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Homework Help: Friction, Motion and Gravity

  1. Jul 22, 2008 #1
    Friction, Motion and Gravity. Need concept and understanding help!

    Three questions, I have a very little idea how to do them. Reason for not knowing is under the attempt part.

    1. The problem statement, all variables and given/known data

    Q1) Paul and Joe have decided to skip their next physics class. Know that their teacher patrols the bottom floor of the school they have decided to escape via a second floor window. A rope is attached to Paul, who has a mass of 75kg. The other end of the rope is led through a frictionless pulley and attached to Joe, who has a mass or 51kg. Paul steps onto his skateboard, and Joe falls out the window. What will be Paul's acceleration if there is a frctional force of 150N acting on the skateboard?

    Q2) Maegan, having a mass of 50kb has been sent into orbit around the earth as an astronaut. She is at an altitude of 300km above the surfae of the earth.
    a. What is the gravitational field intensity (strength) she would experience at this altitude?
    b. What will be the force of gravity acting on her body?

    Q3) Spencer is rushing down Brock street at 40m/s (meters per second) when a dag jumps out of nowhere 120m in from of the car. Being a great driver he is Spencer hits the brakes and comes to a stop in 100m. His car has a mass of 1200kg.
    a. What is the average acceleratio of the car?
    b. What is the kinetic friction as the car slides to a stop?
    c. What is the coefficient of friction between Spencer's tires and the road (his tired are an unknown compound).
    d. How long will it take Spencer to stop?

    2. Relevant equations

    All I know is gravity pulling objects to earth at 9.8m/s2

    3. The attempt at a solution

    Couldn't exactly attemp because my teacher has a stong accent and when I ask for help I mabey understand 20% of the words if I am lucky. I do have notes from copying the chalk board but my teacher almost never does examples, this is why I need help. If you could show me how to do the questions and tell me what exactly is going on it would be greatly appreciated.

    Thank You!
    Last edited: Jul 22, 2008
  2. jcsd
  3. Jul 22, 2008 #2


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    Gold Member

    Hi nubnub and welcome to PF,

    I appreciate that your situation may seem difficult, but with a half decent set of notes you should at least be able to attempt the set question. We will not show you how to do your homework, but we will help you through it.

    Let's start with question one, can you identify the concepts and physical laws involved in this question?
  4. Jul 22, 2008 #3
    To find the friction force on the skateboard with a 75kg weight on it attached to a 51kg weight through a frictingless pully.

    Not entirely sure.
  5. Jul 22, 2008 #4


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    Staff Emeritus
    Science Advisor
    Gold Member

    It might be best to draw a free body diagram for question one.
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