Answer

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**Hint:**To solve this question one needs to know some things about projectile motion. Using the knowledge of the motion of a projectile, form some equations in terms of the angle of projectile and the initial velocity then solve the equation to calculate the angle of projection.

**Formula used:**

${{v}_{y}}=u\sin \theta -gt$

where ${{v}_{y}}$ is velocity of the projectile in vertical direction at time t, g is acceleration due to gravity, u is initial velocity and $\theta $ is angle of projection.

${{v}_{x}}=u\cos \theta $

where ${{v}_{x}}$ is velocity of the projectile in horizontal direction at time t.

$T=\dfrac{2u\sin \theta }{g}$

where T is the time of flight of the projectile.

**Complete step by step answer:**

It is given that the speed of the projectile is minimum after two seconds and we know that the speed of the projectile is minimum at the highest point from the ground. A projectile projected from the ground, takes half of the time equal to its time of flight to reach the maximum height.

This means that $2=\dfrac{T}{2}=\dfrac{\left( \dfrac{2u\sin \theta }{g} \right)}{2}$.

$\Rightarrow \dfrac{u\sin \theta }{g}=2$ …. (i).

It is given that at time $t=1s$, the motion of the projectile makes an angle of ${{45}^{\circ }}$ with the horizontal.

Let the speed of the projectile at this time be v. Therefore, its speed in the horizontal direction is ${{v}_{x}}=v\cos {{45}^{\circ }}=\dfrac{v}{\sqrt{2}}$ and its speed in the vertical direction is ${{v}_{y}}=v\sin {{45}^{\circ }}=\dfrac{v}{\sqrt{2}}$

But we know that after one second, ${{v}_{x}}=u\cos \theta $ and ${{v}_{y}}=u\sin \theta -g(1)$. Therefore, from the above four equations we get that ${{v}_{x}}={{v}_{y}}=u\cos \theta =u\sin \theta -g$.

This means that $g=u\sin \theta -u\cos \theta $

$\Rightarrow u=\dfrac{g}{\sin \theta -\cos \theta }$

Substitute the value of u in equation (i).

$\Rightarrow \left( \dfrac{g}{\sin \theta -\cos \theta } \right)\dfrac{\sin \theta }{g}=2$

$\Rightarrow \sin \theta =2(\sin \theta -\cos \theta )$

$\Rightarrow \sin \theta =2\sin \theta -2\cos \theta )$

On simplifying further we get $\dfrac{\sin \theta }{\cos \theta }=2$

$\Rightarrow \tan \theta =2$

$\therefore \theta ={{\tan }^{-1}}2$

**Therefore, the angle of projection is equal to ${{\tan }^{-1}}(2)$.**

**Note:**The motion of the projectile is influenced by the gravitational force only (in absence of air resistance). Since the direction of the gravitational force is always in the downward direction, the projectile is only accelerated in the downward direction and has a constant velocity along the horizontal direction.If you do not remember the formula for the time of flight, then you can easily derive it using kinematic equations.

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