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Homework Help: Velocity of a freely falling object

  1. Sep 3, 2012 #1
    1. The problem statement, all variables and given/known data
    A certain freely falling object, released from rest, requires 1.65 s to travel the last 39.0 m before it hits the ground.
    (a) Find the velocity of the object when it is 39.0 m above the ground. (Indicate the direction with the sign of your answer. Let the positive direction be upward.)
    (b) Find the total distance the object travels during the fall.

    2. Relevant equations
    distance Δx=vavg(t)

    3. The attempt at a solution
    a) To find the velocity I am simply using the velocity formula, and making a=-9.8 because it is falling down.
    When I put -16.17 into the answer online it response is within 10% of the correct value. This may be due to roundoff error, or you could have a mistake in your calculation. Carry out all intermediate results to at least four-digit accuracy to minimize roundoff error.
    I also tried -16.2, but before I use another try (I only have 3 left) I wanted to make sure that I am close to the answer, so I can try rounding to 16m/s.
    b) I am confused on this one but I got 47.1m and it tells me the same thing, that I am within 10% of the correct value.
  2. jcsd
  3. Sep 3, 2012 #2
    You have distance (-39.5 meters), you have time (1.65 seconds) and you have acceleration (-9.8 meters/s2). You should know the formula s = v0t + at2/2. So you should be able to find v0 from this equation. That's part 1.

    For part 2, you have v0 that the object acquired after falling for some unknown time. For this, you should be able to find out the time. Then use the distance formula again to find out the total distance.
  4. Dec 3, 2013 #3
    Does velocity of freely falling object depends on mass???
  5. Dec 3, 2013 #4
    No. Acceleration due to gravity is a constant g=9.807 m/s^2 for all objects regardless of mass. Only aerodynamic considerations will affect the velocities.
  6. Dec 4, 2013 #5
    what do mean by aerodynamic consoderations???! consider a book and a large car dropped from a certain height. car reaches first and That is due to the surface area of the car more then book and so atmosphEric pressure acting on the car is more , so car reaches ground first. Am I right? But the velocity that the both the bodies hit the ground is same.
  7. Dec 4, 2013 #6
  8. Dec 4, 2013 #7
    A larger surface area in relation to the mass of an object is likely to slow it. Think "wind resistance" slowing the acceleration of an object. In a vacuum, a feather and a bowling ball dropped from any height will hit the ground at the same time. Whereas in actual atmospheric conditions, the feather will fall much more slowly than the bowling ball because the feather has a huge surface area in relation to its mass.
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