What are Kinetic and Potential Energies? Example of jumping.
You can think about the physics of moving objects in two different and equally acceptable ways: in terms of forces, or in terms of energy. Which you use depends on which is more convenient; both will always work. In this section, we'll look at energy in ice skating. It's an energetic section!
Kinetic energy is the energy of motion ("kinetic" means motion). So an ice skater flying across the ice has lots of kinetic energy. When he slams into the boards, that is transferred abruptly into thermal energy, sound, and work done on the skater (ie compressing his chest, rearranging his face, etc.) Seriously, to measure kinetic energy (KE), just measure the mass of the object and its velocity. KE = 1/2 mass*velocity*velocity or 1/2 mv^2.
If the movement is rotational instead of straight-line, then the equations are similar. (See Karen's section on rotation.) The idea is the same - how fast the skater is spinning is directly related to her kinetic energy.
It's easy to "see" kinetic energy in the motion of an object. But what about potential energy? Potential energy is stored energy. Energy can be stored in chemicals, by compressing a spring, or by doing work against gravity (ex: by placing an object on a higher shelf.) Muscles act like springs, so the chemical potential energy of muscles is converted, by the muscle applying a force, into kinetic energy of motion.
In a jump, the skater uses chemical potential energy (muscle power) to gain speed across the ice. When she jumps, she's also converting her chemical potential energy into kinetic energy. As she flies upward, her kinetic energy is converted into gravitational potential energy; as she slows, she gains height above the ice. At the top of her jump, she has no kinetic energy (for a moment!) There, all of her former kinetic energy is now gravitational potential energy. As she falls back down, her potential energy is converted back into kinetic energy. At the moment she hits the ice, all the gravitational potential energy she had at the jump's peak is again kinetic energy, so she hits pretty hard. If you measure the height of her jump, you can determine how hard she pushed off, which is also how hard she smacked down.
Back to the Physics of Ice Skating
by Karen Knierman and Jane Rigby