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A projectile is fired at $45°$ to the horizontal. The elevation angle of the projectile at its highest point, as seen from the point of projection, is:
At max height $H = \dfrac{u^2}{4g}$; horizontal distance $= \dfrac{R}{2} = \dfrac{u^2}{2g}$
Elevation angle: $\tan\phi = \dfrac{H}{R/2} = \dfrac{u^2/(4g)}{u^2/(2g)} = \dfrac{1}{2}$
$\phi = \mathbf{\tan^{-1}\!\left(\dfrac{1}{2}\right)}$
Work done (in atm·dm³) when 1 mol ideal gas expands from 15 dm³ to 20 dm³ against constant external pressure 2 atm:
\(W = -P_{\text{ext}} \Delta V = -2 \times (20-15) = -2 \times 5 = -10\ \mathrm{atm·dm³}\). Negative sign indicates work done by the system (expansion).
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