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Homework Help: I solved 2 simple question but i need to make sure help please?

  1. Dec 23, 2011 #1
    1)- A person lifts a mass of 20.0 kg vertically upwards at a constant speed of 0.200 m/s through a distance of 0.800 m. What is the work done by the person?

    A) 156.8 J
    B) 3.20 J
    C) 16.0 J
    D) None of the other choices is correct.

    I choosed the answer B.
    I used the law : W = F x D = (20 x 0.2) x 0.8 = 3.2
    Am i correct?


    2)- A truck is towing a car whose mass is one quarter that of the truck. The force exerted by the truck on the car is 6000 N. The force exerted by the car on the truck is

    A) 6000 N.
    B) 24000 N.
    C) 3000 N.
    D) 1500 N.

    I choosed the answer D, because the the force exerted by the truck on the car is 6000 N, and the car's mass is one quarter that of the truck, so 6000/4 = 1500 N.
    Am i correct?


    And for this question.. i only need to know the law..

    A 40.0-kg crate is being lowered by means of a rope. Its downward acceleration is 2.00 m/(s)2 . What is the force exerted by the rope on the crate?

    A) 312 N.
    B) 40.0 N.
    C) 472 N.
    D) 232 N.

    Last edited: Dec 23, 2011
  2. jcsd
  3. Dec 23, 2011 #2
    Check out the PDF attachment below!

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  4. Dec 23, 2011 #3


    User Avatar

    Staff: Mentor

    No you didn't. You should have, but you didn't.

    You calculated F as being equal to 20 x 0.2
    but there is no formula in physics that says force = mass x speed

    Think about it some more.

    Not correct.

    Hint: to every action there is an equal and opposite reaction

    The question involves a mass and its acceleration, so
    the law that should spring to mind is F = ma
    but you'll need to draw a diagram of the forces so you can clearly see what is going on.
  5. Dec 24, 2011 #4
    How about the formula: Change in kinetic energy = total work done on body ???
  6. Dec 24, 2011 #5
    The work of weight is special its: m*g*h
    you have 20kgs and g is a constant and you have h so you should be able to easily solve it.
  7. Dec 24, 2011 #6
    Thank you guys.. i solved them all.. all of them the answer is A.. right?
  8. Dec 24, 2011 #7
    Yep that's what it looks like to me
  9. Dec 24, 2011 #8
    for this question.. i solved it by using this law

    mg - T = F

    (40 - 9.8) - T = (40 x 2)

    392 - T = 80

    392 - 80 = T

    312 = T

    Correct ??
  10. Dec 24, 2011 #9
  11. Dec 25, 2011 #10


    User Avatar

    Staff: Mentor

    There certainly is such a formula. What answer would it give you here?
  12. Dec 26, 2011 #11
    Change in kinetic energy is 0. and only work done is by gravity and man

    so -(work by gravity) = work by man ...
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