Transcript Chapter 7 Energy - Alejandro Garcia's Web Site

Chapter 7
Energy
The “money” of physics
18-Jul-15
Physics 1 (Garcia) SJSU
Primitive Economics
Get paid
Do your job
18-Jul-15
Physics 1 (Garcia) SJSU
Modern Economics
Buy stuff
Do your job
Get paid
Using money simplifies economics and accounting.
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Physics 1 (Garcia) SJSU
Why Energy Helps
Motion, in general, is hard to calculate.
Using forces, momentum, acceleration, etc. gets
complicated because they are all vectors (have
magnitude & direction).
Energy is not a vector; it’s just a number.
Can predict motion by figuring out how much
energy that motion will “cost.”
18-Jul-15
Physics 1 (Garcia) SJSU
Potential Energy (PE)
Gravitational potential energy of an object is,
(Potential Energy) = (Weight) x (Height)
Think of potential energy as stored energy
or energy “in the bank.”
Metric unit of energy is Joules (same as for work).
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Physics 1 (Garcia) SJSU
Sample Problem
6 kg
What is potential energy at zero height?
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Physics 1 (Garcia) SJSU
20 m
What is the potential energy of a 6kg bowling
ball at a height of 20 meters?
Kinetic Energy (KE)
Kinetic energy of an object is,
(Kinetic Energy) = ½ x (Mass) x (Speed)2
Kinetic energy is the energy of motion.
A stationary object has zero kinetic energy.
Kinetic energy is related to momentum but
concepts are not the same!
18-Jul-15
Physics 1 (Garcia) SJSU
Sample Problem
20 m
What is the kinetic energy of a 6kg
bowling ball, falling from a height of
20 meters, just as it reaches the
ground?
6 kg
20 m/s
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Physics 1 (Garcia) SJSU
The Big Idea
PE = 1200 J, KE = 0 J
Energy is the “currency” of motion
PE = 600 J, KE = 600 J
20 m
10 m
6 kg
PE = 0 J, KE = 1200 J
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Physics 1 (Garcia) SJSU
Conservation of Energy
Conservation of mechanical energy is
(Potential Energy) + (Kinetic Energy)
stays constant during motion.
Energy “bookkeeping” makes motion simple.
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Pendulum
Energy exchange from PE to KE and back.
Maximum
Height
Maximum
Speed
Can predict speed from height
since PE+KE constant
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Maximum
Height, again
Note: Use this concept in
lab experiment entitled
“Projectiles”
Demo: Don’t Flinch
When pendulum returns, never hits you.
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Movie: Bowling Ball Pendulum
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Physics 1 (Garcia) SJSU
Demo: Interrupted Pendulum
By conservation
of energy we
know that the
pendulum ball
will never rise
above its initial
height.
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Demo: Bunny Bash
www.bunnybash.org
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Physics 1 (Garcia) SJSU
Demo: Bunny Bash
Would the ball strike at a higher speed by falling
straight down instead of swinging as a pendulum?
No, by energy conservation the kinetic energy at
impact only depends on the initial potential
energy (on initial height).
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Physics 1 (Garcia) SJSU
Demo: Bunny Bash
The following year, we simplified the process.
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Demo: Ball Races
Can predict ball speeds along the tracks.
Ball on track B goes the same speed as ball on track A
whenever the two balls are at the same height
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Demo: Blaster Balls
How high does the ping-pong ball go?
After collision
the speed of
ping-pong ball
is 3x larger
Ping
pong
ball
Golf
ball
Speed after collision is 3 times larger so kinetic energy is 9 times greater.
Ping-pong ball rises to 9 times its original height (9x original potential energy).
18-Jul-15
Physics 1 (Garcia) SJSU
Work
Define work done on an object by a force as
(Work) = (Force) X (Distance traveled)
Force acting in direction of motion: Positive work.
Force acting in opposite direction: Negative work.
Force perpendicular to motion: Zero work
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Physics 1 (Garcia) SJSU
Check Yourself
Slaves pull a heavy load.
Work done by slaves is
positive, negative, or zero?
Work done by friction force?
Support
Work done by the ground?
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Physics 1 (Garcia) SJSU
Friction
Pull
LOAD
Work & Energy
When forces do work on an object, the work
done equals the change in energy.
(small force)
X (LONG DISTANCE)
(BIG FORCE)
X (short distance)
Two persons do the same work in different ways.
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Physics 1 (Garcia) SJSU
Jumping
How high you jump depends
on the force and on the
distance over which you
apply that force
This is because the work
done, (Force)x(distance),
equals the energy gained.
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Physics 1 (Garcia) SJSU
Can only push while in contact
with the ground so squatting
helps by increasing distance.
Check Yourself
Compared with going 30 mph, a car going 60
mph has ____ times the kinetic energy.
Four times the KE means ____ times the
work required to stop the car.
Four times the work means ____ times the
distance (same friction force on brakes).
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Physics 1 (Garcia) SJSU
Stopping & Braking Distance
55’
35’
30 mph
90’
Reaction Distance
Braking Distance
Stopping Distance
45 mph
70’
60 mph
220’
At twice the speed, braking distance is four times longer
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Physics 1 (Garcia) SJSU
290’