Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question hel;ppp

  1. Nov 29, 2003 #1
    question...plz hel;ppp

    guys iam takin physics

    nd i dont get physics attt allll!!!
    pllz hlp me with this
    i need to show work as well

    an elvtor weighing 25,000 N is supported by a steal cable. that is the tension in the cable wehn the elavotor is begin accelerated upward at the reate of 3.0 m/s^2? (g= 9.8m/s^2)
    plzzz help[
    Last edited: Nov 29, 2003
  2. jcsd
  3. Nov 29, 2003 #2


    User Avatar
    Science Advisor
    Gold Member

    You need to draw a free body diagram and use Newton's second law. What forces are acting on the elevator?
  4. Nov 29, 2003 #3
    well u c
    i duno how to do all that
    thats wut my prblem is
    can u hlp me out
  5. Nov 29, 2003 #4


    User Avatar
    Science Advisor
    Gold Member

    OK, well I can't just give you the answer to a test question. If you think about the elevator, the tension is pulling it up and the weight is pulling it down. The net acceleration on the elevator is up. Newton's second law says that Fnet = ma. So if you add the forces together (using positive for up and negative for down), you can write a single equation from Newton's second law. After one step of algebra, you can use that to solve for the tension. I hope that helps you out.
  6. Nov 29, 2003 #5
    Trying a basic explanation...

    First off, tension is much the same as force. It's just another name engineers invented when they're talking about a force that is transmitted by a cable, string, or something like that. So what you're looking for is a force, OK?

    Now, the first force that acts on everything on Earth's surface, is the force of gravity. It makes things fall. Experiments have shown that any falling object accelerates at
    g = 9.8 \frac{m}{s^2}
    We know that things fall a lot slower on the Moon. Why is this? It's so because the force of gravity is smaller there (because the Moon is a smaller body than the Earth). So, ISAAC NEWTON concluded that the acceleration is a measure of force, and arrived at his famous 2nd law
    F = ma
    where a is the acceleration produced by a force F acting on a body of mass m.
    That's why the unit of force is called one Newton:
    1 N = 1 kg \cdot 1 \frac{m}{s^2}
    You see it's the unit of mass *times* the unit of acceleration.

    Now back to your problem: Two things are exerting forces on your elevator. One is the Earth (pulling it down), and the other is the elevator's motor (pulling it up). Both forces will add, giving the cable tension (remember: =force) Can you calculate both forces by Newton's second law? And find the sum?
    Last edited: Nov 29, 2003
  7. Nov 29, 2003 #6
    thanx guys
    but if u tell the eqauitns whihc i have to solve
    than i can solve it
    i mean liek the free body diagram
    nd wut equations i need to solve it
  8. Nov 29, 2003 #7
    i jus dont get it [b(]
  9. Nov 29, 2003 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member


    We don't do your homework for you here. We'll help you if you actually make an attempt at it, but we simply won't help you cheat. Sorry.

    Here's a recommendation: open you book, and start reading. Setting up free body diagrams is not difficult to do.

    If you try the problem and post what you've got, you may get some help.
  10. Nov 29, 2003 #9
    ok i got
    f= 25,000(3.0m/s^2)
    is that somwhat right?
  11. Nov 30, 2003 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's what the force would be if there was no gravity. You've got to sum all forces on it.
  12. Nov 30, 2003 #11
    how do i do that?
    common am soo confused
  13. Nov 30, 2003 #12
    u should master Free Body Diagrams thoroughly.
    Consult any standard book

    know more about forces, types of forces, frame of reference etc
  14. Nov 30, 2003 #13
    No, it's wrong.
    25,000 N is not the elevator's mass. Since it's in Newtons, it must be the force of gravity, right? (Did you read my other post?)
    You got to find the mass m by using F = mg, and rearranging that. Do you know how to rearrange an equation to solve for the unknown?
    Next, you use F = ma to find the elevator's force.
  15. Nov 30, 2003 #14
    how do i rearrange that
    nd wut do i subsitute in thos eletters
  16. Nov 30, 2003 #15
    You rearrange like this:
    F = mg | :g
    m= \frac{F}{g}
    Plug in your F, and the g that I told you in my 1st post.

    (You should study more. What I tell you is all very basic (grade 8), and you should know it if you're taking this course.)
    Last edited: Nov 30, 2003
  17. Nov 30, 2003 #16
    this is wut i got

    is that righ?
  18. Dec 1, 2003 #17
    Yes. The motor force is 7653 N (you should always use units). Now that adds to the elevator's weight, giving WHAT?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook