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The deck of a bridge is suspended 255 feet above a river. If a pebble falls off the side of the bridge, the height, in feet, of the pebble above the water surface after [tex]t[/tex] seconds is given by [tex]y = 255 - 16t^2[/tex].

(a) Find the average velocity (in [tex]ft/s[/tex]) of the pebble for the time period beginning when [tex]t = 3[/tex] and lasting the following amount of time:

(i) 0.1 seconds
[tex]\square[/tex] -97.6 [tex]ft/s[/tex]

(ii) 0.05 seconds
[tex]\square[/tex] -96.8 [tex]ft/s[/tex]

(iii) 0.01 seconds
[tex]\square[/tex] -96.16 [tex]ft/s[/tex]

(b) Estimate the instantaneous velocity (in [tex]ft/s[/tex]) of the pebble after 3 seconds.
[tex]\square[/tex] [tex]ft/s[/tex]

Answer :

Let's solve this problem step-by-step.

We start with the formula for the height of the pebble above the water surface, given by:

[tex]\[ y = 255 - 16t^2 \][/tex]

Our goal is to find the average velocity for different time intervals and estimate the instantaneous velocity at [tex]\( t = 3 \)[/tex] seconds.

### (a) Average Velocity Calculations

The average velocity over a time interval from [tex]\( t = 3 \)[/tex] to [tex]\( t = 3 + \Delta t \)[/tex] is calculated as:

[tex]\[
\text{Average Velocity} = \frac{y(3 + \Delta t) - y(3)}{\Delta t}
\][/tex]

Where [tex]\( y(t) = 255 - 16t^2 \)[/tex].

For (i) [tex]\(\Delta t = 0.1\)[/tex] seconds:

1. Calculate [tex]\( y(3) \)[/tex]:
[tex]\[
y(3) = 255 - 16(3)^2 = 255 - 144 = 111
\][/tex]

2. Calculate [tex]\( y(3.1) \)[/tex] (since [tex]\( t = 3 + 0.1 = 3.1 \)[/tex]):
[tex]\[
y(3.1) = 255 - 16(3.1)^2 = 255 - 16 \times 9.61 = 255 - 153.76 = 101.24
\][/tex]

3. Determine the average velocity:
[tex]\[
\text{Average Velocity} = \frac{101.24 - 111}{0.1} = \frac{-9.76}{0.1} = -97.6 \, \text{ft/s}
\][/tex]

For (ii) [tex]\(\Delta t = 0.05\)[/tex] seconds:

1. Calculate [tex]\( y(3.05) \)[/tex] (since [tex]\( t = 3 + 0.05 = 3.05 \)[/tex]):
[tex]\[
y(3.05) = 255 - 16(3.05)^2 = 255 - 16 \times 9.3025 = 255 - 148.84 = 106.16
\][/tex]

2. Determine the average velocity:
[tex]\[
\text{Average Velocity} = \frac{106.16 - 111}{0.05} = \frac{-4.84}{0.05} = -96.8 \, \text{ft/s}
\][/tex]

For (iii) [tex]\(\Delta t = 0.01\)[/tex] seconds:

1. Calculate [tex]\( y(3.01) \)[/tex] (since [tex]\( t = 3 + 0.01 = 3.01 \)[/tex]):
[tex]\[
y(3.01) = 255 - 16(3.01)^2 = 255 - 16 \times 9.0601 = 255 - 144.9616 = 110.0384
\][/tex]

2. Determine the average velocity:
[tex]\[
\text{Average Velocity} = \frac{110.0384 - 111}{0.01} = \frac{-0.9616}{0.01} = -96.16 \, \text{ft/s}
\][/tex]

### (b) Instantaneous Velocity Estimation

The instantaneous velocity is the derivative of [tex]\( y \)[/tex] with respect to [tex]\( t \)[/tex], evaluated at [tex]\( t = 3 \)[/tex]. The derivative of [tex]\( y = 255 - 16t^2 \)[/tex] is:

[tex]\[
\frac{dy}{dt} = -32t
\][/tex]

Evaluate this at [tex]\( t = 3 \)[/tex]:

[tex]\[
\text{Instantaneous Velocity} = -32 \times 3 = -96 \, \text{ft/s}
\][/tex]

Summary:
- Average Velocities: [tex]\(-97.6 \, \text{ft/s}\)[/tex], [tex]\(-96.8 \, \text{ft/s}\)[/tex], [tex]\(-96.16 \, \text{ft/s}\)[/tex]
- Instantaneous Velocity at [tex]\( t = 3 \)[/tex]: [tex]\(-96 \, \text{ft/s}\)[/tex]

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