Download NAME: Honors Physics When is Mechanical Energy not Conserved

NAME:________________________________ Honors Physics When is Mechanical Energy not Conserved So far we have talked about situations when Mechanical Energy (sum of kinetic and potential energies) is conserved. This is only
a special case then there is a close system and your object only interacts elastically. In the majority of situations in real life,
energy is lost or gained to outside sources ... Work.
In the following cases indicate whether energy is conserved or not and why, and if not what could be the source that is doing the
work and is the work positive or negative.
A Case A pendulum swings from rest at a start height of h to the bottom then up to a final height of h. ME conserved? Source of work (+) or (‐)  yes  no XXXXX  +  ‐ Reasoning: The potential energy at the beginning and the end is equal and there is no kinetic at beginning or end, so there is not energy loss. B A pendulum swings from rest at a start height of h to the bottom then back up to a final height of 0.9h.  yes  no Air Resistance  +  ‐ Reasoning: The potential energy at the end is only 90% of that in the beginning, so there is energy loss, so work must have been done. A block slides along a horizontal surface increasing speed from  yes  no engine  +  ‐ 5 m/s to 7 m/s. Reasoning: There is no change in potential energy, but the speed increases so the kinetic energy also has increased. This is a gain in total energy with means there must be some positive work done. C A block slides down a hill at a constant velocity. D  yes  no  +  ‐ Reasoning: The potential energy is decreasing while the speed and kinetic energy remains constant, this means that there is a loss in total energy, and thus negative work done. E A block slides along a horizontal surface at a constant velocity.  yes  no XXXXX  +  ‐ Reasoning: The potential energy is not changing and the velocity is constant so the kinetic energy is not changing either which means the total energy is constant and conserved. F A ball moves (starting at rest) from a height of h to a height of 3h (ending at rest).  yes  no Lifting force  +  ‐ Reasoning: There is no kinetic energy in the beginning or end, but the ball has gained potential energy which means there is an overall gain in total energy, so positive work must have been done. A car (initially moving) slows down to a stop on flat ground. G  yes  no Brakes, friction  +  ‐ Reasoning: There is no change in potential energy and the car has kinetic energy in the beginning but not at the end. So there must be negative work being done on the car. A block with in initial speed of v0 slides from a height h down and up a half‐pipe to its original height and ends with a  yes  no friction  +  ‐ H speed of 0.8 v0. Reasoning: The initial and final potential energies are the same, but the speed at those points is not the same, the final speed is less than the initial speed, so the final kinetic energy is less and so is the total final energy, meaning negative work was done. Honors Physics Mr. Maloney