High School

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An object moves along the [tex]$x$[/tex] axis according to the equation [tex]$x = 2.75 t^2 - 2.00 t + 3.00$[/tex], where [tex][tex]$x$[/tex][/tex] is in meters and [tex]$t$[/tex] is in seconds.

(a) Determine the average speed between [tex]$t = 1.70 \, s$[/tex] and [tex][tex]$t = 3.40 \, s$[/tex][/tex].
[tex]\square \, \text{m/s}[/tex]

(b) Determine the instantaneous speed at [tex]$t = 1.70 \, s$[/tex].
[tex]\square \, \text{m/s}[/tex]

Determine the instantaneous speed at [tex]$t = 3.40 \, s$[/tex].
[tex]\square \, \text{m/s}[/tex]

(c) Determine the average acceleration between [tex][tex]$t = 1.70 \, s$[/tex][/tex] and [tex]$t = 3.40 \, s$[/tex].
[tex]\square \, \text{m/s}^2[/tex]

(d) Determine the instantaneous acceleration at [tex]$t = 1.70 \, s$[/tex].
[tex]\square \, \text{m/s}^2[/tex]

Determine the instantaneous acceleration at [tex][tex]$t = 3.40 \, s$[/tex][/tex].
[tex]\square \, \text{m/s}^2[/tex]

(e) At what time is the object at rest?

Answer :

We are given the position function

[tex]$$
x(t) = 2.75t^2 - 2.00t + 3.00,
$$[/tex]

with [tex]$x$[/tex] in meters and [tex]$t$[/tex] in seconds.

Below are the steps to find the required quantities.

────────────────────────
(a) Average Speed between [tex]$t = 1.70\text{ s}$[/tex] and [tex]$t = 3.40\text{ s}$[/tex]

1. First, compute the positions at the two times:

[tex]\[
x(1.70) = 2.75(1.70)^2 - 2.00(1.70) + 3.00 = 7.5475\text{ m},
\][/tex]
[tex]\[
x(3.40) = 2.75(3.40)^2 - 2.00(3.40) + 3.00 = 27.99\text{ m}.
\][/tex]

2. The displacement is

[tex]\[
\Delta x = x(3.40) - x(1.70) = 27.99 - 7.5475 = 20.4425\text{ m}.
\][/tex]

3. The time interval is

[tex]\[
\Delta t = 3.40 - 1.70 = 1.70\text{ s}.
\][/tex]

4. Therefore, the average speed is

[tex]\[
v_{\text{avg}} = \frac{\Delta x}{\Delta t} \approx \frac{20.4425}{1.70} \approx 12.025\text{ m/s}.
\][/tex]

────────────────────────
(b) Instantaneous Speed at Specific Times

1. The instantaneous velocity is the derivative of [tex]$x(t)$[/tex]. Differentiating, we get

[tex]\[
v(t) = \frac{dx}{dt} = 2(2.75)t - 2.00 = 5.5t - 2.00.
\][/tex]

2. At [tex]$t = 1.70\text{ s}$[/tex]:

[tex]\[
v(1.70) = 5.5(1.70) - 2.00 = 9.35 - 2.00 = 7.35\text{ m/s}.
\][/tex]

3. At [tex]$t = 3.40\text{ s}$[/tex]:

[tex]\[
v(3.40) = 5.5(3.40) - 2.00 = 18.70 - 2.00 = 16.70\text{ m/s}.
\][/tex]

────────────────────────
(c) Average Acceleration between [tex]$t = 1.70\text{ s}$[/tex] and [tex]$t = 3.40\text{ s}$[/tex]

1. The average acceleration is given by

[tex]\[
a_{\text{avg}} = \frac{v(3.40) - v(1.70)}{3.40 - 1.70}.
\][/tex]

2. Substituting the computed velocities:

[tex]\[
a_{\text{avg}} = \frac{16.70 - 7.35}{1.70} = \frac{9.35}{1.70} \approx 5.5\text{ m/s}^2.
\][/tex]

────────────────────────
(d) Instantaneous Acceleration at Specific Times

1. The instantaneous acceleration is the derivative of the velocity function. Since

[tex]\[
v(t) = 5.5t - 2.00,
\][/tex]

its derivative is

[tex]\[
a(t) = \frac{dv}{dt} = 5.5.
\][/tex]

2. Hence, the instantaneous acceleration at any time is

[tex]\[
a(1.70) = a(3.40) = 5.5\text{ m/s}^2.
\][/tex]

────────────────────────
(e) Time When the Object is at Rest

1. The object is at rest when [tex]$v(t) = 0$[/tex]. Set up the equation:

[tex]\[
5.5t - 2.00 = 0.
\][/tex]

2. Solve for [tex]$t$[/tex]:

[tex]\[
5.5t = 2.00 \quad \Longrightarrow \quad t = \frac{2.00}{5.5} \approx 0.3636\text{ s}.
\][/tex]

────────────────────────
Summary of Results:

- (a) Average speed between [tex]$t=1.70\text{ s}$[/tex] and [tex]$t=3.40\text{ s}$[/tex]: [tex]$\boxed{12.025\ \text{m/s}}$[/tex].
- (b) Instantaneous speed at [tex]$t=1.70\text{ s}$[/tex]: [tex]$\boxed{7.35\ \text{m/s}}$[/tex]; at [tex]$t=3.40\text{ s}$[/tex]: [tex]$\boxed{16.70\ \text{m/s}}$[/tex].
- (c) Average acceleration between [tex]$t=1.70\text{ s}$[/tex] and [tex]$t=3.40\text{ s}$[/tex]: [tex]$\boxed{5.5\ \text{m/s}^2}$[/tex].
- (d) Instantaneous acceleration at both [tex]$t=1.70\text{ s}$[/tex] and [tex]$t=3.40\text{ s}$[/tex]: [tex]$\boxed{5.5\ \text{m/s}^2}$[/tex].
- (e) The object is at rest at [tex]$t \approx \boxed{0.3636\ \text{s}}$[/tex].

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