1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Calculate how far the spring is stretched from its position?

  1. Mar 30, 2014 #1
    A 7.5kg block is suspended at rest from a spring (k=45N/m) along a incline as shown below (where theta= 17°)
     

    Attached Files:

  2. jcsd
  3. Mar 30, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Welcome to PF;
    good pic and question.
    please show your best attempt.
     
  4. Mar 30, 2014 #3
    Well I am not sure what to do next
     

    Attached Files:

  5. Mar 30, 2014 #4
    Also I know that F=mg so I get 35N=-45N/m(x)
     
  6. Mar 31, 2014 #5

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Start with a more careful free-body diagram for the block: what are the forces on it?
    (hint: gravity, spring, and only one other...)

    [edit: you appear to have Fk pointing directly upwards ... if this is the force from the spring you need to take another look at the diagram: which direction is the spring pulling on the block?
    Don't know what Fp is supposed to be. Force of a "pull"? What is pulling on the black?]
     
  7. Mar 31, 2014 #6
    Well there is force of gravity pushing down on the block and normal force pushing up than then the the force of the blocks mass is pulling the spring while the spring resists the pull...so that's what I have there in my free-body diagram
     
  8. Mar 31, 2014 #7

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You are very confused about forces.
    You listed four and there are only three - so you have one too many.

    "pushing up" is too vague: the direction of the normal force is "upwards perpendicular to the ramp".

    the spring force points "up along the ramp".

    "gravity" is what we call the pull that something has due to it's mass.
    you don't need a separate force for it. It points directly down.

    redraw your free body diagram to show the correct directions and forces.
     
  9. Mar 31, 2014 #8
    So I changed my free-body diagram. .but I still cant get the answer .48m
     

    Attached Files:

    Last edited: Mar 31, 2014
  10. Mar 31, 2014 #9

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Which direction does the spring pull in?
    Have you never pulled on a spring in your life?

    This is a serious problem - do you not understand what a force is?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted