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Lana71 [14]
2 years ago
6

A particle’s position is ~r = (ct2 − 2dt3 )iˆ+ (2ct2 − dt2 )jˆ, where c and d are positive constants. Find the expressions for t

imes t > 0 when the particle is moving in (a) x-direction (b) y-direction
Physics
1 answer:
Mars2501 [29]2 years ago
7 0

The velocity of the particle is given by the derivative of the position vector:

\vec v = \dfrac{\mathrm d\vec r}{\mathrm dt} = (2ct-6dt^2)\,\vec\imath + (4ct-2dt)\,\vec\jmath

(a) The particle is moving in the <em>x</em>-direction when the <em>y</em>-component of velocity is zero:

4ct-2dt = 2t (2c - d) = 0 \implies t=0

But we want <em>t</em> > 0, so this never happens, unless 2<em>c</em> = <em>d</em> is given, in which case the <em>y</em>-component is always zero.

(b) Similarly, the particle moves in the <em>y</em>-direction when the <em>x</em>-component vanishes:

2ct-6dt^2 = 2t (c - 3dt) = 0 \implies t=0 \text{ or }c-3dt = 0

We drop the zero solution, and we're left with

c-3dt = 0 \implies c=3dt \implies \boxed{t = \dfrac c{3d}}

In the case of 2<em>c</em> = d, this times reduces to <em>t</em> = <em>c</em>/(6<em>c</em>) = 1/6.

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Explanation:

The average speed during any time interval is equal to the total distance travelled divided by the total time.

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