The Art of Animation. Although a static straight line has a certain aesthetic appeal due to its geometric perfection, a monotone linear motion is much less likely to work satisfactorily - in most cases, it's simply boring. Accelerate or decelerate all instances of motion.
Constant speed adds a mechanic feeling to any movement.
Imagine that every element in your composition has a certain mass, and therefore cannot start or stop moving too abruptly. Use curvilinear motion paths instead of linear ones where possible, even if the curve is a simple circle. When moving an object along a linear path, rotating it at some degree during the shift will help to enliven it. Using rotation in combination with acceleration or deceleration will make a motion much more organic. Adding a third dimension to your motion may compensate for its too linear character.