1. November 29th
If a total distance of 750 m is covered in a time interval of 25s, the average speed is ______?
a.  3, 974 mph b.  3 mph c.  30 mph d.  30 m/s
What is the formula you used?
Herriman High Physics

2. December 5th and 6th Define energy, work, and power.
Herriman High Physics

3. December 7th and 8th What is a spring constant?
How is a spring constant calculated?
Herriman High Physics

4. Energy
Chapter 7
Herriman High Physics

5. Energy Facts There are different types of energy
Energy of all types is measured in Joules
Law of Conservation of Energy – Energy can be neither created nor destroyed, merely changed from one form to another
Herriman High Physics

6. Types of Energy (Unit Overview)
Mechanical Potential Energy
Energy of Position
Gravitational
Elastic
Kinetic Energy
Energy of Motion
If it moves it has kinetic energy
Heat Energy
Heat is a form of Energy Transfer
Other Forms of Stored Energy
Chemical
Fuels - usually release energy by combusion
Food – energy released by digestion
Electrical
Generated from other forms of energy
Herriman High Physics

7. Work
The Physics definition of work requires a displacement, i.e. an object must be moved in order for work to be done!
The Applied force which causes the displacement contributes to the work, i.e. in order to contribute to the work, the applied force must be parallel to the displacement.
Herriman High Physics

8. Work: A Mathematical Definition
Work = (Force)(Displacement)
Units of Work = (Newton)(Meter)
1 Newton•Meter = 1 Joule
A Joule is a unit of Energy and it takes energy to do work and work done on an object is stored as Potential Energy!
Herriman High Physics

9. Sample Problem
What work is done sliding a 200 Newton box across the room if the frictional force is 160 Newtons and the room is 5 meters wide?
W = Ff • ΔX = (160 N)(5 m)
800 Joules
Herriman High Physics

10. This can also be written as:
Power
Power = Work/time = Joules/Second
Mathematically there are two formulas for Power:
This can also be written as:
P = Fv
Herriman High Physics

11. Sample Problem
What power is developed by a 55 kg person who does 20 chin ups, h = 0.5 m, in 45 seconds?
P= w/t = FΔd/t = mgh/t
(20(55 kg)(9.8 m/s2)(0.5 m))/45 sec
= Watts
Herriman High Physics

12. Energy of Position: Gravitational Potential Energy
Occurs due to the accelerating force of gravity
Is determined by the position of the object in the gravitational field
Mathematically determined by:
Ep = mgh
mass x gravity x height
Herriman High Physics

13. Sample Problem
What is the potential energy of a 10 kg rock sitting on a cliff 30 meters high? The acceleration due to gravity is 10 m/s2.
Ep = mgh = (10 kg)(9.8 m/s2)(30 m)
2940 Joules
Herriman High Physics

14. Elastic Potential Energy
Bungee cords, rubber bands, springs any object that has elasticity can store potential energy.
Each of these objects has a rest or “zero potential” position
When work is done to stretch or compress the object to a different position elastic potential energy is stored
Alta Physics

15. Elastic Potential Energy
Top picture is “rest position”; x = 0
This is a point where the elastic potential energy = 0
Bottom picture is “stretched position”
Here elastic potential energy is stored in the spring
Ep = ½ kx2 where k is the “spring constant” in N/m
Alta Physics

16. Sample Problem
What is the Elastic potential energy of a car spring that has been stretched 0.5 meters? The spring constant for the car spring is 90 N/m.
Ep = ½ kx2 = (½)(90 N/m)(0.5 m)2
=11.25 Joules
Alta Physics

17. Where Does “K” Come From?
K is the force required to displace a spring 1 meter. So:
K = F/x
Often K is determined by hanging a known weight from the spring and measuring how much it is stretched from its rest postion.
Alta Physics

18. Sample Problem
A spring is hung from a hook and a 10 Newton weight is hung from the spring. The spring stretches 0.25 meters.
What is the spring constant?
If this spring were compressed 0.5 meters, how much energy would be stored?
If this spring were used to power a projectile launcher, which fires a 0.2 kg projectile, with what velocity would the projectile leave the launcher? Assume 0.5 m compression.
Alta Physics

19. Solution K = F/x K =10 N/0.25 m = 40 N/m Ep = ½ Kx2
Ep = ½ (40 N/m)(0.5 m)2 = 5 Joules
Ep = Ek = ½ mv2
5 Joules = ½ (0.2 kg)(v2)
V = 7.05 m/s
Alta Physics

20. Kinetic Energy Kinetic Energy is energy of Motion
Any moving object has kinetic energy
Dependent on the mass of the object and its velocity.
Mathematically expressed as:
Ek = ½ mv2
Herriman High Physics

21. Sample Problem
What is the kinetic energy of a car with a mass of 2000 kg moving at 30 m/s?
Ek = ½ mv2 = (½)(2000 kg)(30 m/s)2
= 900,000 Joules
Herriman High Physics

22. Machines Three types of Simple Machines Mechanical Advantage
Lever – Directions of input and output are opposite
Pulley - Changes direction of an applied force and it multiplies force
Incline Plane – sacrifices distance for force, i.e you use less force but must travel a greater distance
Mechanical Advantage
Machines provide a mechanical advantage, i.e. they increase the force that you can provide
Herriman High Physics

23. Efficiency Efficiency is a measure of how well work is done.
Herriman High Physics

24. Sample Problem
A pulley is used to lift a 500 Newton piano a distance of 3 meters off the ground. If the person pulling on the rope exerts a force of 200 newtons through a distance of 12 meters what is the efficiency of the system?
Efficiency = (FΔd)out/(FΔd)in =
(500 N)(3 m)/ (200 N)(12 m)=
=62.5%
Herriman High Physics

25. Review Questions
Pg 117 #1, 3, 4, 5, 6, 8, 10, 12, 13, 15, 17, 19, 21, 23, 24, 25
Herriman High Physics