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!

Force Question

  1. Mar 4, 2007 #1
    1. The problem statement, all variables and given/known data

    A model rocket of mass 4.8 x 10^2 g accelerates vertically upward at 34.m/s2.
    Calculate the thrust force applied by the rocket if the air resistance is 2.4N

    2. Relevant equations

    F=ma?


    3. The attempt at a solution

    Converted into SI
    rocket mass = 0.48g

    Calculated the Force of the Rocket
    F=ma
    F= 16.32N

    Stuck here can anyone help?
     
  2. jcsd
  3. Mar 4, 2007 #2

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    First make up names FOR EACH SEPARATE FORCE YOU CAN THINK OF AS RELEVANT IN THE PROBLEM!!
     
  4. Mar 4, 2007 #3
    Like...
    Fr = Rocket Force
    Fa = Air resistance
    Fg = Gravity
    ?
     
  5. Mar 4, 2007 #4

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Indeed!!
    Now, set up Newton's 2.law of motion where you include all of these forces EXPLICITLY!
     
  6. Mar 4, 2007 #5
    Im not sure how to find the force of gravity though, is it 9.8?
     
  7. Mar 4, 2007 #6

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Can you please set up in SYMBOLS Newton's 2.law?
     
  8. Mar 4, 2007 #7
    Like what do you mean? Like F=ma?
     
  9. Mar 4, 2007 #8

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Written with the symbols you made up for the three separate forces, please.
     
  10. Mar 4, 2007 #9
    Fr = (0.48kg)(34)
    = 16.32 N

    Fa = ma
    2.4N = ma

    Fg = ma
     
  11. Mar 4, 2007 #10

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Totally wrong!

    You have to learn that we SUM FORCES TOGETHER TO GET THE NET FORCE F!!!

    Thus, Newton's 2.law reads (using big A for acceleration of rocket):
    Fr+Fa+Fg=m*A

    Do you understand this?
     
  12. Mar 4, 2007 #11
    yes, ohh, i thought that since Fa and Fg was going the opposite direction that it would be subtracted from Fr.
    How would i go from there
    like this?

    Fr+Fa+Fg=m*A
    16.32 + 2.4 + Fg = m(34)

    is this correct?
     
    Last edited: Mar 4, 2007
  13. Mar 4, 2007 #12

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    ALWAYS start with setting up Newton's 2.law with names for the separate forces before you start to simplify the "force side" of it by invoking directions of forces, specific force laws etc.

    NOW, let us continue:
    If we let the positive direction be away from Earth, what are the signs of Fr,Fa,Fg and A?

    I saw your EDIT:
    It is incorrect
     
    Last edited: Mar 4, 2007
  14. Mar 4, 2007 #13
    Fr = +
    Fa = -
    Fg = -
    A = +
     
  15. Mar 4, 2007 #14

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Correct!
    So, Fa=-2.4N with direction included, agreed?
     
  16. Mar 4, 2007 #15
    Yup, agreed.
     
  17. Mar 4, 2007 #16

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Now, the force of gravity is nothing else than the WEIGHT of the rocket.
    So, with direction, what is Fg?
     
  18. Mar 4, 2007 #17
    Fg = -0.48N?
     
  19. Mar 4, 2007 #18

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Weight is given by (mass)*(acceleration of gravity), that is with units and direction, we have:
    Fg=-0.48*9.8N

    Now, insert all known quantities into their proper place in Fr+Fa+Fg=mA!
    (Skip the units at this point, if you like)
     
  20. Mar 4, 2007 #19
    Fr = ?
    Fa = -2.4 N
    Fg = -4.704 N
    m = .48 kg
    A = 34 m/s2

    Fr =.48*34 +2.4 + 4.704
    Fr = 23.424 N

    Correct?
     
  21. Mar 4, 2007 #20

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Probably, your logic is completely sound, so it is only calculator mistakes you might have made.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Force Question
  1. Tension Force Question (Replies: 8)

  2. Forces question (Replies: 1)

  3. Forces question (Replies: 1)

  4. Force Question (Replies: 3)

Loading...