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Homework Help: Circular motion

  1. Mar 27, 2010 #1
    Question 1.

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

    The question shows a circle labeled as follows
    -A at the uppermost point
    -B at the most-right point
    -C at the lowest point
    -D at the most-left point

    so, the length along the circle from A to B (and B to C, C to D, D to A) is one quarter of the circumference.

    There is a particle traveling along this circle at constant speed, and completes one revolution in 5.0seconds.

    a) average speed for one revolution
    b) average speed from A to B
    c) average velocity for one revolution
    d) average velocity from A to C
    e) average velocity from A to B
    f) change in velocity from A to C
    g) change in velocity from A to B

    2. Relevant equations

    a = (v^2)/r
    v = wr

    3. The attempt at a solution

    I thought I understood circular motion and the definitions for speed and velocity until I came upon this question :(

    a) Well, speed is velocity without a direction, but when something is moving around a circle, it's direction is constantly changing so I'm unsure how to go about this question

    b) same problem as (a)

    c) I thought this to be zero, since it returns to its original displacement but I'm not sure if it is asking for angular or linear... are definitions different? Or if it isn't mentioned, is it normally linear?

    d) and e) same problem as (c)

    f)&g) thought acceleration was just (v^2)/r... I'm not sure thought because they ask from one place to another, not around the entire circle.

    Question 2..

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

    The turning circle of an F16 jet fighter when flying at a constant speed of 650 metres per second, has a radius of 80km. What is the angle of bank the plane must make to achieve this manoeuvre? (The angular tilt of its wings)

    3. The attempt at a solution

    I.. have no idea. The answer says 28.3 degrees, and I tried finding the angle using the velocity in one direction and acceleration perpendicular to it, with trig, but it comes out as 0.47 degrees (way too low)

    That's it for now I guess >_>

    Thanks ^^
  2. jcsd
  3. Mar 27, 2010 #2
    a&b) the question said "constant speed". You just need the length of the path traveled divided by the speed.

    c-e) average velocity is displacement/time. It's a vector with an x and y component.

    f&g) change in velocity is (final velocity) - (initial velocity)
  4. Mar 27, 2010 #3
    Yeah Willem2 has it spot on. look up the difference between distance and displacement,it's critical to problem 1. Problem one seems to be a insight into definitions.

    For Problem 2, you need to understand, why does the F16 need to bank? what does it achieve by banking? Its taking a turn right? why doesn't a car need to bank in order to make a turn? - Once you figure those out - Draw a diagram with the forces. and remember gravity is always down.... straight down. While lift is perpendicular to the F16.
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