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Forces and Motion

  1. May 29, 2006 #1
    Hello everyone!

    I am having a little trouble with my physics homework and am hoping for someone(s) to confirm if I am on the right track. Please and thank you! :smile:

    1. In a total time of 2.0min, a duck on a pond paddles 22m [36° N of E] and then paddles another 65m [25° E of S].

    Determine the duck's
    i) total distance travelled

    Total = 22m + 65m = 87m

    ii) average speed

    v = d/t = 87m/120s = 0.73m/s

    iii) total displacement
    a² = b² + c²
    65² = 22² +c²
    c = 61

    iv) average velocity
    v = d/t = 65-22/120 = 0.36m/s[E]

    2. The skid marks of a car was 150m from the point where the driver applied the brakes to where the car came to rest. The driver was travelling at 80km/h(22m/s) when the brakes were applied. Calculate the acceleration of the vehicle.

    Vf² = Vi² + 2aΔd
    0 = (22m/s)² + 2a(150m)
    484/150 = 2a
    a = 1.6m/s²

    3. An archer shoots an arrow straight up into the air with an initial velocity of 45m/s . Calculate the maximum height of the arrow.

    Vf² = Vi² + 2aΔd
    0 = (45m/s)² + 2(-9.8m/s²) Δd
    Δd = -2025/-19.6
    Δd = 103m

    Thank you once again!! :biggrin:
  2. jcsd
  3. May 29, 2006 #2
    your first two answers to the first question are right, but the other two need some fixing.

    (iii)From the angles you gave in the question you can't actually use pythagoras' theorem for this question. You have to use another rule, which is the lay of cosines. http://en.wikipedia.org/wiki/Triangle

    This explains it reasonably well and it provides a diagram so you can find the unknown side reasonably easy.

    (iv)Your average velocity is displacement/time, so you will use the displacement value above (iii) and the total time given in the question to calculate the average velocity. Also remembering that it is a vector quantity you will have to give direction and bearing (angle).

    With question 2 when you tak Vi to the LHS of the equal sign, you end up changing the sign so it becomes -484, which will yield a= -1.6 m/s^2. This is what you'd expect 'cause when you are breaking you are decelerating.
  4. Jun 5, 2006 #3
    thank you for the help!
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