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Homework Help: Need help with a physics problem

  1. Nov 1, 2003 #1
    Ok, this is a two part problem.

    Part I: We have a ball at the top of a roof that has a 30 degree angle at the horizontal. We let the ball roll and it takes 2.55 seconds to reach the bottom of the 6.34 meter roof. The ball weighs .35 kg. **DISREGARD ANY FRICTION** I.E friction of the ramp, wind resistance, any type of friction. The main part of this is to find the Final Velocity at the end of the 6.34 meter roof. So you have Vi(initial velocity) = 0, Vf(Final Velocity) = ????, G(gravity) = 9.8m/s^2. The ramp has a 30 degree angle at the horizontal, and it takes 2.55 seconds to get to the bottom.

    Part II: After we find the Final Velocity, we have to figure out how far the ball will travel in the air. After it reaches the final velocity, it falls off the roof 2.51 meters to the ground. The *main part* of the entire problem is to figure out how far it will go off the roof. The Y axis is 2.51 meters, and we need to find out what the X axis will be. We need to find out Dx(distance of x), Vx(Velocity of x), and T(time it takes to reach the ground).

    My friends and I have been having trouble with this problem all day. We keep coming up with wrong, impossible answers. Any help is appreciated. Thanks.
  2. jcsd
  3. Nov 1, 2003 #2


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    OK, so we can't be assuming NO friction if we assume the ball rolls. The problem is a little different depending on whether the ball slides down the roof or rolls down the roof; it's not especially harder either way, just different. Anyway, like most problems, there is more than one way to solve this. You could do a balance the forces and moments acting on the ball to find its acceleration and find its speed at the bottom of the roof. It's probably easier to use the conservation of energy (no work is lost to friction here): The ball has some initial potential energy given by mgh, where h is the distance above some datum you define; if you choose the datum to be the height of the base of the roof (i.e., 2.51 m off the ground), then the ball will end up with 0 potential energy there. Set this potential energy equal to the kinetic energy associated with some velocity, v, of the ball: the translational kinetic energy is given by .5*m*v^2, and the rotational kinetic energy is given by .5*I*ω^2. If I remember correctly, the moment of inertia about a center axis for a solid sphere is (2/5)*m*r^2. Also use the fact that you have a geometric constraint in this problem because the ball is rolling: ω = v/r. So in one line you get the speed of the ball (v) as it leaves the roof and you know that it is directed along the angle of the roof.

    For part 2, break this velocity up into components (horizontal and vertical). There is no acceleration in the horizontal direction and there is constant acceleration downward due to gravity in the vertical direction. So get the time of flight from the given drop distance in the y direction and use that to find the change in x position. I hope this description is enough for you to figure out which equations to use.
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