Jun 23, 2022 Physics 1 (Garcia) SJSU Chapter 7 Energy The “money” of physics
Jan 02, 2016
Apr 20, 2023 Physics 1 (Garcia) SJSU
Chapter 7 Energy
The “money” of physics
Apr 20, 2023 Physics 1 (Garcia) SJSU
Primitive Economics
Do your job Get paid
Apr 20, 2023 Physics 1 (Garcia) SJSU
Modern Economics
Using money simplifies economics and accounting.
Do your job
Get paid
Buy stuff
Apr 20, 2023 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.”
Apr 20, 2023 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).
Apr 20, 2023 Physics 1 (Garcia) SJSU
Sample Problem
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
6 kg
Apr 20, 2023 Physics 1 (Garcia) SJSU
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!
Apr 20, 2023 Physics 1 (Garcia) SJSU
Sample Problem
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
6 kg
20 m/s
Apr 20, 2023 Physics 1 (Garcia) SJSU
10 m
PE = 600 J, KE = 600 J
The Big Idea20
m
6 kgPE = 1200 J, KE = 0 J
PE = 0 J, KE = 1200 J
Energy is the “currency” of motion
Apr 20, 2023 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.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Pendulum
Energy exchange from PE to KE and back.
MaximumHeight
MaximumHeight, again
MaximumSpeed
Can predict speed from heightsince PE+KE constant
Note: Use this concept in lab experiment entitled “Projectiles”
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Don’t Flinch
When pendulum returns, never hits you.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Movie: Bowling Ball Pendulum
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Interrupted Pendulum
By conservation
of energy we
know that the
pendulum ball
will never rise
above its initial
height.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Bunny Bash
www.bunnybash.org
Apr 20, 2023 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).
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Bunny Bash
The following year, we simplified the process.
Apr 20, 2023 Physics 1 (Garcia) SJSU
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
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Blaster Balls
How high does the ping-pong ball go?
Pingpongball
Golfball
After collision the speed ofping-pong ball
is 3x larger
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).
Apr 20, 2023 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
Apr 20, 2023 Physics 1 (Garcia) SJSU
Check YourselfSlaves pull a heavy load.
Work done by slaves is positive, negative, or zero?
Work done by friction force?
Work done by the ground?LOAD
SupportPull
Friction
Apr 20, 2023 Physics 1 (Garcia) SJSU
Work & EnergyWhen forces do work on an object, the work
done equals the change in energy.
(BIG FORCE) X (short distance)
(small force) X (LONG DISTANCE)
Two persons do the same work in different ways.
Apr 20, 2023 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.
Can only push while in contact with the ground so squatting helps by increasing distance.
Apr 20, 2023 Physics 1 (Garcia) SJSU
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).
Apr 20, 2023 Physics 1 (Garcia) SJSU
Stopping & Braking Distance
35’ 55’90’
70’ 220’290’
Reaction Distance
Braking Distance
Stopping Distance
30 mph
60 mph
45 mph
At twice the speed, braking distance is four times longer