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Math Help - Kinematics

  1. #1
    Member classicstrings's Avatar
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    Kinematics

    A particle moving in straight line with constant acceleration of 4m/s^2 has initial velocity of 11m/s

    How far does it travel in 2nd second of its motion?
    How far does it travel in 8th second of its motion?

    An object moving with a velocity of 42m/s is stopped in 7 seconds.
    How far has it travelled?

    Isn't that 42+36+30+24+18+12+6 = 168? But answer is 147.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by classicstrings View Post
    A particle moving in straight line with constant acceleration of 4m/s^2 has initial velocity of 11m/s

    How far does it travel in 2nd second of its motion?
    How far does it travel in 8th second of its motion?
    v=4t+11
    s=2t^2+11t

    (assuming it starts from 0)

    At start of 2nd second (t=1) it is at 13m, at end of 2nd second (t=2) it is at
    30m, so it has moved 30-13=17m.

    Now you should be able to do the 8th second part.

    An object moving with a velocity of 42m/s is stopped in 7 seconds.
    How far has it travelled?

    Isn't that 42+36+30+24+18+12+6 = 168? But answer is 147.
    Average speed is 21m/s, distance travelled in 7 seconds is 7*21=147.
    (using the aversge speed works here if we assume constant decelleration).

    RonL
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  3. #3
    Member classicstrings's Avatar
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    Sorry how did you get average speed is 21m/s?
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  4. #4
    Grand Panjandrum
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    Quote Originally Posted by classicstrings View Post
    Sorry how did you get average speed is 21m/s?
    start speed 42m/s decreasing linearly to 0m/s - average is 1/2 the starting speed.


    (Altenativly think of it this way: the distance travelled is the area under the speed/time
    graph - this is a triangle of height 42 and base 7 so area is 42*7/2)

    RonL
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