Energy Ability to do work or the Ability to make something happen

Kinetic energy Energy something has because it is moving at a certain speed Linear E k related to objects mass and linear speed E k = 1/2m v 2

E k units m (mass) in kg V (velocity) in m/s ◦ Once it is squared, units become m 2 /s 2 Measured in Joules which = Nm which = kg m 2 /s 2 English units will be ft lb

Rotating objects Energy related to inertia (I) and angular speed (  ) E k = ½ I (  

Moment of inertia Depends on ◦ Location of axis around which object rotates ◦ Mass of object ◦ Distribution of mass around axis If mass is unevenly distributed, doesn’t rotate smoothly (vibrates or wobbles) MOI varies with each shape

Moment in inertia (cont.) Center of mass – location at which an object’s mass is assumed to be concentrated If mass is unevenly distributed, doesn’t rotate smoothly (vibrates or wobbles)

Kinetic energy Since E k = 1/2m v 2, objects with more mass have more KE for the same speed Since E k = 1/2m v 2, increasing speed increases KE by a LOT (v is squared!)

Work-energy theorem Work =  KE – the work-energy theorem (proposed by James Prescott Joule). Work is measure of energy transmitted by any kind of force (push-pull,  p,  v or  T) Object’s total kinetic energy changes by amount equal to work done.

Work and kinetic energy Work is measure of energy transmitted by force Work measured by change in KE of moving object ◦ - work means force applied is opposite object’s motion ◦ + work is in same direction as object’s motion

Energy conversions Energy done by machines is usually accomplished by converting energy from 1 form to another Energy can be used, but no work done, if no motion occurs (W = F*d) Examples: ◦ Food (chemical)  mechanical and heat (you, or the engine in a car) ◦ Electric  mechanical  potential (pump)  All energy conversions create wasted energy (heat)

Law of Conservation of Energy Energy is neither created nor destroyed. It only changes from 1 form to another. In other words, (like momentum and mass) energy is always conserved. The total amount before change = total amount after change.

References used en.wikipedia.org/wiki tutor4physics.com/positivenegativework.htm _11.htm _11.htm nservation.asp nservation.asp war.html war.html