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Homework Help: A few quick problems with kinematics

  1. Jun 1, 2012 #1
    Hi there,

    Sorry for so many questions, but I feel frankly stupid for getting these wrong, and yet I have no clue why they're so off. Any ideas of why the answers are what they are? Thanks so much.

    15. A truck starts from rest and accelerates at 1.5 m/s2for 20 s. It then travels at constant speed for 20 s and decelerates at –1.5 m/s2until it stops. How far did the truck travel?

    a. 300 m

    b. 600 m

    *c. 1200 m

    d. 1500 m

    I got 600 because I used x = 1/2at^2 + v0t + x0 with v0 = 0, a = 1.5 m/s^2, t = 20 s, and x = ?

    17. A ball is dropped from the top of a tall building. Just before it strikes the ground it is moving at 34.3 m/s. How tall is the building?

    a. 60 m

    *b. 131 m

    c. 250 m

    d. 125 m

    I got 60 m using v^2 = 2ax with v0 = 0, vf = 34.3 m/s, a = -9.8 m/s^2, and x = ?

    18. A bus leaves a terminal en-route to a destination 200 km away. During the trip, the bus stopped for a 45 minute lunch. The bus arrived at its destination four hours after it departed. The average velocity was:

    a. 0 km/hr

    b. 61 km/hr

    c. 30 km/hr

    *d. 50 km/hr

    I used v = (delta)x / (delta)t with delta x = 200 km, and delta t = 3.25 hr

    19. The position of a particle is given by x ( t ) =24 t – 4 t 2. At what time is the particle at rest?

    *a. 3s

    b. 8s

    c. 10s

    d. 2s

    I got 6, Wolfram got 6, I'm very confused.

    20. The position of a particle as a function of time is given by 4 t +3 t 2–2 t 3. What is the average acceleration over the time interval from 0 to 4s?

    a. 90 m/s2

    *b. –18m/s2

    c. –102 m/s2

    d. 102 m/s2

    Since acceleration is the second-derivative of position, I got 6 - 12t. Which apparently is wrong.

    Thanks again.
  2. jcsd
  3. Jun 1, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Did you include the constant speed segment? (I'd say that none of the answers are correct.)

    Your answer is correct.

    The zero-speed segment counts. Δt = 4 hours.
    The speed is zero, not the position.

    They want the average acceleration, not the instantaneous acceleration.
  4. Jun 1, 2012 #3
    http://img15.imageshack.us/img15/2332/p1010016rn.jpg [Broken]
    Last edited by a moderator: May 6, 2017
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