Answer:

Explanation:
The planet can be thought as a solid sphere rotating around its axis. The moment of inertia of a solid sphere rotating arount the axis is

where
M is the mass
R is the radius
For the planet in the problem, we have


Solving the equation for R, we find the radius of the planet:

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Using the equation;
TE = 1/2mv^2(1+2); where k = 2/5 for a solid sphere; V is the velocity, and m is the mass.
Total energy = 0.5 × 21 × 8² (7/5)
= 940.8 J
The rotational kinetic energy of the sphere is 940.8 J