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Six easy physics problems (I have the answers, just need instructions.)

  1. Nov 3, 2008 #1
    I have a test in a day and I basically know almost everything. We were handed out a packet of about 50 questions for practice. I've gotten everything but these 6 problems.
    I have the answers, but I'm clueless on how to get them. I have an A average in Physics AP and would like to keep that, haha. Could anyone help me figure these problems out? (Instructions) Thanks!

    1. In space, a 70.0-kg astronaut pushes to the left on a spacecraft with a force R. The spacecraft has a total mass of 1.0 x 10^4 kg. During the push, the astronaut accelerates to the right with an acceleration of 0.36 m/s2. Determine the magnitude of the acceleration of the spacecraft. [Answer: 2.5 x 10^-3 m/s2]

    2. A muscle builder holds the ends of a massless rope. At the center of the rope, a 15-kg ball is hung. What is the tension in the rope if both angles θ are 4.5°. (The angles are under, respecting the horizontal line.) [Answer: 940N]

    3. If the coefficient of kinetic friction, μk, between a block and the surface is .30 and the magnitude of the frictional force is 80N, what is the weight of the block? [Answer: 4.0]

    4. A 250-N force is directed horizontally to push a 29-kg box up an inclined plane (27 degrees respect to the horizontal) at a constant speed. Determine the magnitude of the normal force, FN, and the coefficient of kinetic friction, µk. [Answer: 370N and .26]

    5. A 2.0-N rock slides on a frictionless inclined plane. Which one of the following statements is true concerning the normal force that the plane exerts on the rock?
    A) The normal force is zero newtons.
    B) The normal force is 2.0 N.
    C) The normal force is greater than 2.0 N.
    D) It increases as the angle of inclination, q, is increased.
    E) The normal force is less than 2.0 N, but greater than zero newtons. [Answer: E]

    6. Two forces act on a 4.5-kg block resting on a frictionless surface as shown. What is the magnitude of the horizontal acceleration of the block? Link to image: http://img363.imageshack.us/img363/6591/picki0.jpg [Broken] [Answer: 1.8 m/s2]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Nov 3, 2008 #2


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    Welcome to PF. Please note that, in order for us to help you, we first need to see some attempt. So, what have you tried for these problems?
  4. Nov 3, 2008 #3
    Argh, I don't really have the time to say, I was in hopes of just putting this up and waking up in the morning to review what people have said and to apply that. (It's almost midnight here and my mother wants me in bed.)

    I did try multitudes of ways, even by working the problems backwards. I get all the concepts and everything, but these problems are just a bit vague at the moment. I can't contact my teacher tomorrow since we're off of school and there's no one around me that knows physics. I just found this forum five minutes ago and it looked pretty active, so I decided to ask here.

    I hope you understand, thanks and good night. :)
  5. Nov 3, 2008 #4


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    You don't have time to attempt the problems, but you want someone else to do them for you?

    Ok, so what did you try? I would not advise on "working the problems backwards", since if in future you are not given the answer, you will have no idea how to find it. I'm not sure what you mean by saying the problems are vague: they are not.

    Regardless of your situation, we have a blanket policy here that we do not help students that don't show us any work.
  6. Nov 3, 2008 #5
    I don't know what to show, though. They were multiple choice questions and he said that it doesn't take long to solve it. I just don't understand the process for these specific questions.
  7. Nov 16, 2011 #6
    Uhhh I have a question about #4
    So what I did was find the parallel and perpendicular forces of gravity on the box
    But do you add 250sin(27) to 284.2cos(27) To find normal force?
    And how do you find kinetic frictional force?? Is it 250cos(27) - 284.2sin(27) ?
    Last edited: Nov 16, 2011
  8. Nov 17, 2011 #7


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    Welcome to Physics Forums.
    Also correct, yes.

    p.s. Thank you for taking the time to show what you have worked out.
  9. Nov 17, 2011 #8


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    Staff: Mentor

    Unfortunately, that wasn't the OP!
  10. Nov 17, 2011 #9


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    Yes, understood. The OP is long gone.
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