What parts of your body are in control of voluntary movement ?

The brain controls voluntary movement.

The motor cortex. The motor cortex is located in the rear portion of the frontal lobe, just before the central sulcus (furrow) that separates the frontal lobe from the parietal lobe in the brain.

1) See explanation below, 2) False.

Explanation:

1) The velocity of an object is equal to the instaneous rate of change of its position vector in time with respect to a reference point. Each component of the vector represents the rate of change of the position component in time.

2) An object moving in a circle at constant speed has a constant magnitude but a changing direction, meaning that velocity vector is not constant. Hence, the statement is FALSE.