Integration in Kinematics of Linear Motion

8.3   Integration in Kinematics of Linear Motion
 
  • If given the acceleration function, \(a\) that is \(a=\dfrac{dv}{dt}\), the velocity function can be determined by integration
  • In general, the relationship between acceleration function, \(a=h(t)\) and the velocity function \(v=g(t)\) can be simplified as follows:
 
 
  • The same goes with the displacement function, \(s\) that can be obtained by performing two consecutive integrations, which are:
     
    \(v=\int a \ dt \ \text{ and } \ s=\int v \ dt\)    
     
 
Example
     
 

A particle moves along a straight line and passes through a fixed point \(O\) with a velocity of \(12\text{ ms}^{-1}\).

The acceleration, \(a\text{ ms}^{-2}\), at \(t\) seconds after passing through \(O\)is given by \(a=4-2t\).

 
  

Determine the instantaneous displacement, in m, of the particle from \(O\)  

(i)

when \(t=3\),

(ii)

when the particle is at rest.

   

Solution:

\(\begin{aligned} \text{Velocity function, }v \text{ is given by } \ v&=\int a \ dt\\ v&=\int(4-2t) \ dt\\ v&=4t-t^2+c\\ \end{aligned}\)

\(\begin{aligned} \text{When }t=0 \text{ and }v=12, \text{ then, } \ 12&= 4(0)-0^2 +c\\ c&=12\\ \end{aligned}\)

\(\text{Hence, at time }t, \ v=12+4t-t^2\)

\(\begin{aligned} \text{Displacement function, } s \text{ is given by, }\ s&=\int v \ dt\\ s&=\int (12+4t-t^2) \ dt\\ s&=12t+2t^2-\dfrac{1}{3}t^3+c\\ \end{aligned}\)

When \(t=0 \text{ and }s=0\),

\(\begin{aligned} \text{then, } \ 0&=12(0)+2(0)^2-\dfrac{1}{3}(0)^3+c\\ c&=0\\ \end{aligned}\)

Hence, at time \(t,\ s=12t+2t^2-\dfrac{1}{3}t^3\)

(i)

\(\begin{aligned} \text{When }t=3, \ \ s&= 12(3)+2(3)^2-\dfrac{1}{3}(3)^3\\ s&=36+18-9\\ s&=45 \end{aligned}\)

Hence, the instantaneous displacement when \(t=3 \text{ is }45 \text{ m}\).

   
(ii)

When the particle is at rest, \(v=0\)

\(\begin{aligned} \text{Then, }12+4t-t^2&=0\\ t^2-4t-12&=0\\ (t+2)(t-6)&=0\\ \text{Since }\ t \geqslant 0, t&=6 \end{aligned}\)

\(\begin{aligned} \text{When }t=6, \ \ s&= 12(6)+2(6)^2-\dfrac{1}{3}(6)^3\\ s&=72+72-72\\ s&=72\\ \end{aligned}\)

Hence, the instantaneous displacement when the particle is at rest is \(72 \text{ m}\).