Download - GEOCITIES.ws

Work and Energy
1
Why do we do work anyway?
• Newton single handedly invented
mechanics, but missed one concept
– Energy (E)
• the ability to do work
– no wonder he avoided work!
– we’ll get to this in a second
– Work (W)
• has many definitions, but here it is very precise
(remember the beginning of the year)
• force multiplied by distance
– must be the component of force acting in the
direction of motion
2
Working 9 to 5
• MKS units: Joule
– the ability to exert 1 Newton over a distance of
one meter
– 1J=1N*1m
• find units that are all m, kg, sec
• Work examples
•
•
•
•
weightlifting: the clean and press
pulling a wagon
waitress
earth and moon
• Efficiency
– ratio of work in to work out
– can this ever be greater than 100% ?
3
More Power
• Tim Taylor: More Power (grunt, grunt)
• Power (P)
– the amount of work done per unit time
– derive: P = F*v
– unit is watt
• one watt is defined as one joule of work done in
one second
• find units in m, kg, sec
4
Anyone have a Energy Bar?
• Energy (E)
– ability to do work
• hammer swinging
– found in many forms
• electromagnetic, mechanical, heat, nuclear,
sound
– also measured in joules
– can change forms, but can’t be created or
destroyed
• more on this later
– two types: potential and kinetic
5
Neighbor Paul Energy
• potential energy (PE)
– the energy of an object due to position or
configuration
– two special types:
• gravitational potential energy
• elastic potential energy
6
Gravity, It’s Still the Law
• Gravitational potential energy
– energy obtained due to work done against
Earth’s gravitational field
• When we raise or lower an object, the
force required to do so is = weight = mg
– as long as we’re close to earth; g is constant
• We raise it a distance h
• W = F*x
• W = mg*h = PE
7
Elastics (not a book in the Bible)
• Elastic Potential Energy
– energy obtained due to work done from a
spring or rubber band
– force required to stretch or compress a
spring is given by Hooke’s law
• F=kx
– k is a spring constant, different for each spring
» unit for this is N/m
– Work done to stretch a spring is given by
½kx2
– this also equals what?
– Lab: Finding a spring constant
8
Keith Energy
• kinetic energy (KE)
– energy an object has because of its motion
– depends on reference frame
– KE = ½mv2
•
•
•
•
if v = 0, then what is KE?
if I double the velocity, then what happens to KE?
how can I get KE to be 9 times as big?
how can I get KE to be 1/16 as big?
• work-kinetic energy theorem
– the amount of work done on a system is = the
change in KE of an object
9
Conservation, Part 2
• total energy = kinetic energy + potential energy
• I raise a student, mass 50 kg, to a height of 2 meters and hold him.
– what is the PE? KE? E?
• Now I drop him, and right before impact, all his potential energy is
converted to kinetic energy
– from this you can find his velocity at impact
• This is true for all closed systems and is called
the conservation of energy
– The total energy of any closed system is conserved
• E = KE + PE
• for a closed system, E = 0
10
Roller coaster
• Can we use this in the real world?
– Well, let’s ride a roller coaster, baby, baby
– Can a roller coaster ever be higher than its
original point, assuming no outside forces act
on it?
• Can you find the velocity of the roller coaster at
any point
– Lab: Cart on a ramp (pg. 225)
11
A Change Will Do You Good
• As said before, energy is conserved in a
closed system
– but energy can change forms
• can never be created nor destroyed
– neither can mass
– In fact, energy and mass are related
– E = mc2
• this is from SR
• energy is mass!
• how much energy is in a 70 kg object
12
Footnote
• Conservative vs. Nonconservative
• Republican vs. Democrat
• Bush vs. Kerry ?
• No we’re talking about forces
• Conservative force
– force where mechanical energy remains constant
(ex. gravity)
– can “get it back”
• Nonconservative force
– force that converts mechanical energy into
another form, i.e. heat (ex. friction)
• for this there is a footnote to the conservation of energy
– delta E = delta KE + delta PE + internal energy
13
Collisions
• Types of collisions
– completely elastic
• kinetic energy is conserved
– completely inelastic
• kinetic energy not conserved, two masses
clump together
– none of the above
• a little bit of this, a little bit of that
• Is momentum conserved? How about
total energy?
• Again the ballistic pendulum
14
Executive toy
• One ball hits on one side, one ball flies off
the other side
• What is conserved?
– momentum and KE
• Why doesn’t two balls fly off the other end
with the same speed?
– hint find momentum before and after
– find KE before and after
15