1. Chapter 7 Energy The “money” of physics 23-Apr-17
Physics 1 (Garcia) SJSU

2. Primitive Economics Do your job Get paid 23-Apr-17
Physics 1 (Garcia) SJSU

3. Modern Economics Buy stuff Do your job Get paid
Using money simplifies economics and accounting.
23-Apr-17
Physics 1 (Garcia) SJSU

4. 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.”
23-Apr-17
Physics 1 (Garcia) SJSU

5. 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).
23-Apr-17
Physics 1 (Garcia) SJSU

6. 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
23-Apr-17
Physics 1 (Garcia) SJSU

7. 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!
23-Apr-17
Physics 1 (Garcia) SJSU

8. 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
23-Apr-17
Physics 1 (Garcia) SJSU

9. The Big Idea Energy is the “currency” of motion PE = 1200 J, KE = 0 J
6 kg
Energy is the “currency” of motion
20 m
PE = 0 J, KE = 1200 J
23-Apr-17
Physics 1 (Garcia) SJSU

10. Conservation of Energy
Conservation of mechanical energy is
(Potential Energy) + (Kinetic Energy)
stays constant during motion.
Energy “bookkeeping” makes motion simple.
23-Apr-17
Physics 1 (Garcia) SJSU

11. Can predict speed from height
Pendulum
Energy exchange from PE to KE and back.
Maximum
Height
Maximum
Speed
Maximum
Height, again
Can predict speed from height
since PE+KE constant
Note: Use this concept in lab experiment entitled “Projectiles”
23-Apr-17
Physics 1 (Garcia) SJSU

12. Demo: Don’t Flinch When pendulum returns, never hits you. 23-Apr-17
Physics 1 (Garcia) SJSU

13. Movie: Bowling Ball Pendulum
23-Apr-17
Physics 1 (Garcia) SJSU

14. Demo: Interrupted Pendulum
By conservation
of energy we
know that the
pendulum ball
will never rise
above its initial
height.
23-Apr-17
Physics 1 (Garcia) SJSU

15. Demo: Bunny Bash
23-Apr-17
Physics 1 (Garcia) SJSU

16. 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).
23-Apr-17
Physics 1 (Garcia) SJSU

17. Demo: Bunny Bash The following year, we simplified the process.
23-Apr-17
Physics 1 (Garcia) SJSU

18. 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
23-Apr-17
Physics 1 (Garcia) SJSU

19. 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).
23-Apr-17
Physics 1 (Garcia) SJSU

20. 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
23-Apr-17
Physics 1 (Garcia) SJSU

21. 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
23-Apr-17
Physics 1 (Garcia) SJSU

22. 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.
23-Apr-17
Physics 1 (Garcia) SJSU

23. 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.
23-Apr-17
Physics 1 (Garcia) SJSU

24. 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).
23-Apr-17
Physics 1 (Garcia) SJSU

25. Stopping & Braking Distance
35’
55’
30 mph
90’
Reaction Distance
Braking Distance
Stopping Distance
45 mph
70’
220’
60 mph
290’
At twice the speed, braking distance is four times longer
23-Apr-17
Physics 1 (Garcia) SJSU