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College level homework help.

  1. Oct 11, 2006 #1
    I have been assigned the following four questions, and am absolutely lost as to how to solve them. I am NOT looking for someone to solve them for me, however any general hints and tips, as well as pointing at other similar examples or equations would be wonderful! I want to be able to understand these on my own, not just copy someone else's work. Thanks in advance!

    1. A baseball player hits a home run that just barely clears the 12-m-high fence in right fi eld—92 m from home plate. If the ball was at a height of 1 m when it was hit, and its initial path after it left the bat was at an angle of 40° above the horizontal, what was the ball's initial speed when it left the bat?

    2. While installing an antenna on your roof, you lost your footing and slid off the roof. Fortunately, you placed a safety net whose center is positioned at a horizontal distance of 3.75 m, measured from a point directly below the edge of the roof—and indeed you landed in the center of the net. If the roof slopes downward at a 30° angle below the horizontal, and your speed as you left the roof was 4 m/s, how high was the edge of the roof above the level of the safety net?

    3. At the same moment when his teammate punts (kicks) the ball, a football player starts from rest—at a point exactly alongside the kicker—and runs down the level fi eld with constant (non-zero) acceleration,
    toward the opponent who is waiting to catch the ball. If the ball leaves the kicker's foot at an angle of 76.0° above the horizontal, what acceleration does the player need in order to arrive and tackle the opponent just as he catches the ball? (Assume the ball is caught at the same height as that from which it was kicked.)

    4. A train and a car collided in the night at a rural crossing on level ground. The train track is straight and runs northeast/southwest. At the point of collision (the crossing) the train was traveling northeast at a constant speed of 30 m/s. The road runs east/west, and at the point of collision, the car was traveling east at a constant speed of 20 m/s. What was the speed of impact?
  2. jcsd
  3. Oct 11, 2006 #2
    have you drawen diagrams for all these. that will help tremendously.
  4. Oct 11, 2006 #3
    So how much have you covered in Physics?

    Btw, I cant really help you out until you post on how far youve got or at least your thoughts on each question.
  5. Oct 11, 2006 #4
    I've attempted diagrams for each of the problems, but I'm still having issues with what formulas to use, since each I try seems to have a piece missing that I need. I tried making a table of varaiables to find, but still no luck. I know most of these have to do with sin/cos/tan type situations with all the angles posted. These problems were given to us after we'd just finished kinematics in two dimensions, so I know they probably have something to do with finding separate x and y values for horizontal and vertical distances.. Hope that helps.
  6. Oct 12, 2006 #5
    ok so for the first one, what equations do you have for x direction and y direction. And since this is a projectile problem what can you assume about acceleration in the y and x direction.
  7. Oct 12, 2006 #6
    I've had trouble trying to post this due to a terrible internet connection. This is my third try. Hope you're still interested.
    I'll give you some tips first on tackling 2D kinematics questions:
    1. Resolve the variables into 2 perpendicular directions, normally horizontal, x and vertical.
    2. Set a convention on which direction the variables are positive like velocity and displacement upwards is positive.
    3. Set a zero reference point.
    4. Remember that the only variable that is constant in both directions is time.
    5. Normally, we take horizontal acceleration as zero and vertical acceleration as 'g'. I prefer 9.81 m/s^2.
  8. Oct 13, 2006 #7
    For the first question, you can set the zero reference point at the 1 metre above the ground where the the ball was hit. That would mean that vertical displacement would be 11m while horizontal displacement 92m. Since you have the angle, you can find the vertical and horizontal components of the velocity. Take 'u', and then simply find it!
  9. Oct 13, 2006 #8
    I had a few TAs help me out, and I think I got things squared away. Thanks everyone for the help and the tips! Every little bit gets me one step closer to that A on the midterm. :)
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