In circular motion, the velocity may remain constant but the direction of motion will change. If our automobile is going down the road at a constant 60 MPH and it goes around a curve in the road, the auto undergoes acceleration because its direction is constantly changing while it is in the curve. Roller coasters and other amusement park rides produce rapid changes in acceleration (sometimes called centripetal acceleration) which will cause such effects as "g" forces, "weightlessness" and other real or imaginary forces to act on the body, causing dramatic experiences to occur. Astronauts experience as much as 7 "gs" during lift-off of the space shuttle but once in orbit it appears that they have lost all their weight. The concept of "weightlessness" in space is a highly misunderstood phenomena. It is not caused by the fact that the shuttle is so far from the Earth; it is produced because the space shuttle is in free fall under the influence of gravity. The shuttle is traveling 17,400 MPH around Earth and it is continually falling toward Earth, but the Earth falls away from the shuttle at exactly the same rate.
Science EncyclopediaScience & Philosophy: 1,2-dibromoethane to AdrenergicAcceleration - History, Linear Acceleration, Circular Acceleration, Force And Acceleration