Question
Show that there are two values of time for a projectile when it is at same height. Also show that the sum of these two times is equal to the time of flight.

OR

Prove the following relations by calculus method:

  1. $\text{s}=\text{ut}+\frac{1}{2}\text{at}^2$
  2. $\text{v}^2-\text{u}^2=2\text{as}$

Answer

For projectile motion equation for y-coordinates

$\text{y}=\text{u}\sin(\theta)\text{t}-\frac{1}{2}\text{gt}^2$

Solving this for t (using quadratic formula)

$\text{t}=\frac{\text{u}\sin\theta\pm\sqrt{\text{u}^2\sin^2\theta-2\text{gy}}}{\text{g}}$

Without loss of generalityl, let

$\text{t}_1=\frac{\text{u}\sin\theta}{\text{g}}+\frac{\sqrt{\text{u}^2\sin^2\theta-2\text{gy}}}{\text{g}}$

$\text{t}_2=\frac{\text{u}\sin\theta}{\text{g}}-\frac{\sqrt{\text{u}^2\sin^2\theta-2\text{gy}}}{\text{g}}$

for $\text{t}_1+\text{t}_2=\frac{2\text{u}\sin\theta}{\text{g}}$

which is equation of time of flight.

OR

  1. Consider an object moving in a straight line with uniform acceleration 'a', let at any instant of time 't', dx be the displacement of the objects.

$\therefore$ instantaneous velocity, $\text{v}=\frac{\text{dx}}{\text{dt}}$ or dx = vdt.

dx = (u + at)dt $(\because$ v = u + at)

let x0 and x be the displacements of the obhect at time 'zero' and 't'.

$\int\limits^{\text{x}}_{\text{x}_0}\text{dx}=\int\limits^{\text{t}}_0(\text{u}+\text{at})\text{dt}$

$=\text{u}\int\limits^{\text{t}}_0\text{dt}+\text{a}\int\limits^{\text{t}}_0\text{t dt}$

$(\text{x})^{\text{x}}_{\text{x}_0}=\text{u}(\text{t})^\text{t}_0+\text{a}\Big(\frac{\text{t}^2}{2}\Big)^\text{t}_0$

$\text{x}-\text{x}_0=\text{ut}+\frac{1}{2}\text{at}^2$

If x = x0 = s (distance) covered by the object.

$\text{s}=\text{ut}+\frac{1}{2}\text{at}^2$

  1. Consider a particle moving in a straight line with initial velocity 'v' and acceleration 'a'.

Then, $\text{a}=\frac{\text{dv}}{\text{dt}}=\frac{\text{dv}}{\text{dx}}\times\frac{\text{dx}}{\text{dt}}=\frac{\text{dv}}{\text{dx}}\times\text{v}$

$\text{a dx}=\text{v dv}$

Integrating,

$\int\limits^{\text{x}}_{\text{x}_0}\text{a dx}=\int\limits^{\text{V}}_\text{u}\text{v dv}$

$\Rightarrow\ \text{a}|\text{x}|^{\text{x}}_{\text{x}_0}=\Big[\frac{\text{v}^2}{2}\Big]^\text{V}_\text{u}$

$\text{a}(\text{x}-\text{x}_0)=\frac{\text{v}^2}{2}-\frac{\text{u}^2}{2}$

$\text{v}^2-\text{u}^2=2\text{a}(\text{x}-\text{x}_0)$

$\text{v}^2-\text{u}^2=2\text{a}\text{s.}$

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