Energy & Work

Energy• What is energy?

• The capacity to do work

• What is Work?• The transfer of energy from one physical system

to another

Bottom line: it is the outward manifestation of energy which causes a change in something (e.g. changes in motion or position).

• Energy is measured in Joules

• Joules = newton*meter = kg*m2/s2

• In English units we call them calories or kilocalories

Types of energy

• Kinetic energy – energy of motion

• Potential energy – stored energy

Kinetic Energy

• Mechanical energy – energy inherent in objects in motion

• Light energy – highly organized packets of energy called photons

• Electrical energy – energy generated by charged particles

• Heat energy – atomic and molecular motion create heat

Potential Energy

• Positional energy – energy stored due to position of object relative to some force

• Electrical energy – stored charged particles

• Bond energy – energy stored in the chemical bonds holding 2 or more atoms together

Laws of thermodynamics

• laws of the movement of heat?...why heat?

• Push your book of the desk!– 1) Positional energy to simple kinetic energy– 2) positional energy also converted to heat due to friction – 3) book hits ground, the kinetic energy is converted to

heat energy in the book and the ground – 4) pick it back up and bond energy is converted into

mechanical energy in my muscles – 5) both bond and mechanical muscle energy are

converted to heat– 6) the heat is eventually radiated into the atmosphere

Laws of thermodynamics

• 1st Law - Conservation of Energy• Energy is neither created nor destroyed• Energy within a system is only converted from one

form of energy to another• Energy can be moved into or out of a system• Total energy in the universe is a constant

Laws of thermodynamics

• 2nd Law – Entropy• Energy naturally flows from more ordered forms of energy to

less ordered forms of energy

• Corollaries:– The total disorder in any closed system always

increases.– Every conversion of energy from one form to another

converts some energy into disordered energy in the form of heat.

Back to Work

• If work is the transfer of energy and energy can be neither created or destroyed…….the work done within a closed system……..

»Decreases over time?

»Remains constant ?

How is Work measured?

• The force applied to an object times the distance the object moves due to that force.

• W = F*d (Recall, F=ma)

• Peculiarities:– 1) Movement perpendicular to the direction of the

force does not result in work– 2) Supporting an item does not result in work. If the

distance moved is zero, then there is no work.

• Moving a body is doing work

• There are a variety of ways of dealing with forces and distance that relate to the ease of accomplishment of work

• Humans, being animals, have made a habit of using these ways to accomplish work more easily in the form of Simple Machines

Animal Movement

A machine is a device that helps make work easier to perform

• Transferring a force from one place to another

• Changing the direction of the force

• Increasing the magnitude of a force

• Increasing the distance or speed a force is applied

4 Basic Simple Machines

• Levers

• Inclined planes

• Pulleys

• Wheel and axle

Incline Plane

Vertical distance

Horizontal distance

How much force is required to lift a 50kg weight 3 meter off of the ground?

50kg

F = m a

F = 50 kg (9.8 m/s2)

= 490N3 m

How much work vs. gravity needs to be done to move a 50 kg weight up 3

meters?

50kg

3 m 5 m

Work = F*d = 490N*3m = 1470 Joules

How much force do you need to apply to move the same weight up the ramp?

50kg

3 m 5 m

Work = F*d F = W/d = 1470J/5m = 294 N

How much easier is it to do the work (1470J) with the inclined plane than without?

3 m 5 m

Mechanical advantage = force without incline / force with incline

490 N

294 N

Force w/o

Force w= = 1.66

For inclined planes you get the same number by:

Length of plane

Height of plane=

5 m

3 m= 1.66

Mechanical Advantage

• force produced by the machine divided by force applied to the machine

–F out / F in

Levers

Effort Force

Resistance force

d e

Effort Force

Resistance force

d e

d r

What is the mechanical advantage of the lever system?

ER

d e

5 kg15 kg

F out

F in

15 kg * 9.8 m / s2

=

5 kg * 9.8 m / s2

If the mechanical advantage is 3 :1 can you calculate dr and de?

ER

5 kg15 kg

d e

If you need to move the Fr 9 meters what is the distance the Fe will have to move?

This is a different type of lever; what is different from the previous

lever?

F e F r

Where are dr and de?

F e

F r

drde

This is the last type of lever; what is different?

F e F r

Where are dr and de?

F e

F r

dedr