Kinetic Energy

Origin of Work  Work describes a process.  Where does work come from, and go to?  The ability to do work can come from our bodies, electricity, chemicals, nuclear reactions, and motion.  Energy measures the state of an object, and its ability to do work.

Energy of Motion  Consider the work done on an object with constant acceleration in a straight line.  The distance is related to the acceleration. F=ma d The work done becomes kinetic energy: (1/2) mv 2

Changing Energy  The definition of kinetic energy applies to all objects. If an object speeds up it gains kinetic energy. If an object slows down it loses kinetic energy.  This matches the work done on or by an object. Work done on the ball v2v2 v1v1 Work done by the ball v4v4 v3v3

Work-Energy Principle  The change in kinetic energy is due to net work done on the object. W net = K 2 – K 1 =  KW net = K 2 – K 1 =  K The sign of work is positive when energy is addedThe sign of work is positive when energy is added

Using Kinetic Energy  A man lets the 300 kg piano pick up speed from 2.0 m/s to 4.0 m/s over 3.0 m. What work is done on the piano? The initial kinetic energy is K 1 = (0.5)(300 kg)(2.0 m/s) 2 = 600 J. The final kinetic energy is K 2 = (0.5)(300 kg)(4.0 m/s) 2 = 2400 J. The work done on the piano is 1800 J. mg v1v1 v2v2 xx

Limits of Kinetic Energy  At very high speed the work-energy principle fails. Nothing goes faster than the speed of lightNothing goes faster than the speed of light Kinetic energy can continue to increaseKinetic energy can continue to increase Contradiction – energy increases but not speedContradiction – energy increases but not speed  At speeds close to the speed of light we use Einstein’s special relativity. next