- #1
hatelove
- 101
- 1
Homework Statement
A ball is dropped that's 85 feet above the ground. The ball's initial speed is 5 feet/second, and the acceleration of the ball is 32 feet/sec^2. How long does the ball travel before hitting the ground?
Homework Equations
distance = rate * time
The Attempt at a Solution
So the speed of the ball I have as:
[t = time]
-5 - 32t
Which I believe makes sense because the ball is initially traveling @ -5 ft/sec, and after 1 second, the ball is traveling -37 ft/sec, then after 2 seconds the ball is traveling @ -69 ft.sec, etc.
I assume we use d = rt (which is distance = rate * time).
So the total distance the ball travels is -85 feet which is equal to the total rate of -5 - 32t * time. The equation I set up like this:
-85 = (-5 - 32t) * t
I set up the quadratic like so:
-32t^2 - 5t + 85 = 0
I solved it and got:
(1/64) * (sqrt(10905) - 5) which is approx. 1.55 seconds.
But the solution says it's 2.15 seconds, and I found out that it's because the solution needs to have taken the average velocity or something, meaning the average of:
-5 ft/sec and (-5 ft/sec - (32 ft/sec(t)))
So the average turns out to be (-5 ft/sec - 16 ft/sec * t) and I can go on with the rest of the solution and I get about 2.15 seconds as the answer.
What I don't understand is why we take the average velocity of the initial and the relative velocities.
I thought -5 - 32t was relative to the distance and time that the ball had traveled. For example:
At 0 seconds:
d = (-5 - 32(0))(0)
d = (-5 - 0))(0)
d = (-5 ft/sec))(0 seconds)
d = 0 ft.
So while the ball has not yet traveled, it has a velocity of -5 ft/sec.
After 1 second:
d = (-5 - 32(1))(1)
d = (-5 - 32)(1)
d = (-37 ft/sec)(1 second)
d = -37 ft.
After 1 second, the ball was traveling at -37 ft/sec, and has traveled -37 feet towards its destination so far.
After 2 seconds:
d = (-5 - 32(2))(2)
d = (-5 - 64)(2)
d = (-64 ft/sec)(2 seconds)
d = -128 ft.
The ball is traveling at -64 ft/sec but has already past the -85 ft. mark and presumably through the ground, -43 feet more underground (but that doesn't count right now in this scenario).
So when the ball has reached its final destination after 1.55 seconds:
d = (-5 - 49.6(1.55))(1.55)
d = (-81.88 ft/sec)(1.55 seconds)
d = -84.63 ft.
The ball was traveling -81.88 ft/sec right when it hit the ground -85 feet down.
Just for example's sake, I'll calculate the math for when the ball was halfway to its mark:
(-85ft / 2) = (-5 - 32(t))(t)
-42.5 ft. = -32t^2 - 5t
0 = -32t^2 - 5t + 42.5
t = 1.07696 seconds
So after 1.07696 seconds, the ball was halfway to the ground, and:
d = (-5 - 32(1.07696))(1.07696)
d = (-39.46272 ft/sec)(1.07696 seconds)
d = -42.4997709 feet
Which means that halfway to the target (-42.5 feet down), the ball had a velocity of -39.46272 ft/sec at 1.07696 seconds after the ball started to travel.
So as a recap:
At 0 feet = ball is traveling at -5 ft/sec after 0 seconds of traveling
At -37 feet = ball is traveling at -37 ft/sec after 1 second of traveling
At -42.5 feet = ball is traveling at -39.5 ft/sec after 1.07696 seconds of traveling
At -85 feet = ball is traveling at -81.88 ft/sec after 1.55 seconds of traveling
So what is the purpose of taking the average velocity?
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