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Homework Help: Help 3 simple questions need to know the law, 2 question need to make sure only

  1. Dec 27, 2011 #1
    1)- An object is being acted upon by three forces and moves with a constant velocity. One force is 60.0 N along x-axis, the second is 75.0 N along the y-axis. What is the magnitude of the third force?

    A- 15.0 N
    B- 96.0 N
    C- 48.0 N
    D- 20 N


    2)- A chatcher stops a ball traveling at 40 m/s in a distance of 20 cm and feels a force of 600 N against hid glove. What is the mass of the ball?

    A- 0.30 kg
    B- 0.25 kg
    C- 0.15
    D- 0.20 kg


    3)- How much energy is needed to charge the speed of a 1600 kg sport utility vehicle from 15.0 m/s to 40 m/s ?

    A- 1.10 M
    B- 40.0 kJ
    C- 1.10 MJ
    D- 0.960 MJ


    * Now i have 2 questions i just want to make sure about the answers.

    4)- How much work is required to stretch a spring 25.2 cm if its force constant is 12.3 N/m ?

    A- 1.05 J
    B- 3.10 J
    C- 0.391 J
    D- 0.381 J

    i used the law: W = F x D,
    F = 12.3 N/m,
    D = 25.2 cm = 0.252 m,
    W = 12.3 x 0.252 = 3.09 J
    so the answer is B.

    5)- John's mass is half the mass of Kim. They both start walking and John moves twice as fast as Kim. What is the ratio of the kinetic energy of Kim to the kinetic energy of John?

    A- 1/8
    B- 1
    C- 2
    D- 1/2

    now if John's mass is 1 then Kim's mass is 2, and if John's velocity if 2 the Kim's velocity is 1

    John: Ek = 0.5 x 1 x 4 = 2

    Kim: Ek = 0.5 x 2 x 1 = 1

    so the answer is D.
  2. jcsd
  3. Dec 27, 2011 #2


    User Avatar

    Staff: Mentor

    Q1,2,3 you have not shown your effort, so we can't say whether you have the right method or not.

    Q4. w=Fd is correct, providing F is constant. For a spring, F=kx, meaning the force it exerts keeps increasing as you stretch it. So you have to use your knowledge of calculus as well.

    Q5. correct
  4. Dec 27, 2011 #3
    I will not touch the first three questions. You have to show your attempt or at least describe what is it that you are unsure of or do not understand.

    Question 5 is correct.

    Question 4, however is not. k = 12.3 N/m is the force/spring constant, not the force! This means that this spring obeys the law |F| = (12.3) x, where F is the force required to stretch/compress the spring by a distance x. For instance, the amount of force required to stretch this spring by 25.2cm is give by F = 12.3 x 0.252 = 3.09 N.
    Another flaw in your reasoning is simply using W = Fd. In this case, the force applied on the spring to stretch it is not constant throughout! At an extension of 0.100m, the force is 1.23N. Thus, the work done on the spring is instead given by [itex]\frac{1}{2}kx^{2}[/itex]. (You can easily verify this by integration or by considering the area under the F vs x graph)

    The answer is thus [itex]\frac{1}{2}(12.3)(0.252)^{2} = 0.391 J[/itex]
  5. Dec 27, 2011 #4
    Q1 and 2 and 3 im not understanding how to solve them anymore!!
    for Q1 f = ma.. okay but what about three forces.
    for Q2 im looking for a law and i didn't find.
    for Q3 there is 2 speeds, i didn't understand
    i only need some help
  6. Dec 27, 2011 #5

    What do you think ? Think about the direction in which the net force will be acting. Also think about Pythagoras theorem.

    Are you aware about kinematic equations ? Hint : Apply the best suited equation on motion to evaluate for deceleration in the ball.

    Work done = Energy manifested.
    Hmm think about the formula of work-energy theorem i.e gain or loss in kinetic energies.

    Last two questions have already been given direction by others.
    Last edited: Dec 27, 2011
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