QOTD: Write a list of 10 examples your idea of work. Demo : create a work equation using a spring scale, string and an agenda book.
Work is done only when a force moves an object • A force acting on an object and causing it to move a distance is work • Not every force is work..if you push against the wall it does not move..that is not work! • Work = force X distance • Work is measured in Joules • If you pick up a bag of groceries and walk across the room the work is picking up the groceries not the walking. What is work?
The object must move some distance as a result of your force • The force you exert must be in the same direction as the objects motion. • Ie: the groceries • You walk
What’s work? • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car?
What’s work? • A scientist delivers a speech to an audience of his peers. No • A body builder lifts 350 pounds above his head. Yes • A mother carries her baby from room to room. No • A father pushes a baby in a carriage. Yes • A woman carries a 20 kg grocery bag to her car? No
Work can be determined by calculating • Force used x distance moved = amount of work • Therefore what is the formula for work?
Work = force x distance Joule – is the SI unit for work. Newton = force Meters = distance Therefore if you exert: 1 Newton of force for 1 meter of distance = 1 joule of work or 1N/m
Work is done when a force is exerted through a distance. A student lifts a bag of books that weighs 135 N. If the bag is lifted .75 m, how much work does the student do? F = 135 N d = .75 m W = Fd W = (135 N)( .75 m) W = 101.25 J
A +24 N force is applied to an object that moves 10 m in the same direction during the time that the force is applied. How much work is done to the object?
Find the equation for POWER 1. Attach a string and spring scale to a large book. 2. Pull the book .5m slowly. Use a stopwatch to determine time . 3. Record the time and distance on a data table. 4. Repeat 1- 3 – but this time pull the book faster. 5. Repeat 1-3 even faster. Force-N distance -m time- s
Power tells you how fast something is happening..how fast the work is being done • Power = work/time or Power = Force X Distance Time • Power is measured in watts (W) • One watt is equal to 1 joule per second of work divide joules/seconds Power
Power – the rate at which energy is transferred. P = W P = power Watts t W = work Joules t = time seconds 1 Watt (W) = 1 J/s
How much power must a motor have to operate a pump that raises 1500 kg of water every minute a distance of 12 m?
1. m = 1500 kg t = 60 s d = 12 m 2. Equations- P = W/t W = Fd F = mg 3. Plug and chug- F = mg = (1500 kg)(9.8 m/s2) = 14,700 N W = Fd = (14,700 N)(12 m) = 1.76 x 105 J P = W/t = (1.76 x 105 J)/(60 s) = 2940 W
What is Energy? It turns out that energy is so fundamental, like space and time, that there is no good answer to this question. However, just like space and time, that doesn't stop us from doing very useful calculations with Energy We may not be able to define energy, but because it is a conserved property of nature, it's a very useful idea.
Potential Energy Potential Energy (PE): Stored energy due to position Examples: rock on a cliff, battery, food, gasoline, stretched rubber band, apple hanging in a tree
Gravitational Potential Energy A barbell of mass "m" is lifted vertically upwards a distance "h" by an outside force. How much work does that outside force do on the barbell? Fapp W = Fdparallel Since a = 0, Fapp = mg W = (mg) dparallel Since F and d are in the same direction ...and dparallel = h W = (mg) h W = mgh mg
Gravitational Potential Energy But we know that in general, Eo + W = Ef. If our barbell had no energy to begin with, Eo = 0, then W = Ef But we just showed that we did W=mgh to lift the barbell... so mgh=Ef The energy of a mass is increased by an amount mgh when it is raised by a height "h".
Gravitational Potential Energy The name for this form of energy is Gravitational Potential Energy (GPE). GPE = mgh One important thing to note is that while changes in gravitational potential energy are important, their absolute value is not.
Gravitational Potential Energy You can define any height to be the zero for height...and therefore the zero for GPE. But whichever height you choose to call zero, changes in heights will result in changes of GPE. For example, the floor level can be considered zero energy or the ladder level can be zero. 0.5 m 0 m 0.5 m 0 m
Gravitational PE Gravitational PE (GPE): Energy stored by objects that are above the earth’s surface (objects that can fall) Depends on mass, acceleration and height GPE increases with height
Gravitational PE GPE = mass gravity height GPE = m g h = weight height GPE = m (kg) 9.8 m/s2 h (m) j = 1 Nm
9 What is the change of GPE for a 5.0 kg object which is raised from the floor to a final height of 2.0m above the floor? GPE=mgh GPE= (5kg)(9.8)(2m) GPE=98 J answer
10 As an object falls, its GPE always _____. A increases B decreases C stays the same B answer
11 What is the change of GPE for a 8.0 kg object which is lowered from an initial height of 2.0 m above the floor to a final height of 1.5m above the floor? GPE=mgh GPE= (8)(9.8)(-0.5) GPE= -39.2 J answer
12 What is the change in height of a 2.0 kg object which gained 16 J of GPE? GPE=mgh h = GPE/mg h = 16/(2)(9.8) h = 0.82m answer
Kinetic Energy Kinetic Energy (KE): Energy in the form of motion Depends on mass and velocity of moving object. Object in motion has ability to do work http://www.youtube.com/watch?feature=player_detailpage&v=0ASLLiuejAo
Kinetic Energy The energy an object has by virtue of its motion is called its kinetic energy. The symbol we will be using for kinetic energy is KE. Like all forms of energy, it is measured in Joules (J). The amount of KE an object has is given by: KE = 1/2 mv2
Kinetic Energy KE = ½ mass velocity2 KE = m V2 2 (j) = (kg) (m/s) 1 j = 1 kg m/s
13 As an object falls, its KE always _____. A decreases B increases C stays the same. B answer
14 A ball falls from the top of a building to the ground below. How does the kinetic energy (KE) compare to the potential energy (PE) at the top of the building? A KE = PE B KE > PE C KE < PE D It is impossible to tell. C answer
15 What is the kinetic energy of a 12 kg object with a velocity of 10 m/s? KE = 0.5 mv2 KE = (0.5)(12)(100) KE = 600 J answer
16 What is the mass of an object which has 2400 J of KE when traveling at 6.0 m/s? KE = 0.5 mv2 m = KE / (0.5)(v2) m = 2400 / (0.5)(62) m = 133.33kg answer
17 A 3 kg object has 45 J of kinetic energy. What is its velocity? KE = 0.5 mv2 v2 = KE/0.5m v2 = 45 / 0.5 (3) v = 5.48 m/s answer
18 If the speed of a car is doubled, the KE of the car is: A quadrupled B quartered C halved D doubled A answer
19 Which graph best represents the relationship between the KE and the velocity of an object accelerating in a straight line? C A KE KE B D v answer D v KE KE v v
20 The data table below lists mass and speed for 4 objects. Which 2 have the same KE? A A and D B B and D C A and C D B and C D answer
Elastic Potential Energy Energy can be stored in a spring, this energy is called Elastic Potential Energy. Robert Hooke first observed the relationship between the force necessary to compress a spring and how much the spring was compressed.
Elastic Potential Energy The energy imparted to the spring by this work must be stored in the Elastic Potential Energy (EPE) of the spring: EPE EPE = 1/2 k x2 Like all forms of energy, it is measured in Joules (J).
21 Determine the elastic potential energy stored in a spring whose spring constant is 250 N/m and which is compressed 8 cm. EPE= 1/2 Kx2 X=distance compressed K=spring constant EPE = 0.5 kx2 EPE = 0.5 (250)(0.082) EPE = 0.8 J answer
22 What is the spring constant of a spring that is compressed 5 cm and has 0.65 J of elastic potential energy stored in it? EPE = 0.5 kx2 k = EPE/0.5x2 k = 0.65 / 0.5 (0.052) k = 520 N/m answer
k = 1176 N/m 25 The same 3 kg mass compresses the same spring 2.5 cm. How much elastic potential energy is stored in the spring? EPE = 0.5kx2 EPE = (0.5)(1176)(0.0252) EPE = 0.368 J answer
Law of Conservation of Energy The law of Conservation of Energy: Energy cannot be created or destroyed. It may be transformed from one form into another; however, the total amount of energy in the universe remains constant. (Transformers)