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Homework Help: Help me please (linear motion) HELP Please!

  1. Jun 27, 2008 #1
    1. The problem statement, all variables and given/known data

    1.Two runners accelerate from rest with the acceleration of runner A twice that of runner B. After accelerating for a given time, the speed of runner A will be twice that of runner B.
    is this right or wrong?

    2.An ball is thrown vertically upward. At the top of its flight, both the ball's velocity and acceleration are zero.
    Is this right or wrong?

    3.An object is thrown downward with a speed of 10 m/s from a bridge. Accelerating under gravity (9.8 m/s2), the object strikes the water 1.8 s later. From what height above the water was the object thrown? (Assume 2 significant digits.)
    what is the right answer?

    4.A stone is thrown downward from a window. Accelerating under gravity (9.8 m/s2), the stone hits the ground 2.6 s later with a speed of 20.0 m/s. From what height above the ground was it thrown?
    what is the right answer?

    5.An 80.4-km trip takes a time of 0.75 h to complete. The average speed, expressed in the correct manner, is?

    6.An rockis thrown vertically upward at 25.0 m/s. If it experiences an acceleration due to gravity of 9.8 m/s2 [down], what is the rock's velocity 3.0 s later?

    2. Relevant equations

    v2=v1+a(change in time)
    (delta D)= v1(change in time) +1/2a (change in time)squared
    there are some more that i probably dont know

    3. The attempt at a solution

    1. i got that number 1 is wrong
    2.i got that number 2 is right
    3. i got 27m
    4,5,6 i had no clue

  2. jcsd
  3. Jun 27, 2008 #2
    for question 4 i got 85m
  4. Jun 27, 2008 #3
    for question 5 i got
    107.2 i dont know which way is right is it
    1.1 × 102 km/h
    107.2 km/h
    4 )
    1 × 102 km/h
    1.072 × 102 km/h
  5. Jun 27, 2008 #4
    for 6 i got -4.4
    i used v2=v1 +a(change in time)
    i got -4.4
    Therefore it has hit the ground?
  6. Jun 27, 2008 #5


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    Homework Helper

    1) is right. 2) is wrong. You tell me why. That's a really bad start. Those were easy. And it doesn't get better after that. You don't understand this stuff well enough for a quick answer to really help you. Ask these questions again, one at a time, when you have the time to actually learn this subject.
  7. Jun 27, 2008 #6
    nvm i got it all
  8. Jun 28, 2008 #7


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    Science Advisor
    Homework Helper

    That's good. You learn quickly.
  9. Jun 28, 2008 #8
    im stupid sometimes its so easy i think it cant be like that lol
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