Aplanet moves fastest in its orbit around the sun when it is at which position?

When it's closest to the sun.

Explanation:

The force of gravity acting on a planet is equal to its mass times its centripetal acceleration.

Fg = m v^2 / r

The force of gravity is defined by Newton's law of universal gravitation as:

Fg = mMG / r^2

Therefore:

mMG / r^2 = m v^2 / r

MG / r = v^2

v increases as r decreases.  So the planet is moving fastest when it's closest to the sun, also known as the perihelion.

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