Chapter 2: Energy and Matter “Does energy really matter?” Sept. 2006.
Chapter 7 Energy Introduction Universe is made up of matter and energy. Energy is the mover of...
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Transcript of Chapter 7 Energy Introduction Universe is made up of matter and energy. Energy is the mover of...
Introduction
Universe is made up of matter and energy.
Energy is the mover of matter.It has several forms. To
understand this concept we will begin with a closely related physical concept.
1. WORK
Now instead of a force for how long in time we consider a force for how long in distance.
Work = Force x distance or W = F.dUnits - Joules (J) or ft.lbBTU = 778 ft.lb (energy of one wooden
kitchen match)Pushing on a wall and wall doesn’t move
(no work done on the wall)
W
Power = Work/time or P = W/t
Units - J/s =
Watt
2. POWER
550 ft.lb/s = 1
hp
1 hp = 746 J/s = 746 W
1 BTU/hr = 0.293 W
100 W bulb = 0.1341 hp
250 hp engine = 186,450
W
A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2.5 ft. Which lifting required the most work?
(a) 10 lb weight
(b) 20 lb weight
(c) same work for each lifting
(d) not enough information is given to work the
problem
Two cars, A and B, travel as fast as they can to the top of a hill. If their masses are equal and they start at the same time, which one does the most work if A gets to the top first?
(a) A
(b) B
(c) they do the same amount of work
3. MECHANICAL ENERGY
When work is done on an object, the object generally has acquired the ability to do work.
This is called energy and it has the same units as work.
Two Types of Mechanical EnergyPotential EnergyKinetic Energy
Potential Energy
Energy of position or configuration
Demo - Dart gunDemo - Dart gun
Other examples - Springs, bow, sling shot, chemical energy, and gravitational potential energy
The latter is GPE = mgh
The potential energy of an object
depends on a reference position.
It represents the work done against
gravity to put the mass m in its
position h above some reference
position.
It is an energy of position.
Kinetic Energy
It is an energy of motion.
221 mvKE
It is a square law. Total Work (work done by all forces
acting on mass m) = KE
Work-Energy Theorem
The net work done on an object is equal to the change in the kinetic energy of the object.
Net Work = KE
A 20 pound weight is lifted 4 feet. The change in potential energy of the weight in ft.lb is
(a) 20(b) 24(c) 16(d) 80(e) 5
4. CONSERVATION OF ENERGY
Galileo's inclines
Demo - Bowling ball pendulumDemo - Bowling ball pendulum
Demo - Loop the loopDemo - Loop the loop
Video - Pole VaultingVideo - Pole Vaulting
Energy lost due to friction is actually
not a loss; it is just a conversion.
Energy cannot be created or
destroyed.It may be transformed from one form
into another, but the total amount of energy never changes.
Energy Conservation in Satellite Motion
(Next slide)
Ellipse
Ellipse
ParabolaH
yperbola
Energy is conserved alongall of these paths.
Perigees
Apogees
Circle
Video ClipsVideo Clips
Driving NailsDriving Nails
Water WheelWater Wheel
Roller CoasterRoller Coaster
Condition for Conservation of Mechanical Energy
No work can be done on the object by a nonconservative force.
A nonconservative force is a force that converts mechanical energy into another form.
Example: Friction
An object of mass 6 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest?
(a) 180 Joules(b) 2700 Joules(c) 36 Joules(d) 5 Joules(e) 180 N
5. Machines
If no losses then
work input = work output
(F.d)input = (F.d)output
Examples - levers, block and tackle, etc.
Demo - Block and tackle Demo - Block and tackle
Demo - Hydraulic liftDemo - Hydraulic lift
6. EFFICIENCY
Efficiency = work done/energy usedUseful energy becomes wasted
energy with inefficiency.Heat is the graveyard of useful
energy.EER = energy efficiency ratio
It is the output capacity(BTU/hr)/input energy(Watts)(Output capacity represents energy moved.)
ip
0fp
I
Thief absorbs all the kinetic energy.
if ppp
This is the impulse applied to the bullet.
This is the impulse applied to the man.
This is the impulse applied to the man.This is the impulse from previous slide.
ip
Thief does not absorb all the kinetic energy
in this latter example.
fp ip
-
I
if ppp
This is the impulse applied to the bullet.
Rubber bullets versus lead bullets
Slow and fast football players with different masses.
Consider head-on with one having twice the mass but half the speed of the other.
2mv21 v)m2(
21KE
22 )v2(m
21KE
Kinetic energy is not a vector and cannot cancel out.
The kinetic energy of the big slow person is
The kinetic energy of the small fast person is
2mv2
8. SOURCES OF ENERGY
Except for nuclear and geothermal power, the source of practically all our energy is the sun.
Nuclear powerGeothermal powerSolar powerWind power