Physics Projectile Question, don't know what i'm doing wrong...?

Question

A golf ball is hit at ground level. The ball is
observed to reach its maximum height above
ground level 8.5 s after being hit. 0.65 s after
reaching this maximum height, the ball is
observed to barely clear a fence that is 744 ft
from where it was hit.
The acceleration of gravity is 32 ft/s
2
.
How high is the fence?
Answer in units of ft


I've tried answering this question and i just can't figure out what i'm doing wrong..

this is what I have:

Vf = Vo + AT
so    -Vo = -32(8.5)
or Vo =272 

X = (1/2)AT^2+VoT
plugging in values: x=1/2(-32)(8.5+.65)+(272)(8.5+.65)=2342.4

but that isn't the right answer....
at first I thought i had the times mixed up but I tried switching them and it was still wrong... 

anyways any help would be appreciated

Paul · Accepted Answer

1.  If the ball was going straight up and down your equation would be correct, but it needs a correction since the travel is vertical AND horizontal.
2. Your calculation doesn't show where the time, t, gets squared.
3. If the peak is reached in 8.5 seconds, the ball hits the ground in 17 seconds
- (1/2)(32)(17^2) + K(17) = 0 (K = Vo x sin(theta)
-4624 + 17K = 0
17K = 4624
K = 4624/17 = 272
h(t) = -16t^2 + 272t
h = -16(9.15^2) + 272(9.15) = -1339.56 + 2488.8 = 1149.24 feet (height of fence)
this answer doesn't appear to be realistic; however, neither does it appear realistic that it takes the golf ball 8.5 seconds to reach its peak height.

jimmymae2000 · Answer

V up = AT = 32* 8.5 = 272 ft/sec
Height = 272/2 * 8.5 = 1156 feet
Displacment down = AT^2/2 = 32 * .65^2/2 = 6.76 ft
Fence height = 1156 ft - 6,76 ft = 1149.24 ft

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Physics Projectile Question, don't know what i'm doing wrong...?

2 Answers