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Homework Help: Some Physhics Help

  1. Oct 17, 2006 #1
    I have some questions which I don't know how to do do. Please can someone help me do it please? I only have 20 mins to finish bunch of homework and out of all the questions, these are majors ones that I dont' know how to do.

    1. The force of air resistance on a raindrop is 4.8 x 10^-5 N when it falls with a terminal velocity of 5.4 m/s.

    The acceleration of gravity is 9.8 m/s^2.
    What is the mass of the raindrop? Answer in units of kg.

    2. A small piece of Styrofoam packing material is dropped from a height of 2.47 m above the ground. Until the terminal speed is reached, the acceleration is given by a = g - bv. After falling 0.64 m, it reaches terminal speed, and the Styrofoam takes an extra 2.2 s to reach the ground.

    What is the value of the constant b? Answer in units of s^-1.

    3. (Part of #2) What is the acceleration when the speed is 0.15 m/s? Answer in units of m/s^2.
  2. jcsd
  3. Oct 17, 2006 #2


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    After it reached terminal velocity, it doesn't accelerate anymore. What does Newton's laws tell you about the net force acting upon the raindrop?

    You are given how long it takes to reach the ground from a height you can calculate when it reaches terminal velocity, so you can calculate the terminal velocity. Then you can find b by again considering the forces acting on the package.

    a=g-bv. You know g, v is given, you calculated b above. So plug away.
  4. Oct 17, 2006 #3
    #1 is just a [tex] \sum F = ma [/tex]. It says terminal velocity so it implies that no acceleration exists, which means you just sum the forces = 0. Plug in the 2 forces and solve for mass.
  5. Oct 17, 2006 #4
    Wow guys thanks alot. I love this website. I am going to refer this site to alot of my freinds. I will also try to help out other people who need help. Thanks. I have more questions comming up. lol :biggrin:
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