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Velocity at a specific time

A particle moves along a line with position function s(t)=t2t−2s(t) = \dfrac{t^2}{t-2} for t>2t > 2, where tt is measured in seconds and ss in meters.

Find the velocity of the particle at t=5t = 5.

Hint

Use the quotient rule to differentiate s(t)=t2t−2s(t) = \dfrac{t^2}{t-2}, then substitute t=5t = 5 into the resulting expression for v(t)v(t).

Step-by-step solution

Apply the quotient rule: if s(t)=t2t−2s(t) = \dfrac{t^2}{t-2}, then v(t)=s′(t)=(2t)(t−2)−t2(1)(t−2)2v(t) = s'(t) = \dfrac{(2t)(t-2) - t^2(1)}{(t-2)^2}. Expanding the numerator gives 2t2−4t−t2=t2−4t2t^2 - 4t - t^2 = t^2 - 4t, so v(t)=t2−4t(t−2)2v(t) = \dfrac{t^2 - 4t}{(t-2)^2}.

Substitute t=5t = 5: v(5)=25−20(3)2=59v(5) = \dfrac{25 - 20}{(3)^2} = \dfrac{5}{9}. So the velocity at t=5t=5 is 59\dfrac{5}{9} meters per second (positive, meaning the particle is moving in the positive direction).

Behavior as t approaches 2 from the right

Describe what happens to the particle's position s(t)s(t) as t→2+t \to 2^+, and explain what this means physically for the particle's motion.

Hint

Consider the sign and size of the numerator t2t^2 and denominator t−2t - 2 as tt approaches 2 from values greater than 2.

Step-by-step solution

As t→2+t \to 2^+, the numerator t2→4t^2 \to 4 (a positive constant), while the denominator t−2→0+t - 2 \to 0^+ (a small positive number). Thus s(t)=t2t−2→+∞s(t) = \dfrac{t^2}{t-2} \to +\infty.

This means the particle's position grows without bound as tt approaches 2 from the right, so the particle races off toward positive infinity in a finite amount of time as t→2+t\to 2^+; there is no well-defined finite position or velocity at t=2t=2 itself, consistent with the domain restriction t>2t>2.

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