A change in rate of motion or direction?

Velocity is the rate at which the position changes. for an object to have constant velocity, speed and direction must not be changing. A change in velocity can be either a change in how fast something is moving or change in the direction of movement.

in simple terms, the principle states that the buoyancy force on an object is equal to the weight of the fluid displaced by the object, or the density of the fluid multiplied by the submerged volume times the gravitational acceleration, g. this is also known as upthrust.