Transcript Work,Energy,Power, New0
Work…
In everyday speech work has a very general meaning. In describing motion in physics, work has a very specific meaning.
Chair Example
Standing Walking No work is done on the chair
Work is defined as the product of the force applied to cause motion and the distance the object moves in the direction of the force.
Work is done only when components of a force are parallel to a displacement
FORMULA
W = fd IN DIRECTION OF MOTION The symbol for work is W Work has 2 acceptable units
Nm
Joules (J)
JOULE
Lifting an apple about 2ft is a Joule
3 good push-ups is about 1000J
Situations that affect the sign of work
Force is in the direction of motion Force opposes motion Force is 90° to motion Object is not in motion Positive Work Negative Work No Work No Work
10N Moves 2m W = fd W = 20J of Work
Notice direction of motion is the same as the applied force
10N How would you solve this? Force applied is NOT in the same direction as the objects motion.
Y X 60° 2m Think back to vectors and use the component of the force applied in the direction the object moves.
Y X 10N 60° COS θ = adj/ hyp COS 60° = force parallel to motion 10N force para. = COS 60° (10N) force para. = 5N 2m w = F(parallel) D 5N (2m) = 10Nm
W = Fd (COS θ)
Always measure angle with horizontal!
The above formula works in every case θ = 0° θ = 90° No work because no motion in direction of force
Pg. 170 Problems 1-4
Section Review p. 171 With your Neighbor, answer questions 2, 3, 4
Section Review Answers 2 the neighbor, twice as much 3 a-negative B-positive C- negative 4 a-yes B- no C-yes
Energy
The Stuff that makes things move
The ability to do work
Has the units of Joules (J)
There are 2 kinds of mechanical energy
Kinetic Energy
This is the energy associated with an objects motion.
KE depends on mass and velocity When the object is treated as a particle, the formula for KE is… KE = ½ mV 2 manipulated V = 2KE/m M = 2KE/V 2
KE is a scalar quantity The SI unit for KE is the Joule, yes the same as for work Look at sample prob. 5B Page 173
DO practice problems
5B 1-5 on page 174
Work- Kinetic Energy Theorem
The net work done on an object is equal to the change in the kinetic energy of the object
W net W net = ΔKE = KE final – KE initial fd(cos θ) = ½ mV 2 The KE of an object is equal to the work that moving object can do
This theorem allows us to think of KE as the work an object can do as it comes to rest, or the amount of energy contained in the moving object The KE of the moving hammer can do work KE = Work done (net) fd = ½ mv 2 some of the energy is sound, heat and light (if spark)
Practice Problems 5C p.176 #1 and 4 only
Potential Energy
1.
STORED ENERGY There are two kinds of potential energy GRAVITATIONAL POTENTIAL ENERGY 2.
This is the energy associated with an object due to the position of the object.
ELASTIC POTENTIAL ENERGY
Gravitational Potential Energy (PE g ) The energy associated with an object due to the objects position relative to a gravitational reference Wh = PE g = mgh = mass x gravity x height acceleration gm = w Has the unit of joules
Elastic Potential Energy (PE elastic )
The energy associated with a stretched or compressed elastic object
Spring, bungee cord, rubber band
Elastic Potential Energy
Overhead
(springs)
In both the compressed and stretched example, energy is stored PE elastic
= ½ KX 2 K = spring constant
X = distance stretched or compressed Practice Problems 5D 1-3 pg. 180
Conservation of Energy
To say something is conserved is to say it remains constant. Something can change form and still be conserved.
burning log: matter and energy are conserved.
5Kg 5Kg ASH
Pendulum
Energy is transferred from one form to another As the pendulum swings, PE is transferred to KE. As the bob swings upwards KE is stored as PE PE = max KE = min PE = min KE = max PE = max KE = min
PE = mgh 10M A falling egg Mass = .1kg Height = 10m PE = 10 J KE = 0 J PE = 5 J KE = 5 J PE = 0 J KE = 10 J
Mechanical Energy
The sum of Kinetic Energy and ALL forms of Potential energy associated with an object or group of objects ME is not a unique form of energy. Its merely a way of classifying energy ME includes KE and PE
Mechanical Energy
ME is different from non mechanical energy (nuclear, chemical, thermal, internal, electrical) ME = Σ KE + Σ PE
ME = ½ mv 2 + mgh (if PE is NOT present, elastic)
Conservation Of Mechanical Energy
Conservation of Mechanical Energy can also be written as…
ME i = ME f ½ mv i 2 + mgh i = ½ mv f 2 + mgh f True when friction can be ignored
The Law of Conservation of Energy: The total energy of a closed system is constant.
Often is the case that KE or PE i or PE f that is the case… i or KE f will be zero. When mgh = ½ mv 2 2mgh = v 2 m V = 2gh h = V 2 2g
Problems p.185
Look at sample problem 5E
Practice problems 5E #s 5,2,1
HOMEWORK!!
28-31, 33, 34a
All on page 195 of packet
Power
This quantity also has a very specific meaning in science that can be confused by common English usage Power is the rate of doing work
That is to say that power is the rate at which energy is transferred
Power
Power is work done divided by the time taken to do the work Power = Work = fd P = w
Time t t Power is measured in watts (W) J/s
A watt is a small unit, 1 watt is about what is needed to lift a 2N glass of water .5m to your mouth in 1 second.
Watts
Since watts are so small, we sometimes use Kilowatts
1 KW = 1000W
Watts are metric
Horse power is traditional
1 Horse power = 746 Watts
Watts
Watts are named after James Watt, the inventor of the steam engine
Practice Problem
An electric motor lifts an elevator that weighs 12000N a distance of 9m in 15sec
What is the motors power in watts?
What is the motors power in kilowatts?
Given Formula Solution f = 12000N P = fd/t P = 12000(9) d = 9m t = 15s A. P = 7200 W 15 P = ?
B. 7.2 KW
Sample Problem p. 188 Pg. 189 5F 5, 4, 3, 2