**Uniformly accelerated motion is motion with a constant, uniform change in velocity.** This often, but does not always, include a change in speed. For instance, an object falling in a vacuum under the influence of gravity has nearly uniform acceleration with a change in speed, while a planet orbiting a star in a circular orbit has uniform acceleration with no change in speed.

Any acceleration of an object necessarily implies a net force in the direction of acceleration. In an environment with negligible friction, a constant force of uniform magnitude produces constant and uniform acceleration. In the case of gravitational acceleration mentioned above, the constant force is gravity, which accelerates an object toward the gravitational center of another mass. This is true even if the initial direction of motion is opposite to the gravitational force, like a ball thrown straight up, because deceleration in one direction is equivalent to acceleration in the opposite direction.

In the case of an orbiting object such as a planet, the force of gravity accelerates the body not in the direction of motion, nor in the opposite direction, but at a right angle to it. This is similar to the centripetal forces that hold together the materials of a spinning wheel.