Factors such as angular momentum, the moment of inertia, angular acceleration, and the skater's center of mass determines if a jump is successfully completed. Unlike jumping from dry land, which is fundamentally a linear movement, jumping on the ice is more complicated because of angular momentum. For example, most jumps involve rotation. Scientist James Richards from the University of Delaware stated that successful jumps depend upon "how much angular momentum do you leave the ice with, how small can you make your moment of inertia in the air, and how much time you can spend in the air". He found that many skaters, although they were able to gain the necessary angular momentum for takeoff, had difficulty gaining enough rotational speed to complete the jump. For example, a skater could successfully complete a jump by making small changes to his or her arm position partway through the rotation, and a small bend in the hips and knees allows a skater "to land with a lower center of mass than they started with, perhaps eking out a few precious degrees of rotation and a better body position for landing". A skater tends to spend the same amount of time in the air regardless of whether he or she completes triple or quadruple jumps, but his or her angular momentum at the start of triples and quadruples is slightly higher than it is for double jumps. The key to completing higher-rotation jumps is how a skater controls the moment of inertia.