certain!

    (but yes, it should be)
     
  19. Dec 11, 2011 #18
    As for the elevator problem. Would i set it up as 700N=65kg(a)?
     
  20. Dec 11, 2011 #19

    e.bar.goum

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    That seems like the thing to do, doesn't it?
     
  21. Dec 11, 2011 #20
    Alright. So 700/65=10.76... Which would mean 10.76=a. but it is saying thats not right. What else could i do?
     
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