Physics- Energy Transformations, November 25, 2008

Energy Transformations

– We have talked about kinetic, potential, and elastic energy- these are all components of mechanical energy

– Mechanical energy can be conserved or lost, depending on the forces involved

– example: hockey puck that has been hit and slides across the ice- will eventually stop

– example: a perfect pendulum will continue to swing for a very long time

Conservative Forces do work on an object in a way that the amount of work done is independent of the path taken

– Example: gravity is a conservative force
– takes the same amount of work to live a mass a given height, regardless of path that is traveled

Non-conservative forces are path dependent

Example: friction
– The work done against a frictional force depends on whether the path is straight, curved, zig-zagged, etc (consider a crate across the floor)

Work done by conservative forces result in energy changes that are independent of path and are reversible

Work done by non-conservative forces is not reversible

Conservation of Mechanical Energy

when all the work done throughout the energy transformation is done by conservative forces, mechanical energy is conserved

gravity and perfectly elastic materials are conservative forces; no real material is perfectly elastic but come close

The Law of Conservation of Mechanical Energy

When all the work done throughout a process is done by conservative forces, the total mechanical energy of the system after the process is equal to the total mechanical energy of the system before the process.

Ek1 + Eg1 + Ee1 = Ek2 + Eg2 + Ee2

Example: dropping a rock- gravitational potential energy is being transferred into kinetic energy right until it hits the ground

We can use what we know about conservative forces and the conservation of total energy to solve for changing and transforming energy systems.

For homework, please complete page 287 #1-8.


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