Conservation of Energy

Conservation of Energy• We stated earlier that when one object

does work on another it changes the motion (kinetic energy) of the second object.

• This relationship between work and kinetic energy is known as the Work-Kinetic Energy Theorem and can be expressed as:

• W net = ΔE

TME = Mechanical Energy (in the form of potential energy)

Conservation of Energy

• In order to solve problems you will be given appropriate information to solve for either work or both initial and final kinetic energy. You can then use the theorem to solve for other quantities.

Conservation of Energy

• In the absence of friction or external work, mechanical energy is conserved, although it may be converted from one type to another.

• KEi (initial kinetic energy)+GPEi (intial gravitational potential energy)+EPEi (initial elastic potential energy)= KEf + GPEf + EPEf

• Let’s look at an animation that shows this idea:

• http://www.physicsclassroom.com/mmedia/energy/ce.html

Conservation of Energy

• This law, unlike the motion equations already studied, can be used to solve problems when the acceleration is not constant.

• Let’s do a practice problem (5D) on pg. 181

Conservation of Energy

• When friction is involved, some of our mechanical energy can be converted into invisible forms of non-mechanical energy. Mechanical energy is no longer conserved, but TOTAL ENERGY IS ALWAYS CONSERVED

• Let’s do a practice problem on pg. 185

Power• When work is put into an object,

energy is transferred from one system to another.

• The rate that this happens is described as Power

• P = W / time• The units for power are Joules/second,

which are known as Watts (W)(don’t get confused between W=watts and W=work—one is a unit and the other a symbol)

Power

• Another way to define power is force multiplied by velocity

• P = (F)(v)• The units work out the same:• (Newton) x (meters/seconds) is the

same as (Newton x meter) / (s)• We’ll do a practice problem on page

188.