13 thoughts on “Homework #6”

  1. rmroz

    I just went online to Mastering Physics to start homework #6, and it was not up yet. I just wanted to know if it is supposed to be up yet, or if it is coming up soon. Thanks!

  3. physics131-willocq Post author

    Several students have asked two questions about the written homework problem.

    Q1. Should we include the weight of the ball and its associated normal force?

    A1. NO. We are assuming that only the 3 forces given act on the ball.

    Q2. After calculating the x- and y-components of the acceleration, do I then proceed with the calculation of the magnitude of the acceleration vector and use that to calculate the distance traveled in 3 seconds?

    A2. NO. You need to consider the horizontal and vertical parts of the motion as independent (just like we have done for projectile motion). This means you will need to calculate the horizontal and vertical displacements separately using the corresponding acceleration in each direction. Only at the very end do you then compute the distance traveled by combining the horizontal and vertical displacements (treat those like x- and y-components of the overall two-dimensional displacement vector).

    HTH.

  6. mlindema

    on the online homework problem 5.50, I got the initial acceleration but I don’t understand how to do part B. Do you have any hints, I feel really lost.

  7. physics131-willocq Post author

    The second part of this problem follows along the same line as the first part. You need to use Newton’s end law along the vertical axis:

    Sum F_y = m a_y

    where Sum F_y is the sum over all forces acting in the vertical direction. In the second part, you are given the engine thrust (it is assumed constant) and the acceleration of the rocket a_y. The only unknown quantity in the equation above is m, the mass of rocket (it also enters the weight of the rocket). So your job is to solve for m.

    At the end, remember that the question is what mass of fuel is spent whereas solving the equation above gives you the mass of the rocket (fuel included) at the moment when it has the acceleration given.

  8. rdetroy

    I am also having trouble with part 2 to problem 5.50. I tried doing what you explained above however, I continue to get the wrong answer, even after subtracting the final mass that I found from the initial mass that was given. I wasn’t sure if the sum F_y is still the same as part 1 or whether weight is nolonger a force acting on the rocket. Let me know if you can clarify any of these problems any further.

  9. physics131-willocq Post author

    For part 2 of this rocket problem, you still need to consider both engine thrust and rocket weight as forces acting on the rocket.

    Remember that the weight of the rocket decreases as fuel is spent.

  10. mdobrien

    Could you clarify whether our lowest online hw will be dropped when calculating grades? The syllabus says that the online hw score will be computed from the sum of our 10 best scores…I suppose my question is more bluntly asking if we can expect more than 10 assignments or not? Thanks.

  12. mbellero

    on the problem with the hanging speaker, after working through the whole problem with hints and getting the right numbers I am still getting the wrong answer once plugged into the equation, the response says right track but check your geometry but the hints says the geometry is right.

  13. physics131-willocq Post author

    I suspect you may have trouble translating the vertical component of the tension into the magnitude of the tension vector.

    Solving Newton’s 2nd law in the vertical direction allows you to find the vertical component of the tension in each cable. You then need to use trigonometry to translate the vertical component of the tension into the total magnitude of the tension. Remember that the direction of the tension force is exactly the same as that of the cable hanging from the ceiling.

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