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Homework Help: Noob in AP Physics ): Help with HW please?

  1. Sep 7, 2010 #1
    1. The problem statement, all variables and given/known data

    I've managed to solve a few, but a few major problems are bugging me (quite a few, unfortunately.)

    The first question is this:

    "A golfer takes two putts to sink his ball in the hole once he is on the green. The first putt displaces the ball 1.10 m east, and the second putt displaces it 8.27 m south. What displacement would put the ball in the hole in one putt? Answer in units of m"

    Easy, pythagorean theorem... however, the second part, is confusing me:

    "What is the direction (measured from due east, with counterclockwise positive) of the
    displacement? Answer in units of ◦."

    I drew it out, and managed to get something like 277.5764903.. not sure if it's correct, I got ~82.4 on the angle, and then subtracted that from 90 and then added 270 (because I'm assuming they're telling us to start at E... which is "0")?


    The 2nd confusing question...

    "The eye of a hurricane passes over Grand Bahama Island. It is moving in a direction 88.5◦ north of west with a speed of 43.7 km/h. Exactly 3.10 hours later, the course of the hurricane shifts due north, and its speed slows to 20.2 km/h, as shown. How far from Grand Bahama is the hurricane 4.87 h after it passes over the island? Answer in units of km."

    I'm in a tad of a hurry to finish my PreCalc homework, so I'll post up what I did for the hurricane question later. Please help me out! I get vectors OK, but this has gotten me stumped.

    Thanks alot!
    Jake.
     
  2. jcsd
  3. Sep 7, 2010 #2
    I drew it out, and managed to get something like 277.5764903.. not sure if it's correct, I got ~82.4 on the angle, and then subtracted that from 90 and then added 270 (because I'm assuming they're telling us to start at E... which is "0")? Looks correct. 277.58 degrees or -82.42 degrees so looks good on both counts!

    I'll help you with the hurricane problem when you can post what you've attempted later on^^
     
  4. Sep 8, 2010 #3
    1. I took TAN = y/x = 8.27/1.1, then got the Arctan and subtracted it from 360, which gave me the same answer.

    2. Is "88.5◦ north of west" equal to a heading of 358.5? Which is 1.5 off due north. So calculate an x and y for this new position. Then it travels due north for 4.87 - 3.1 hours. Add this value to the y. Solve for the new value of the hypotenuse.
     
  5. Sep 8, 2010 #4
    Hay guys, I got down n' dirty and figured that one out (171.blah), and finished a few more. Now this one (the directions really) are confuddling me... this is the actual question:

    A pedestrian moves 4.3 km east and then 14 km north. Find the magnitude of the resultant displacement vector. Answer in units of km.

    I got the answer of 14.65... haoever, the followup question has be bewildered.

    "What is the direction of the displacement vector (using the counter-clockwise angular di
    rection to be positive, within the limits of −180◦ to +180◦)? Answer in units of ◦."

    :(

    I'm completely lost on that one... help pl0x? D:
     
  6. Sep 8, 2010 #5

    PhanthomJay

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    It seems similar to the first problem...sketch it out...but the problem seems to be looking for the angle as measured ccw from the negative x axis....sort of a strange convention...
     
  7. Sep 8, 2010 #6
    Got it, thanks!

    Now this little bugger... I'm pretty sure I have the answer, but the computer denies it?

    "A car travels 15.3 km due north and then 28.2 km in a direction φ = 49.9 ◦ west of north. Find the magnitude of the car’s resultant displacement. Answer in units of km."

    My answer was 41.10226241... but it denies it :(
     
  8. Sep 9, 2010 #7

    PhanthomJay

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    First, don't put in so many decimal points. You've only got 3 significant figures in the problem, so you should round off your answer to just one decimal point. But beyond that, you seem to have your sines and cosines mixed up. The car travels 15.3 km north, then at an angle of 49.9 degrees west of north, which is 49.9 degrees counterclockwise from the y (north) axis.
     
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