Chapter 7 Energy

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Chapter 7 Energy
The money of physics
2
Primitive Economics
Do your job
3
Modern Economics
Using money simplifies economics and accounting.
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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 its just a number.
• Can predict motion by figuring out how much
  energy that motion will cost.

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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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Sample Problem
6 kg

• What is the potential energy of a 6kg bowling
  ball at a height of 20 meters?
• What is potential energy at zero height?

20 m
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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!

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Sample Problem
6 kg

• What is the kinetic energy of a 6kg bowling ball,
  falling from a height of 20 meters, just as it
  reaches the ground?

20 m
20 m/s
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The Big Idea
PE 1200 J, KE 0 J
6 kg
Energy is the currency of motion
20 m
PE 0 J, KE 1200 J
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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.

Can predict speed from height since PEKE constant
Note Use this concept in lab experiment entitled
Projectiles
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Demo Dont Flinch

• When pendulum returns, never hits you.

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Movie Bowling Ball Pendulum
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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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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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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).
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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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Check Yourself

• Slaves pull a heavy load.
• Work done by slaves is positive, negative, or
  zero?
• Work done by friction force?
• Work done by the ground?

Support
Pull
Friction
LOAD
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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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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.

Can only push while in contact with the ground so
squatting helps by increasing distance.
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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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Stopping Braking Distance
35
55
30 mph
90
Reaction Distance
Braking Distance
Stopping Distance
45 mph
70
220
290
60 mph
At twice the speed, braking distance is four
times longer