An object of mass $1 \mathrm{kg}$ is thrown downwards from a height of $20 \mathrm{m}$. The initial speed of the object is $6 \mathrm{m} {\mathrm{s}}^{-1}$.
The object hits the ground at a speed of $20 \mathrm{m} {\mathrm{s}}^{-1}$. Assume $\mathrm{g}=10 \mathrm{m} {\mathrm{s}}^{-2}$. What is the best estimate of the energy transferred from the object to the air as it falls?

A.  $6 \mathrm{J}$

B.  $18 \mathrm{J}$

C.  $182 \mathrm{J}$

D.  $200 \mathrm{J}$

Calculate the speed of the probe in terms of $c$, relative to Earth.

B

$u=\frac{0.5c+0.8c}{1+{\displaystyle \frac{0.5c\times 0.8c}{c^{\mathit{2}}}}}$ ✓

$u=0.93\mathrm{c}$ ✓

Allow all negative signs for velocities

Award [2] marks for a bald correct answer