4.1-4.4 Energy

Before we start, let’s see how much you remember…• List as many different types of energy as

you can

• What is energy?

• What is work?

• Is work a scalar or a vector quantity?

• What does positive work mean?

• What does negative work mean?

Why ask?

• Think very carefully about your own definition or understanding about energy

• Have you ever questioned the definition given regarding energy and work in texts before?

Improving your understanding

• It seems that we take the understanding of energy for granted – it seems like such a natural concept to us – a thing that we have always experienced, or understood

• But how much of it do we really understand?

• Have you questioned this since you began studying it formally?

The official definition

• Work:• The energy transferred to an object when a force

acting on the object moves through a displacement (NOTE: the force must be in the same axis as the displacement)

• Energy:• The ability to do work• Though these definitions seem cyclical, it

should be apparent that force and energy are linked together – where one exists, so does the other

Does this give you a better understanding?• It is important to develop your own

understanding of energy in order to better understand its characteristics and how it governs interactions in the physical world

• Like force, it can be a very abstract concept and system of thinking – but it is governed by only a few simple rules and simple equations

Work – the link between force and energy• Let’s begin by understanding that energy is

a “thing” that can flow from one place to another

• This characteristic is best seen in heat transfer – but it can be applied when dealing with any type of energy

• If energy is a “thing” that can move from one place or object to another – how does it do it?

Now it’s here – not it’s not

• I want you to think of this scenario – a rolling chair or cart sitting in the middle of the room

• Someone comes by and pushes it – what is the sequence of events?

Some simple points to point out…• The chair was at rest to start

• The chair moved forward

• The chair came to a stop

• From a dynamics point of view – we know how to describe these events

• Can you describe them in regard to energy?

Energy

• How did the energy flow?

• Energy possessed by the person

• Was transferred to the chair because the person did work on the chair

• The chair gained kinetic energy and moved

• Friction did work on the chair as it moved removing energy

• The chair lost kinetic energy and came to a stop

One way to picture energy

• A good way to become good at analyzing energy is developing a way to “see” it

• Being able to understand how an object comes to possess it – basically, how it flows from one object to another, will help you a great deal with the problems

Law of Conservation of Mass and Energy• In any isolated system:

• Energy can be converted to different forms, but it cannot be created or destroyed

What’s an isolated system?

• A group of objects that are separated from outside influences

• It means that nothing can enter into the system to take energy away – and therefore give the false impression that energy suddenly disappeared

Understanding energy means understanding the picture

• Note that in the example of the chair – you could have analyzed it:

• Without the person in the system – and the chair starting with a specific kinetic energy that we assume came from another source

• Or with the person in the system

We can assume a starting point

• We don’t have to consider the initial energy transfer in the question

• Note that in order to explain the chair’s behaviour successfully, we have to keep the presence of friction in – or else the chair’s energy would’ve disappeared completely and we wouldn’t be able to account for it

Gravitational potential energy

• Eg = gravitational potential energy

• This is the energy found in objects that are above the surface of the earth

• But how does an object that is up high have energy?

• Why is it called potential energy?

Different energies

• This is how you can come to view the presence of certain types of energy – like the gravitational potential energy of an object raised above a reference point

The end picture• When you look at

something above the ground, you are looking at something with STORED energy – because something had to give it energy for it to move against gravity

• Why is it STORED or POTENTIAL energy?

• Because if you give it the right conditions – that is, remove the support under it – the object moves a distance and falls back to earth – releasing the energy

The question is how it got there

• In order to get an object above ground, something had to come along to lift it up against gravity

So the force of gravity does work!

• Remember: W = F∆d• Fg is a force like all others• When something is lifted, we apply a force to

move an object up – so we give the object energy

• It becomes stored because we stop gravity from pulling down on it by putting something under it

• If we allow gravity to do so – it will apply a force and cause something to fall that same distance back, using up the energy we stored in it

So there are 2 reasons why things fall• Now we can look at falling objects and

describe them two ways

1. An object falls because gravity applies a force to it and pulls it towards the earth

2. An object falls because it loses the energy that was stored in it

So how much energy can you store?• Think about how “hard” it is to lift something• What does it depend on?• IS IT HARDER TO LIFT A 10 kg MASS OR A 20

kg MASS?• Eg increases as mass of the object increases• IS IT HARDER TO WALK UP 10 FLIGHTS OF

STAIRS OR 20 FLIGHTS OF STAIRS?• Eg increases as the displacement from the

ground increases

Something important to know

• It doesn’t matter what path the object took to reach the height – two objects can take two different paths – if their heights are the same, their Eg will be the same in the end

• Both objects have the same Eg since their h is the same

Kinetic energy – how do you have energy in speed?• Think about every situation where an object

moves• Something applies a force to it – and the

object moves a distance• This is the definition of work • So when you look at an object that has

speed – you know it has energy because in order for it to have speed – something MUST have come along and did work to it

Driving a car

• A moving car is a good example of how we generate kinetic energy

• The engine burns gasoline • The engine applies a force

to turn the tires• The tires push off the

ground • The car gains speed

CHEMICAL ENERGY

KINETIC ENERGY

So think about energy when you’re driving a car

• When you drive a car, all the kinetic energy that the car gets comes from the gasoline you burn

• Based on that, what factors affect the amount of gasoline you use?

• IF YOU DRIVE AT SLOWER SPEEDS, HOW MUCH GASOLINE DO YOU BURN?

• Kinetic energy increases with speed• IF YOUR CAR IS LIGHTER, HOW MUCH

GASOLINE DO YOU BURN?• Kinetic energy increases with mass

Let’s see what you really know…• A student is carrying a backpack, and

walks across the room

• Is the student doing work on the backpack?