Applied Physics Chap 4 Energy1 Chapter 8: Energy The universe is composed of two substances called matter and energy which are interrelated on some fundamental.

Applied Physics Chap 4 Energy 1

Chapter 8: Energy The universe is composed of two substances called matter and energy which are interrelated on some fundamental level (Einstein’s law E = mc2 )

But: we really don’t know what Energy is.


Video: Introduction to energy and work


matter possesses Inertia and Gravity

matter forms all the stuff that makes up our world.

Matter is measured by its mass in kg.

Energy appears in a variety of different forms.

Energy can be changed from one form into another

Energy causes changes in matter

What we do know about matter:

What we do know about energy:


Energy: The ability to cause a change in matter.

Different types of Energy:

Thermal Energy: Energy from heat and fire

Mechanical Energy: energy from motion or position

Chemical Energy: Burning fuel, food, batteries

Nuclear Energy: Energy from the atom

Electrical Energy: Energy from electrical interactions

Radiant Energy: Energy in the form of light, etc.Work: the transfer of energy from one kind to another through motion


Video: energy exchanges


Video: Physical energy exchanges


Mechanical energy: is energy that results from the position of something (called Potential Energy) or from its motion (called Kinetic Energy).

Potential Energy PE stored energy

Gravitational PE: is energy stored in an objects height above the ground

Chemical PE: is energy stored in the position of atoms in a molecule

Elastic PE: is energy stored in a spring or rubber band

Kinetic Energy: KE, energy in motion.


Video: Kinetic and Potential Energy


Gravitational Potential Energy depends upon:

the mass of the object being lifted (in kg).

the height the object is lifted to (in meters).

PEg = Gravitational Force (weight) x Change in height.

PE = Fg h

Since Fg = mg then PEg also = mg h

hm

Fm


Units of Energy: Joule 1 N 1 m = 1 Joule

1 Newton pushing something a distance of 1 meter performs 1 Joule of work

James Joule: 1818 - 1889

The English physicist James Prescott Joule (1818-1889) proved that mechanical and thermal energies are inter-convertible on a fixed basis, and thus he established the great principle of conservation of energy.


The product of the force on an object and how far the object is moved by that force.

Wk = Force distance

Work measures how much energy is converted from one form into another

Since work is equal to the quantity of energy transferred, then the units for work would also be JOULES.

WORK: The transfer of energy through motion.


Video: energy and work


WORK DONE AGAINST FRICTION:

Fm

Fm

d

Calculating work done against friction:

Work = Force of Friction x distance moved.

Wk = Ff d

Work is done by changing chemical energy to kinetic energy to thermal energy which is lost to the surrounding air.


Work done lifting a mass to a height.

h

F

As the object is lifted, work is done converting chemical energy (in muscles) into gravitational potential Energy which is stored in its position.

on the ground, the box has 0 J of PEg

After it is lifted the box has PEg = mgh

Since mg = Weight then:

PEg = W h


The faster something is moving, the more kinetic energy it contains. KE = ½ mv2

KE depends on:

The square of the object’s velocity in m/s

The objects mass in kg.

KINETIC ENERGY Mechanical energy that comes from the motion of an object.


Recall: work is done when energy is changed from one form to another through motion.

M = 1500 kg

V = 15 m/s

Work is done accelerating an automobile because, energy stored in gasoline is changed to KE by burning it in the engine.

The amount of work done is equal to the KE of the car after it reaches a top speed.


Kinetic energy and the automobile

M = 1500 kg

V = 15 m/s

J168750/151500 221 smkgKE

M = 1500 kg

V = 30 m/s

J675000/301500 221 smkgKE

Since all the KE comes from gasoline, it takes 4 times as much gasoline to travel at 30 m/s as at 15 m/s.

Double the speed

Increase KE by 4 times as much.


Power: is the rate that work is done

To increase power, just do the same amount of work in a shorter time.

Similarly, taking longer to do work requires less power.

time

workPower

power is given the units of Watts. Named after James Watt the inventor of the steam engine.


Law of conservation of energy.

Energy cannot be created or destroyed.

It can only be transformed from one form into another, leaving the total amount of energy unchanged.

Law of conservation of Mechanical energy: The sum total of KE + PE in a system does not change. but at any point energy is either PE KE

ME = Constant

KEi + PEi = KEf + PEf


Conservation of energy in a Pendulum.

B

As the weight falls toward point B. PE is converted into KE until at Point B all the energy is Kinetic.

When Pendulum is released from point A, it has PE but no KE.

A

As the weight rises toward point C. KE is converted back into PE until it stops moving at point C.

C


Height = 10 m

At the top KE = 0 J

PE = mgh = (5 kg)(9.8)(10m) = 490J

ME = KE + PE = 0 + 490J = 490 J

At the bottom PE = 0 J

KE = 490J

ME = KE + PE = 0 + 490J = 490 J

Conservation of Mechanical energy

Consider a heavy rock at the top of a cliff


Conservation of Energy and the Roller coaster

No PE

No KE

Work is done increasing the car’s PE

PE changes to KE


Conservation of energy: The Pile Driver.

A.) Work is done lifting the weight, converting chemical energy into potential energy.

B.) When the weight falls it PE is converted into KE

C.) When the weight hits the piling it does work driving the piling into the ground. Energy is converted from KE to thermal energy through friction with the ground.


Work, Machines and Mechanical Advantage

For example, to accomplish work Wk = F d we could

use a large force to push an object a short F d

or a small force times a long distance F d as long as the product of force and distance equal the same amount of work

We use machines because they permit us to do the same amount of work while using a smaller amount of force at the expense of a longer effort distance.

Wk = F d = F d


We need to lift a 50N box to a height of 2.0 m. The work we need to do is:

Wk = Fd = 50N 2m = 100 J

F2 m

50N

However its easier to use a ramp

Using a ramp does the same work with less force.

2 m

50 N

50 N

We could lift it straight up:

Jm

J

d

WkF 10

10

100

The force we would need:


Mechanical Advantage: MA: The ratio of the output force of a machine to the input force you have to exert.

1510

50to

N

N

F

R

forceEffort

forceResistanceMA

Effort Force F is the force a worker has to apply to lift an object

Resistance force R is either the weight of the object or the force needed to move the object.

Mechanical advantage is a multiplier. You multiply the effort force by the MA to find how much weight can be lifted.


Simple Machines:

Simple machines devices that use the principle of mechanical advantage to permit accomplishing a certain amount of mechanical work with less force.

There are six basic kinds of simple machines: Levers,

pulleys, wheel and axel,

screw, wedge and

inclined plane. In combination, many other types of compound machines can be constructed.


Levers: a device composed of a rigid arm called the lever arm, and a rest point called a fulcrum.

Fulcrum

Input Force F

Resistance force R

Effort Length E

Resistance Length L


Fulcrum: point around which the lever rotates.

Effort Length E : the distance from the point where you apply force to the fulcrum.

Resistance Length L. The distance from the point where the output force is applied to the fulcrum.

Effort force: F The amount of force that the worker or the machine has to apply to do the work

Resistance force: R Either the weight of the object or the force that must actually be used


F

R

L

EMA

forceeffort

forceResistance

lengthresistance

lengtheffortMA

Mechanical advantage for a lever.

Mechanical advantage is equal to either:

The ratio of effort length to resistance length

OR

The ratio of resistance force to effort force.


Screw: An inclined plane wrapped around a cylinder.A screw is simple a long inclined plane that is rotated to continue moving an object upward. When coupled with a wheel and axel like a screw driver, it allows a large quantity of force to be applied in lifting an object up or down.

Wedge: A double inclined plane that is used to turn a downward force sideways. Example: a log splitting wedge takes the downward force from the maul and redirects it at a 900 angle in each direction to force the log apart.


Resistance Length L

Effort Length E

Wheel and Axel: a special kind of lever.

Effort Length

Resistance Length


Effortlength

Resistance length

Example: Resistance Length = 10 cm and Effort Length = 20 cm.

12 tocm10

cm20

ArmResistance

ArmEffortMA

Wheel and Pulley


Gears are determined like unequal diameter pulley’s as a ratio either of the radius of the gears or as a ratio of the number of teeth on each gear. A

14 :20

80

A Teeth

B TeethMA


Pulley systems: a special type of wheel and axel


300 N

Effort force

= 300 NMA = 1

300 N

Effort force

= 150 NMA = 2

300 N

Effort force

= 100 NMA = 3

Mechanical advantage in pulleys.

Applied Physics Chap 4 Energy1 Chapter 8: Energy The universe is composed of two substances called matter and energy which are interrelated on some fundamental.

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