You have the correct answers for both max height = 0.784m and initial vertical velocity = 3.92 m/s. But this part is incorrect: *Another formula for Vi is Vi=gt*
Vi=(9.8)(0.8)
Vi= 7.84 It seems you are trying to use the formula v2-v1 = at , but what this gives you is the change in velocity of the jumper while in the air. He starts upward at +3.92m/s, and ultimately lands going at -3.92 m/s, so his change in velocity is v2-v1 = -3.92m/s - 3.92 m/s = -7.84m/s. This approach would work if you applied the fact that v2 = -v1:
v2-v1 = -gt,
v2 - v1 = -2v1, so -2v1= -gt,
v1 = gt/2 = 9.8m/s x 0.8s x 1/2 = 3.92 m/s.
As for why a basketball player can only jump 0.784 meters - it's good that you are checking your answers for "reasonableness," but I would argue that it's not an unreasonable answer. When a 6-1/2 foot player dunks the ball he only needs to jump a bit over 1 meter off the ground. so at least the answer is in the ballpark (to use a baseball metaphor). If you had gotten an answer of 190 meters then you would have good reason to think there must be an error somewhere.
One other point - it helps to get in the habit of consistently using positive values for things going up and negative values for things going down, and hence the acceleration due to gravity is always -9.8m/s^2, it's never positive.
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Last edited by ChipB; Aug 22nd 2012 at 01:38 PM.
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