Newton’s laws of motion allow us to analyze many kinds of motion. However, the analysis is often complicated, requiring details about the motion that we simply do not know. Here is an example: A puck is sent sliding along an inclined friction less track that includes several ups and downs (hills and valleys) of various shapes. The puck’s initial speed is 4.0 , and its initial height is 0.46 m. Using Newton’s second law, can you calculate the puck’s speed when it reaches the end of the track, at zero height? No, not without the details of how the incline varies all along the track, and then the calculations can be very complicated.
For an object of mass m whose speed v is well below the speed of light, we
define kinetic energy as
The SI unit of kinetic energy (and every other type of energy) is the joule named for James Prescott Joule, an English scientist of the 18oos. It is defined directly from in terms of the units for mass and velocity.
Thus, the flying duck has a kinetic energy.