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Supposedly easy physics problem =( [projectile]

  1. Jan 15, 2015 #1
    You launch a projectile with an initial velocity of 4 m/s at an angle of 60 degrees and it hits a target at a distance of 1.0 m in the horizontal direction.
    a. what is the y coordinate of the target
    b.how long does it take for the projectile to hit the target
    c.what is the y- component of its velocity just before it impacts the target
    d. and what is its velocity ... magnitude and idrection, just before it impacts the target ?


    this is what i did

    1. drew graph
    2.cos60=vox/4m/s
    vox=2m/s ; ax=0
    3.sin60= voy/4m/s
    voy= 3.46 m/s ; ay = -9.8ms2
    4.time to travel and hit the target.
    1.0/2 = .5 seconds
    5. y coordinate you can find the equation with
    y=voyT+1/2ayT^2
    plugging in what i know, i got .51

    im REALLY really confused.. please enlighten me
     
  2. jcsd
  3. Jan 15, 2015 #2

    haruspex

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    Looks ok so far (a and b)
    What equation can you write down for the vertical motion that involves final velocity?
     
  4. Jan 15, 2015 #3
    v^2=v0^2+2a(x-x0) is that it ?
    im really confused...
     
  5. Jan 15, 2015 #4

    haruspex

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    It will do, though it is not ideal here. It uses the final height (x) determined earlier, so plugging in the numerical value for that leads to some accumulation of rounding errors. It would be better to use an equation with initial velocity, acceleration and time. Yes, time is also a calculated value, but it was determined precisely.
     
  6. Jan 15, 2015 #5
    what can i do do instead ?
     
  7. Jan 15, 2015 #6

    haruspex

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    How does velocity change with time under a constant acceleration?
     
  8. Jan 15, 2015 #7
    im sorry but im not really catching on... can you explain your thought process so therefore i can understand the other previous problem better... i know this question should be REALLY easy... i feel like im missing out on something that is really obvious
     
  9. Jan 15, 2015 #8

    haruspex

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    Let's start with the basics. What is acceleration? If a car goes from stationary to 20 m/s in 10 seconds, what is its average acceleration?
     
  10. Jan 15, 2015 #9
    rate of change ? 1
     
  11. Jan 15, 2015 #10

    haruspex

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    Rate of change of what?
    1 what?
     
  12. Jan 15, 2015 #11
    velocity ms2.. i
     
  13. Jan 15, 2015 #12

    haruspex

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    Yes, acceleration is the rate of change of velocity. Put that in an equation. If the velocity changes by ##\Delta v## in time ##\Delta t##, what is the average acceleration?
     
  14. Jan 15, 2015 #13
  15. Jan 15, 2015 #14

    haruspex

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    OK, but that notation is usually reserved for continuous changes, dv/dt being the instantaneous acceleration. Here we are concerned with a change in speed over some significant period of time, so we use the ##\Delta## notation instead.
    So we have ##a = \frac{\Delta v}{\Delta t}##.
    Rearrange that in the form final speed = initial speed plus something.
     
  16. Jan 15, 2015 #15
    Is it a coincidence that I have the same physics homework as you? I don't know, but so far you're wrong. Your first mistake is the time. Yours is for 2m/s not 4m/s; this means that the time it should take to travel one meter is .25 seconds not .5
    This being said, I ended up with a y coordinate of 3.2 meters.
     
  17. Jan 15, 2015 #16
    i really appreciate you helping me but i dont understand how this is going to help me for the problem...
    a=dv/dt
    aDt=(v-v0)
    adT-v=-v0

    -aDt+v=v0
     
  18. Jan 15, 2015 #17

    haruspex

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    Good.
    In the vertical direction, you know the acceleration, the time to reach the target, and the initial speed. Plug those into the equation above.
     
  19. Jan 15, 2015 #18
    acceleration is 0
    initial speed is 4m/s
    time .5
    is that correct? after that, what do i need to do ?
     
  20. Jan 15, 2015 #19
    the time should be 0.25 seconds ._.
     
  21. Jan 15, 2015 #20
    who's ur teacher ?
     
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