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Test tomorrow on free fall projectiles and force need some help with these questions.

  1. Apr 28, 2009 #1
    1. The problem statement, all variables and given/known data

    1)A 3.0kg chair at rest is pulled across the floor by a 20N force.If the co-efficient of friction between the floor and chair is 0.25, calculate:
    a)the net force on the chair. answer=13N
    b)the displacement of the chair after 2.0sm answer=8.4m


    2. Relevant equations

    Fn=m*a Fn=Fa+Ff Fg=m*g Ff=m*FT(flipped for perpendicular)

    V2=V1+a*t d=V1t+1/2a*t^2

    3. The attempt at a solution

    I cnat get it started like Im lost with what equation to pick.
     
  2. jcsd
  3. Apr 28, 2009 #2
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    First step is to draw a free body diagram. Did you get that far?

    After that, I'm kind of confused about why you can't get it started. You already provided the equations. Just start substituting in what you do know, and solve for what you don't.
     
  4. Apr 28, 2009 #3
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    ya I drew the diagram whats the first equation that I use?
     
  5. Apr 28, 2009 #4
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    [tex]\Sigma[/tex]F=ma
     
  6. Apr 28, 2009 #5
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    so is it 20=3.0*a
     
  7. Apr 28, 2009 #6
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    No. You didn't sum up the forces. You only listed the "pulling" force, ignoring everything else. That's the point of the free body diagram, to list all of the forces and account for them all.
     
  8. Apr 28, 2009 #7
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    is Fg 29.4 and I forgot but doesnt Fg=F with the perpendicular and you seem like a pretty good person helping everyone out thanks for the help.
     
  9. Apr 28, 2009 #8
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    You're still missing a force. Yes, normal force and weight cancel out. You've got the pulling force, but what else are you missing?
     
  10. Apr 28, 2009 #9
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    im missing the force of friction right but I got it form Ff=.25*29.4 and it equaled 7.35 is this right?
     
  11. Apr 28, 2009 #10
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    In which direction?
     
  12. Apr 28, 2009 #11
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    if I add 6.67 and 7.35 with the eqn. Fn=Fa+Ff I get 14 but the answer is 13.
     
  13. Apr 28, 2009 #12
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    well does the direction matter
     
  14. Apr 28, 2009 #13
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    Where did you get 6.67?

    Is force a vector or a scalar quantity?
     
  15. Apr 28, 2009 #14
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    I got 6.67 from 20=3*a but I guess thats wrong right but was the 7.35 right Im not sure what you mean by what direction like the friction is going against the applied force if that what you mean
     
  16. Apr 28, 2009 #15
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    Ignore the 6.67, because that equation wasn't correct. You forgot to include the force of friction, which you just identified, and in any case, diving by mass will just give you acceleration. You're not looking for acceleration just yet. You just want the net force.

    Now, what I mean by direction is what direction the force is acting. As I'm sure you learned in class, force is a vector quantity. What that means is it has both magnitude and direction. The force of friction always acts to OPPOSE motion, which makes logical sense.

    Take a look at the normal force and weight. If you have a book sitting on a table, not moving, the only two forces are the normal force and it's weight. The weight is m*g. The normal force is -(m*g). Note the negative sign. That shows direction. I'm not sure if you're taking calc based physics or not, but there's a trigonometric way to express that. Anyway...

    Use the equation [tex]\Sigma[/tex]F=m*a there. You get m*g + (-m*g) = m*a.

    It's not accelerating, so you get mg - mg = 0 which makes sense.

    You should be able to apply this formula to your current problem. First, it's asking you to sum up the forces on the chair. You already know the weight and normal force cancel out. You know about the pulling force, you also calculated the force of friction.

    Sum them up, and note the direction of the force of friction.
     
  17. Apr 28, 2009 #16
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    Well wouldnt this be Fn=2.45*7.35 but that doesnt work Im sorry for being such a nuisance but Im not very good with equations. It would probably just work better if you show me the work and then I can try and understand it that way.
     
  18. Apr 28, 2009 #17
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    oh wait sry
     
  19. Apr 28, 2009 #18
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    it would be Fn=2.45+7.35 but that wouldnt work either
     
  20. Apr 28, 2009 #19
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    Where did you get 2.45 from?
     
  21. Apr 28, 2009 #20
    Re: Test tomorrow on free fall projectiles and force need some help with these questi

    I thought you said to put 7.35 over 3
     
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