ENERGY AND WORK

Chapter 7

Energy:

The ability to do work

Types –  Kinetic – energy of motion  Mechanical – done by machines  Potential – stored energy (gravitational or elastic/flexed solids)  Chemical – found in the bonds between atoms

Types cont.

 Thermal energy – heat or internal kinetic  Sound – vibrations in matter carried on waves  Light – electromagnetic radiation  Electrical – moving and static electrical charges  Nuclear of atoms – found within the nucleus

Mechanical Energy and Work



Work – a force exerted during a distance. W = F*d



Work and Energy… You must have energy to do work.



Unit is Joules

(J)

.

Mechanical Work

230N

A weightlifter attempting to lift 230N (about 50 lbs) must pull with a force equal to the weight of the barbell to get it off the ground.

At the top of his lift, he has raised the bars 1 meter.

1 m

How much work has he done?

W = F*d W = 230N * 1m W = 230 J What unit do we use?

Joules

230N

Cont.

 How much work to raise the bar over his head to the height of 2.5 m? *    W= 230N*2.5m

W= 575J If we doubled the weight to 460N, what happens to the amount of work?

2.5

m It would double as well! Directly Proportional

* If his arms were long enough!

F

Work and Vectors (direction)

Work is a vector, which means it has a direction.

Work can only be done if the force and distance directions are the same.

NO WORK DONE!!!

distance

Power



Amount of work done per the amount of time it takes to do it.

Work done (J) Power = Time (s) Unit of power = J/s = 1 watt (W) 1 kilowatt (kW) = 1000 W

Law of Conservation of Energy

  1 st Law of Thermodynamics  Energy can be changed from one form to another, but it can NOT be destroyed.  When a change of energy occurs, the final energy is equal to the beginning energy.

The ultimate source of ALL energy on Earth?

Conservation of Energy: bow and arrow

    The bow stores potential energy in its bent limbs when drawn.

This energy is transformed into kinetic energy as the arrow is released. PE of the bow = KE of arrow PE & KE = work the archer had to use to draw the bow

Where does the energy go???

 Even though the arrow is now at rest (no KE) and the bow is not drawn (no PE), NO ENERGY HAS BEEN DESTROYED!

   Where did it go?...

The target did “Work” to the arrow by bringing it to rest with friction … a force which was exerted over a distance (the depth the arrow entered the target) Whenever energy is “lost” it is ALWAYS converted to HEAT.

Potential Energy: Stored!

 Gravitational    PE g = mass * gravity * height Higher an object = ____PE Greater the mass = ____PE  Greater gravity = ____PE  Elastic  PE elastic = distance stretched x weight   Greater the weight = ____PE Greater the distance stretched = ____PE

Kinetic Energy: Motion!

 KE = ½ mass * velocity 2  The more massive the object = ____KE  The faster the object = ____KE  Why does velocity have a greater role in KE than mass?

The mass is cut in half but the velocity is squared!

Other thoughts

 What would happen to the depth of penetration if the arrow were moving faster?

 What happens to the speed of the arrow when shot from a “stiffer” (harder to draw) bow?

 What happens to the speed of the arrow if it weighs more?

 What happens to the speed of the arrow if it is longer? (bow pushes it for a longer distance)

 

Law of Conservation of Energy Again!!!

Energy can be changed from one form to another, but it can NOT be destroyed. This means that when a change of energy occurs, the final energy is equal to the beginning energy.

Law of Conservation of Energy – YET AGAIN!!!

  KE i + PE i = KE f + PE f In other words, the sum total of KE +PE before and after some kind of transformation MUST be equal.

Conversions

   How do you change kg to N?

x10     How do you change W to kW?

/10 3

Law of Thermodynamics - Entropy

  Entropy – a change of a usuable form of energy into an unusable form… HEAT (thermal energy) Whenever energy is “lost” it is ALWAYS converted in to HEAT.