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Homework Help: Motion in two dimensions and radial acceleration

  1. Feb 4, 2006 #1
    Hi all! Brand new to these forums. Hopefully I fit in. I'm in my first physics class ever this semester. Physics 1301 Heat and Mechanics.
    I'm having a hard time keeping up with everything. I'm looking for some help with a few questions of my homework, because I'm just stuck.

    1)The radius of the earth's orbit around the sun (assumed to be circular) is 1.50 \times 10^8 km, and the earth travels around this orbit in 365 days.a)What is the magnitude of the orbital velocity of the earth in m/s? b) What is the radial acceleration of the earth toward the sun?

    a) I don't know where to start!! I've read this section of the book word for word and I can't find a thing related to |v|.
    b)All the equations I have require me to know velocity. I should be able to do this alright with v^2/radius

    2)Henrietta is going off to her physics class, jogging down the sidewalk at a speed of 3.75 m/s. Her husband Bruce suddenly realizes that she left in such a hurry that she forgot her lunch of bagels, so he runs to the window of their apartment, which is a height 35.9 m above the street level and directly above the sidewalk, to throw them to her. Bruce throws them horizontally at a time 6.00 s after Henrietta has passed below the window, and she catches them on the run. You can ignore air resistance. a)With what initial speed must Bruce throw the bagels so Henrietta can catch them just before they hit the ground? Take free fall acceleration to be g = 9.80 m/s^2. b) Where is Henrietta when she catches the bagels?

    a)The only thing I could think to do was y-y(initial)= v(initial)t+1/2*g*t^2 and solve for v(initial). t=6seconds g=9.8m/s^2 y-y(initial)=35.9m
    b)I'm guessing this would be the x=x(initial)=v(initial)t+1/2at^2

    Thanks in advance for your help.
  2. jcsd
  3. Feb 4, 2006 #2
    speed = distance / time
  4. Feb 4, 2006 #3
    I tried that. The homework is submitted online so I can enter in an answer and find out right away if it's correct. That formula wasn't correct.
  5. Feb 4, 2006 #4
    are you sure about the right units and equations?
    show me what you did
  6. Feb 4, 2006 #5
    part 1 of problem one is definitely speed=distance/time, but the radius isn't the total distance travelled. try to figure out a way to find the total distance travelled in 1 whole revolution round the sun.

    for part 2, you can use the equations of pseudo forces for circular motion.
  7. Feb 5, 2006 #6
    Code (Text):

    1.50X10^8 km    1000 m   1 day      4756 m
    --------------  -------  ------   =  --------
    365 days         1 km     86,400 s   seconds
    which comes up as wrong.
    @aalmighty: I don't understand.
  8. Feb 5, 2006 #7
    do you know the equation for the circumference of a circle? what path does the planet travel?
  9. Feb 5, 2006 #8
    I just had an "Ah ha!"
    Thank you.

    Now any help for problem 2?
    Last edited: Feb 5, 2006
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