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11 A particle travels in a straight line so that, t seconds after passing a fixed point A on the line, itsacceleration, a ms–2, is given by a = –2 – 2t. It comes to rest at a point B when t = 4.(i) Find the velocity of the particle at A. [4](ii) Find the distance AB.

Question

11 A particle travels in a straight line so that, t seconds after passing a fixed point A on the line, itsacceleration, a ms–2, is given by a = –2 – 2t. It comes to rest at a point B when t = 4.(i) Find the velocity of the particle at A. 4 Find the distance AB.

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Solution

(i) To find the velocity of the particle at point A, we need to integrate the acceleration function to get the velocity function. The acceleration a a is given by:

a=22t a = -2 - 2t

The velocity v v is the integral of the acceleration with respect to time t t :

v=adt=(22t)dt v = \int a \, dt = \int (-2 - 2t) \, dt

Integrating term by term:

v=2dt+2tdt v = \int -2 \, dt + \int -2t \, dt v=2tt2+C v = -2t - t^2 + C

where C C is the constant of integration. We need to determine C C using the given information. The particle comes to rest at t=4 t = 4 , so v(4)=0 v(4) = 0 :

0=2(4)(4)2+C 0 = -2(4) - (4)^2 + C 0=816+C 0 = -8 - 16 + C C=24 C = 24

Thus, the velocity function is:

v=2tt2+24 v = -2t - t^2 + 24

To find the velocity at point A (when t=0 t = 0 ):

v(0)=2(0)(0)2+24 v(0) = -2(0) - (0)^2 + 24 v(0)=24ms1 v(0) = 24 \, \text{ms}^{-1}

(ii) To find the distance AB, we need to integrate the velocity function to get the position function. The velocity v v is given by:

v=2tt2+24 v = -2t - t^2 + 24

The position s s is the integral of the velocity with respect to time t t :

s=vdt=(2tt2+24)dt s = \int v \, dt = \int (-2t - t^2 + 24) \, dt

Integrating term by term:

s=2tdt+t2dt+24dt s = \int -2t \, dt + \int -t^2 \, dt + \int 24 \, dt s=t2t33+24t+D s = -t^2 - \frac{t^3}{3} + 24t + D

where D D is the constant of integration. We can set D=0 D = 0 since we are measuring the distance from point A (where s=0 s = 0 when t=0 t = 0 ):

s=t2t33+24t s = -t^2 - \frac{t^3}{3} + 24t

To find the distance AB, we evaluate s s at t=4 t = 4 :

s(4)=42433+24(4) s(4) = -4^2 - \frac{4^3}{3} + 24(4) s(4)=16643+96 s(4) = -16 - \frac{64}{3} + 96 s(4)=1621.33+96 s(4) = -16 - 21.33 + 96 s(4)=58.67m s(4) = 58.67 \, \text{m}

Therefore, the distance AB is approximately 58.67m 58.67 \, \text{m} .

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