physics 101 chapter energy work

8/12/2019 Physics 101 Chapter Energy Work

1/42

Chapter 7 - Energy

Work, Power, Potential and Kinetic Energy and

Conservation of Energy


2/42

Energy

What is energy? I dont know. We observe transfers of energy or

the effects of energy.

We cant see most forms, But we can calculate a

value for it in its many different forms. (Thermal,

electrical, chemical, mechanical, light, etc.)

Ability to do work? (This really tells us what it

does, not what it is.) It may go in and out of a system, but it is (as far as

we know) never lost from the universe.


3/42

Related Concept: WORK

Work = net force x distance

FdW

Objects motion changes in the direction of the

applied net force (Fig. 7.1)

Amount of work done is proportionalto both the

applied net force and the distance the object moves

Units of Work:

Metric: Newton-meter (Nm)1 Nm = 1Joule (J)

British: Foot-pound (ftlb)

Fig 7.4


4/42

Fig 7.1


5/42

Fig 7.2


6/42

Only the component of force

that is parallelto the directionof the motion does work


7/42


8/42

Doing work

The object must move.

Only the part of the force that is in the direction of the motion

of the object does work.


9/42

Two categories of Work

Work done against another force

Lifting against the force of Gravity

Pushing against the force of friction

Work done to change the speed of an object Speeding up or slowing down your car

Can energy be transferred from one object or

source to another?

Yes! When one object does work on another, this

istransfer of energy!


10/42

Work is a Scalar Quantity

Work can have a sign, but the sign doesnt mean the direction

of the work as a vector

Work is positive if the net force would tend to cause the object to

speed up Net Force is in thesame direction as velocity .

Work is negativeif the net force would tend to cause the object to

slow downNet Force is in the oppositedirection as the velocity.


11/42

What is the sign on the work done by gravity

on the roller coaster?

When determining the sign ignore the other forcesWork done by friction is negative both ways


12/42

What is thesignof work done

byfriction?!!!


13/42

Power

Power is howfasta person does work, or how

fast energy is transformed from one form into

another.

P = Work done/time interval

Units?

Joule/sec(J/s); 1 J/s =1 Watt (1 W).

1 kilowatt = 1 kW = 1000 W

1 hp is approximately 750 W or kW.

t

W

P


14/42

More power (to ya)

One Watt of power is the work done in lifting 1N

(1/4 pounder) one meter in one second.

More power means the ability to do work faster,or doing the same amount of work in less time.

It also means we are using energy faster!


15/42


16/42

When electricity flows through a light bulb, electricity

is converted to light and heat

The rate at which this conversion is made is called power

On the light bulb you will see the power rating, In this instance40 Watts

This means that 40 J of electrical energy is converted to light

and heat each second that the bulb is on.


17/42

Mechanical Energy

Two most common forms:

Potential Energy. The ability to do workby virtue of

the positionor conditionof an object.

Kinetic Energy. The ability to do workbecause of the

motionof an object.


18/42

Potential Energy

Stored energy: a stretched or compressed spring,a stretched bow, electrons in molecules (chemical

energy).

Gravitational potential energy: energy possessedby an elevatedobject.

To elevate the object one must do work against

the force of gravity (the weight of the object).


19/42

Calculating Gravitational

Potential Energy

Lets lift a mass (m) straight up a distance h.

The upward force required to move m at constant velocity

is equal to its weight or mg.

So Work = F x d

Here: Force = mg

d = height (h)

therefore

PEg= (mg) x h > > > PEg= mgh


20/42

Gravitational Potential Energy

Note: h is above some chosen reference level

See figure 7.4 and 7.6

The thing that is significant is that there is a

change in PE (a change in elevation).

PEcan be transformedinto kineticenergy, theenergy of motion.


21/42

See figure 7.6. Increase or decrease i n

PEgis independent of the path taken to

get there. Path matters not!

Fig 7.6


22/42

Fig. 7.6


23/42

Fig 7.7


24/42

Kinetic Energy

The energy of motion KE = mv2

Units?

(1 kg) (1 m/s)2= (1 kg) 1 m2/s2=

(1 kgm/s2) 1 m = 1 Nm = 1 J

It is the work requiredto bring an object from rest toa given speed(positive work), or the work required

by the object (or against it) the object can do whilebeing brought to rest(negative work).

Point: Work required to move or to stop motion!


25/42

XXXKinetic Energy

2

21 mvKE

The KE of the object depends

On both the mass of the object

And its speed

Q. Object A and object B move with the same speed. If object B has

three times the mass of object A, how much more KE does it have?

A. KE is directly proportional to mass, so 3x the mass gives 3x the KE.

Q. Object A and object B have the same mass. If object B moves with three

times the speed of object A, how much more KE does it have?

A. KE is directly proportional to the speed squared, so 3x the speed gives

32x the KE or 9x the KE.


26/42

Table, p.120


27/42

Work and Kinetic Energy Work-Energy Theorem

Work doneby the net force equalsthe change in kinetic energyof

the object.

Worknet= KE

Work is not energy, but a way of transferring energy from one

place to another or one object to another.


28/42

Work and Kinetic Energy

The final speed of the object depends on the

Net work done on the object

When net work is positive, the object

Will speed up and KE increases

When net work is negative,

The object will slow down and

KE decreases

When net work is zero, the object

Moves with constant velocity and

The KE doesnt change


29/42

XXXWork and Kinetic Energy

Q. An 8-kg object is sliding across a floor at a speedof 10 m/s. What is the objects KE?

Q. Object B has twice the speed of object A and the same mass. Howmuch more work does it take to stop B than to stop A?

A. The work needed to do this is equal to the change in KE. When B has 2xthe speed it has 4x the KE. In stopping, it loses all the KE, so it loses 4xthe KE. Therefore it will require 4x the work to stop B.

JmvKE 400)10)(8(2

1

2

1 22


30/42

Work and Kinetic Energy

A 50-N pail is being lifted 6-m by a rope with a tension of 75-N. Find

the change in the pails KE during this process. Is it an increase or decrease?

Find the work done by eachforce: the tensionand gravity.

WT

= T d = (75N)(6m) = +450J

Wg= Weight d = -(50N)(6m) = -300J (why is this negative?)

Wnet = WT+ Wg= 450J300J = +150J

Apply the work-energy formulaWnet= KE = +150J

This is an increasein KE. The rope puts in 450J of energy while

Gravity removes 300J of energy. This leaves a net gain of 150J of KE.

75 N

50 N


31/42

Conservation of Energy

Energy cannot be created or destroyed; itmay be transformed from one form to

another, but the total amount never changes.

For a given system, it may be transferred in

or out.

Examples:

Figure 7.12 the circus diver

Pendulum

Vibrating Spring

Roller Coaster


32/42

ME = PE + KE

Fig. 7.10


33/42

ME = PE + KE

Fig. 7.14


34/42

PE

PE + KE

PE + KEPE + KE

KE

ME = PE + KE


35/42


36/42


37/42

First Class Lever

Fig. 7.15


38/42

First Class Lever

Fig. 7.16


39/42

Second Class Lever


40/42

Third Class Lever


41/42

Summary

It just keeps . . .

getting . . .

Worse!


42/42

Law of Lever

physics 101 chapter energy work

Documents