June 2016 p42 q2
3752
A particle P moves in a straight line, starting from a point O. At time t s after leaving O, the velocity of P, v m s-1, is given by v = 4t2 - 8t + 3.
- Find the two values of t at which P is at instantaneous rest.
- Find the distance travelled by P between these two times.
Solution
(i) To find when P is at instantaneous rest, set the velocity v to zero:
\(4t^2 - 8t + 3 = 0\)
Factor the quadratic equation:
\((2t - 3)(2t - 1) = 0\)
Solving gives \(t = 0.5\) and \(t = 1.5\).
(ii) To find the distance travelled, integrate the velocity function:
\(s = -\int (4t^2 - 8t + 3) \, dt\)
Integrate to find the displacement:
\(s = -\left[ \frac{4}{3}t^3 - 4t^2 + 3t \right]_{0.5}^{1.5}\)
Evaluate the definite integral:
\(s = -\left[ \left( \frac{4}{3}(1.5)^3 - 4(1.5)^2 + 3(1.5) \right) - \left( \frac{4}{3}(0.5)^3 - 4(0.5)^2 + 3(0.5) \right) \right]\)
\(s = -\left[ 0 - \left( -\frac{2}{3} \right) \right] = \frac{2}{3}\) m
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