EnergyEnergythe ability (capacity) to do workthe ability (capacity) to do work

Energy comes in many forms:Energy comes in many forms:mechanical, electrical , magnetic, solar,mechanical, electrical , magnetic, solar,

thermal, chemical, etc...thermal, chemical, etc...

The SI unit of energy is the The SI unit of energy is the JouleJoule..

Energy, like work, is a Energy, like work, is a scalarscalar, and, andMay be positive or negative.May be positive or negative.

Kinetic EnergyKinetic Energyenergy of motionAll moving objects thatAll moving objects that

have mass have kinetic energy.have mass have kinetic energy.

KE = 1/2 mvKE = 1/2 mv22

mm – mass of the object in – mass of the object in kgkg vv – speed of the object in – speed of the object in m/sm/sKEKE – the kinetic energy in – the kinetic energy in JJ

Work-Energy TheoremWork-Energy Theoremthe net work done on an object is

equal to its change in kinetic energy

W KEnet

Learn more about theLearn more about theWork-Energy TheoremWork-Energy Theorem

here and and here..

A net force causesA net force causesan object to change its KE becausean object to change its KE because

a net force causes an object to accelerate,a net force causes an object to accelerate,and acceleration means a change in velocity,and acceleration means a change in velocity,

and if velocity changes, KE changes.and if velocity changes, KE changes.

Due to friction, energy is transferred both into the floor and into the tire when the bicycle skids to a stop.

a. An infrared camera reveals the heated tire track on the floor. b. The warmth of the tire is also revealed.

9.6 Work-Energy Theorem

When a car brakes, the work is the friction force supplied by the brakes multiplied by the distance over which the friction force acts.A car moving at twice the speed of another has four times as much kinetic energy, and will require four times as much work to stop. The frictional force is nearly the same for both cars, so the faster one takes four times as much distance to stop. Kinetic energy depends on speed squared.

9.6 Work-Energy Theorem

Typical stopping distances for cars equipped with antilock brakes traveling at various speeds. The work done to stop the car is friction force × distance of slide.

9.6 Work-Energy Theorem

Kinetic energy often appears hidden in different forms of energy, such as heat, sound, light, and electricity.

• Random molecular motion is sensed as heat. • Sound consists of molecules vibrating in rhythmic patterns. • Light energy originates in the motion of electrons within

atoms. Electrons in motion make electric currents.

9.6 Work-Energy Theorem

Potential EnergyPotential Energyenergy of position or conditionenergy of position or condition

gravitational potential energygravitational potential energy

PEPEgg = mgh = mgh mm – mass of object in – mass of object in kg kg g g – acceleration of gravity in – acceleration of gravity in m/sm/s22

h h – height of object, in – height of object, in mm,, from some arbitrary reference pointfrom some arbitrary reference pointPE PE – gravitational potential energy in – gravitational potential energy in JJ

Law of Conservation of EnergyLaw of Conservation of Energy

““Energy can be neither created nor destroyed.Energy can be neither created nor destroyed.It may only change forms.”It may only change forms.”

all types of energy before the event all types of energy before the event = = all types of energy after the event all types of energy after the event

Examples:Examples:• A dropped object loses gravitational PE as it gains KE.A dropped object loses gravitational PE as it gains KE.• A block slides across the floor and comes to a stop.A block slides across the floor and comes to a stop.• A compressed spring shoots a ball into the air.A compressed spring shoots a ball into the air.

Potential energy will become the kinetic energy of the arrow.

9.7 Conservation of Energy

The study of the forms of energy and the transformations from one form into another is the law of conservation of energy.For any system in its entirety—as simple as a swinging pendulum or as complex as an exploding galaxy—there is one quantity that does not change: energy. Energy may change form, but the total energy stays the same.

9.7 Conservation of Energy

Part of the PE of the wound spring changes into KE. The remaining PE goes into heating the machinery and the surroundings due to friction. No energy is lost.

9.7 Conservation of Energy

When the woman leaps from the burning building, the sum of her PE and KE remains constant at each successive position all the way down to the ground.

9.7 Conservation of Energy