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!

Homework Help: Mass Free Fall

  1. Jul 24, 2010 #1
    A 10 kg mass is in free fall with no air resistance. In order to slow the mass at a rate equal to the magnitude of g, an upward force must be applied with magnitude:


    F=ma



    F=(10kg)(9.81m/s2) = approximately 100N

    So in order to slow the mass at a rate equal to g it should be a force less than 100N?? Im not sure how to figure this out. Help!!!

    Thanks

    Jerry Zink
     
  2. jcsd
  3. Jul 24, 2010 #2

    rock.freak667

    User Avatar
    Homework Helper

    Let's start with the forces on the body falling.

    There are three forces acting or well two since we are ignoring air resistance. What are these two and what direction are they going in (up or down)?

    Since the body is moving downwards, the resultant force, ma, is downward. Can you formulate an expression for the resultant force ma?
     
  4. Jul 24, 2010 #3
    The two forces are

    1. gravitational force on the 10kg mass "going down"
    2. The upward force "going up"

    So the upward force must go against the force of gravity plus the force of the mass.

    The mass has a force of F=ma = 100N downward

    Since the upward force is in the opposite direction it has to counteract the downward force and slow it down to = g

    Im just having trouble formulating the expression for the upward force.
     
  5. Jul 24, 2010 #4
    Would the upward force be :

    F=m2a ?

    F=(10kg)(2)(9.81m/s2) = 196N

    is this correct?
     
  6. Jul 24, 2010 #5

    rock.freak667

    User Avatar
    Homework Helper

    Correct.
    Right but remember, the force of gravity is the 'force of the mass'.

    So you will have one going down and one going up (don't worry with the formulas for now, just use U for upwards and W for downward). If the mass is moving downwards, what direction should the resultant force be in?
     
  7. Jul 24, 2010 #6
    The resultant force should be in the upward direction
     
  8. Jul 24, 2010 #7

    rock.freak667

    User Avatar
    Homework Helper

    The mass is falling in which direction?
     
  9. Jul 24, 2010 #8
    OH! So the resultant force is in the downward direction.
     
  10. Jul 24, 2010 #9

    rock.freak667

    User Avatar
    Homework Helper

    Right so your resultant is

    ma = W-U

    and you identified the downward force as the force of gravity. So what is W equal to?

    They want the resultant acceleration to be equal to g. So what is 'a'?
     
  11. Jul 25, 2010 #10
    so W = ma + u

    a = 9.81m/s2

    so W is equal to the downward force of 100N?

    So this means u = W - ma

    U = 100N - (10kg)(9.81m/s2)

    U = 1.9N

    Is this right?

    Are you sure its not this "ma = U-W"

    This would make more sense because then U is = to around 200N
     
  12. Jul 25, 2010 #11

    rock.freak667

    User Avatar
    Homework Helper

    If your upward force is greater then your object is moving upwards and not falling.

    Normally what you have is

    ma = W-U-Fair resistance

    Sorry though, I interpreted one part wrong. They want the mass to fall at 'g' right? Which is essentially with the force of gravity, which is constant. Since it's acceleration is constant, the resultant force is ?
     
  13. Jul 25, 2010 #12
    Is the resultant force still the force of the mass 100N downwards?
     
  14. Jul 25, 2010 #13

    rock.freak667

    User Avatar
    Homework Helper

    No, the resultant force is the resultant of W and U. Since the mass is not accelerating anymore, the resultant force is zero.
     
  15. Jul 25, 2010 #14
    ?? So if the resultant force is zero how do we find the upward force?
     
  16. Jul 25, 2010 #15

    rock.freak667

    User Avatar
    Homework Helper

    If we have ma=W-U and ma=0, then wouldn't W-U=0 ?
     
  17. Jul 25, 2010 #16
    So if the upward force is equal to the downward force W=U

    Fw = (10kg)(9.81m/s2)

    W = 100N
    U = 100N

    This doesnt seem right?
     
  18. Jul 25, 2010 #17

    rock.freak667

    User Avatar
    Homework Helper

    Why not?
     
  19. Jul 25, 2010 #18
    Then wouldn't that implicate that the mass is not moving?
     
  20. Jul 25, 2010 #19

    rock.freak667

    User Avatar
    Homework Helper

    Not in this case. The body is falling. Which means it is traveling at a constant velocity.
     
  21. Jul 25, 2010 #20
    That makes sense, but the answer in my book says the upward force is equal to 200N. This is what is throwing me off. Maybe the book is wrong.

    I really appreciate your help rock.freak667

    Can you make any sense of the 200N answer?

    Thanks again

    Jerry Zink
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook