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Homework Help: 9th grade physics-just a little trouble

  1. Sep 26, 2004 #1
    9th grade physics--just a little trouble!!

    okay ive been sick lately and i need some help on this stuff:
    22. Show mathematically that an object can have a negative posiiton but a positive velocity.
    23. On a position time graph, compare the instantaneous velocities of an object when the tangent to the curve slopes upward to the right, when the tangnent slopes downward to the right, and when the tangent is horizontal.
    24. A stunt car is driven along a flat train car. The stunt car is moving toward the engine of the train. how would you calculate the velocity of the stunt car relative to Earth?
    25. Suppose a runner is running aboard an ocean liner at +12 km/h toward the southeast. what is the velocity of the ocean liner relative to the runner?
  2. jcsd
  3. Sep 26, 2004 #2
    Post your thoughts, concerns, etc.

    This forum -- as far as i know -- is not made to give answers without seeing any work.
    Please share what you are having trouble with and we will help you.


    If you are truly stuck, think about the definitions of terms like "acceleration" and "velocity". Compare "velocity" with "speed", "displacement" with "distance".

    Position is the closest distance from the final position to the initial position.

    Velocity is positive if you travel in the positive direction. Set up your reference point.

    When two objects travel beside each other, their velocities are "v2-v1". Therefore, if you travelling at 50 km/h beside a car with a velocity of 60km/h, it will feel like the car beside you is travelling at 10 km/h and you are still.
    If the same car is going the opposite direction, it will feel as if the car travelling is going 110 km/h.
    Last edited by a moderator: Sep 26, 2004
  4. Sep 26, 2004 #3
    pretty much everything :( i dont know the equation for 22..i know what the separate things are and stuff, i just dont know how to put it together. i could write an example but not an equation...
    the next one, i dont know how to graph instantaneous velocities. are they straight lines or points? the tangent is the average velocity right? then the next one, like whoa, i dont need an answer just an explanation..what does it mean relative to the earth? do i need the earths speed or soemthing? i think once i have the explanation to what that one is about and stuff, then i can probably answer the next one. im sorry if i sound like im mooching :P
  5. Sep 26, 2004 #4
    GRaphing instantaenous velocities is very easy. Simply graph a position time graph, and if it is curved: pick a poiint, and draw a straight line along that point, but not touching the curved line at any other point. Then, find the slope of that straight line (the tangent). That slope is the instantaneous velocity at half the time of the tangent.
  6. Sep 26, 2004 #5
    Alright, so 22, if i said that a car started at a 0 mile marker, it woudl b -1 from the 1 mile marker. say that was the origin, and it was heading there again or soemthing like that, at 20 mph..would that b right?
  7. Sep 26, 2004 #6
    well wouldnt all the tangents just go straight down? i mean..how do i know what direction????
  8. Sep 26, 2004 #7
    okay i think i got it... 'if the tangent slopes upto the right, then the velocity is increasing...if it slopes down to the right, its decreasing, and if its vertical, then its constant.' should i put that is it right?
  9. Sep 26, 2004 #8
    if the tangent slopes up to the right, then the velocity is positive. If it goes down to the right, then it's negative. If it is vertical, there is no defined slope.
  10. Sep 26, 2004 #9
    24. youd find how fast the train is moving, then the car, and subtract right?
  11. Sep 26, 2004 #10
    24. or how fast the earth is....wait the train or the earth?
  12. Sep 26, 2004 #11
    earths velocity is ever changing how could u find that?!
  13. Sep 27, 2004 #12
    omg im talkin to myself...
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