1. Not finding help here? Sign up for a free 30min 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!

A 10 kg box rest on a table

  1. Mar 31, 2015 #1
    1. The problem statement, all variables and given/known data
    weight of the box 10 kg on table at rest.
    2. Relevant equations
    deternmine weight of box and normal reaction acting on it.
    also determine w and Fn if force 40N pushes down the box.

    3. The attempt at a solutions
    i did like
    w=mg =NORMAL REACTION(R)
    and
    40=10 * acceleration due to gravity.
    i feeel like this is wrong.
     

    Attached Files:

  2. jcsd
  3. Mar 31, 2015 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Hello Nahsor, welcome to PF :smile: !

    ## W = mg ## seems excellent to me. And if the box doesn't move, the normal force has to be ##F_N = - mg##, because then the net force on the box (in a vertical direction) is ##F_{\rm net} = F_{\rm gravity} + F_N = mg - mg = 0##.

    And your feeling of doubt in the next part is very justified.

    Where you find the 10 in your next expression is a mystery to me.
    Simply continue the reasoning that if the box doesn't move, the net force has to be 0. Only now htere is an extra force acting. W is related to the box only, so it is and it stays ## W = mg ##.
     
  4. Mar 31, 2015 #3
    is this the answer for the next part
    Fn=40N+10*9.8
     
  5. Mar 31, 2015 #4

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That's correct. You found it from ##F_{\rm net} = 0 = -40 \;{\rm N} + mg + F_N ##

    As you can see, I use the convention up = positive.
    And mg is pointing downwards, so if I write ##F_{\rm gravity} = mg## I have to take g = -9.8 m/s2.
    Result: ##F_N = 40 \;{\rm N} - 10 \;{\rm kg} * (-9.8 \; {\rm m/s^2}) ## .

    Important tip: always use dimensions -- and check them !
     
  6. Mar 31, 2015 #5
    THANKS FOR URS HELP BVU
     
  7. Mar 31, 2015 #6
    I UNDERSTOOD NOW FORCE UPWARD=FORCE DOWNWARD
     
  8. Mar 31, 2015 #7

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes, that's all there's to it.
     
  9. Mar 31, 2015 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    One small correction to the question statement. 10kg is the mass of the box, not the weight. The weight is 10g N.
     
  10. Apr 4, 2015 #9
    oh yus yus.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: A 10 kg box rest on a table
  1. 2.7 kg wood box (Replies: 11)

Loading...