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Help with some homework on Vectors

  1. Sep 18, 2006 #1
    I have just a few homework problems that I just don't know where to start with at all. If there is anything anyone can do to push me in the right direction I would really appreciate it. I'm just not sure where to start on these. I have done a little work on some of them and come up with some guesses, but they are mostly that, guesses. Thanks in advance for any help at all.



    16- A bridge that was 5 m long has been washed out by the rain several days ago. How fast must a car be going to successfully jump the stream? Although the road is level on both sides of the bridge, the road on the far side is 2m lower than the road on this side.
    a. 5 m/s b. 8 m/s c. 13 m/s d. 25 m/s

    I think im supposed to use the pythagorean therom for this, and I came up with an answer close to A, but it wasnt exact. so im not sure


    18- A boat moves at 10 m/s relative to the water. If the boat is in a river where the current moves at 3 m/s, how long does it take the boat to make a complete round trip of 1000 m upstream followed by a 1000m trip downstram?
    a. 200 s b. 203 s c. 210 s d. 220 s

    Im pretty sure this one is D 220 s, but not totally positive.



    29- Superguy is flying at treetop level near Paris when he sees the Eiffel Tower elevator start to fall (the cable snapped). His X-ray vision tells him Lois LaTour is inside and needs help. If Superguy is 1.0 km away from the tower, and the elevator falls from a height of 240m, how long does Superguy have to save Lois, and what must be his average velocity? (Superguy needs only touch Lois before she hits the ground because he is magically immune to the laws of physics and anyone he touches can also instantly become immune.)
    a 3 s, 333 m/s b. 5 s, 200 m/s c. 7 s, 143 m/s d. 9 s, 111 m/s

    I figure he has to travel 1028 m to get to her just before she hits, now do i need to calculate her terminal velocity? if so Im not really sure how to go about that, I missed a few classes and the book is a little unclear.


    32- Wiley Coyote has missed the elusive road runner once again. This time, he runs off the edge of the cliff at 25 m/s horizontal velocity. If the canyon is 100 m deep, how far from the edge of the cliff does the coyote land?
    a. 112 m b. 225 m c. 337 m d. 400 m

    not sure where to begin at all with this one.



    38- A ball is rolled off a table with an initial speed of 0.24 m/s. A stop watch measures the ball's trajectory time from the table to the floor to be 0.3 s. What is the height of the table? (neglect air resistance)
    a. 0.11m b. 0.22m c. 0.33m d. 0.44m

    everything i tried on this one I came up with nothing close to the answers, so i'm really not sure where to start.
     
  2. jcsd
  3. Sep 18, 2006 #2

    andrevdh

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    16- Once the car has left the bridge it will act like a projectile.

    18- How did you calculate the speed of the boat?

    29- I would assume that the elevator falls out of rest under the influence of gravity alone.
     
  4. Sep 18, 2006 #3
    For qns 16, find the time the driver has before he drops lower than the 2m.
    equate the time to your horizontal motion.
     
  5. Sep 18, 2006 #4
    for qns 29-- don't complicate matters by taking into account the distance away from the ground super man is. i don't think there is a tree of 240 m in height is there? the distance is still 1000m.
     
  6. Sep 18, 2006 #5
    Qns 32 is excalty the same as Qns 16

    For Qns 38, look up the net for some kinematics equation, there should be 4, use one of them.
     
    Last edited: Sep 18, 2006
  7. Sep 18, 2006 #6
    any feedback?
     
  8. Sep 18, 2006 #7
    ok, I found my equation, but im not sure exactly what Vf is going to equal for problem 38 and on 16 im confused what my Vi should be, is it just 0 m/s?

    http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L6b.html

    btw, my backpack was stolen so I dont have a book and I'm trying to do this all with the internet as my only resource.
     
    Last edited: Sep 18, 2006
  9. Sep 18, 2006 #8
    Let's start with the first one:

    How long does it take (any object) to fall 2m starting from rest? (Integrate a = d^2x/dt^2 twice and rearrange.)
     
  10. Sep 18, 2006 #9

    Hootenanny

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  11. Sep 18, 2006 #10
    ok so im pretty sure i need to use a kinematic equation I am trying to use d = v0*t + (1/2)at^2 to find how long it will take the car to fall 2 meters in the "d" direction (the car has no initial velocity (v0) so I end up with 2 = 1/2(9.8)(t^2)

    here is where I think I mess up. I haven't taken algebra in years and I come out with 0.64 s, is that right? and after that do i need to do something similar with the other vector (5m)?
     
  12. Sep 18, 2006 #11

    Hootenanny

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    Perhaps a mentor could merge the two posts (this one and the one in intro physics?)
     
  13. Sep 18, 2006 #12
    no need, I think im just gonna start a new post with only one or two questions at a time, this is just confusing me too much. btw, is it better to post in here or the general physics forum?
     
  14. Sep 18, 2006 #13

    Hootenanny

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    If its homework related post them here. Otherwise chose the appropriate physics forum.
     
  15. Sep 18, 2006 #14
    Looks OK so far. Now, how fast do you need to be moving to cover 5m in 0.64s?
     
  16. Sep 18, 2006 #15
    should i use v = d/t, and solve for v?

    if so I get 7.8 m/s
     
  17. Sep 18, 2006 #16
  18. Sep 18, 2006 #17

    Doc Al

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    I merged the two threads. (Once is enough!)
     
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