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Physics Problems, could you tell me if I did them right?

  1. Sep 17, 2009 #1
    1. A parent pushes a baby stroller from home to daycare along a level road with a force of 38 N directed at an angle of 30° below the horizontal. If daycare is 0.83 km from home, how much work is done by the parent?

    (38N)(cos(-30))(.83km or 830m)=????

    2. You have a 0.750 kg picture frame on a mantle 1.50 m high. How much work was required to lift the frame from the ground to the mantle?

    0.750*9.80=7.35N(1.50)=11.025 J

    3. You lower a 2.50 kg textbook (remember when textbooks used to be made out of paper instead of being digital?) from a height of 1.81 m to 1.50 m. What is its change in potential energy?

    1.81-1.50=.31
    (2.50kg)(9.80)(.31)

    =7.60 J

    Please if someone could confirm these, plz be sure of the answer before responding, Thanks so much
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Sep 17, 2009
  2. jcsd
  3. Sep 17, 2009 #2
    The two last ones look correct, but your calculation on the first one doesn't make sense. Please explain, you're correct that E = F * d, but why is d = 10 km?

    Wouldn't it be that d = 830 m and that F = 38 * cos 30? (cos 30 = cos -30) Then E = 27 kJ
     
  4. Sep 17, 2009 #3
    My equation is W=(Fcos(theta))(change is direction x)

    is it (38)(cos(-30))(.83)=27.3144 J
     
  5. Sep 17, 2009 #4
    Haven't seen the above equation you're refering to before, but in the lower one: Remember, you must use 830 m and not 0,83 km, the definition of force is kg * m/s^2.
     
  6. Sep 17, 2009 #5
    alright.

    Could someone else work these problems and confirm if i did them right
     
  7. Sep 17, 2009 #6
    I'm sorry, I'm a little confused. The formula (W = F cos(theta) * Change in direction) is the same as E = F * d, it's just a different notation.

    I'm still curious where 10km came from though.
     
  8. Sep 17, 2009 #7
    You're correct dude i know, it was a keying error
     
  9. Sep 17, 2009 #8
    I just want multiple opinions on all these problems
     
  10. Sep 17, 2009 #9
    I understand, that's always a smart thing to do ;)
     
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