1. Chapter 4 Work and Energy

2. 4.1 - Work and Machines Work -
Force x distance is equal to the change in energy
Work = force x distance
Amount of work depends on
How much force
How far the force causes the object to move (no movement, no work)
Force must be in the same direction as motion to count as work.
Units of work are Newton-meter or Joules (J)

3. Your turn – Work or No Work
A book falls off a table and free falls to the ground.
A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed.
A rocket accelerates through space.
A teacher applies a force to a wall and becomes exhausted.

4. Let’s Calculate Work
How much work is needed to push an object that weighs 350 N a distance of 4 meters?
A dancer lifts a 400 N ballerina overhead a distance of 1.4 m. How much work is done?
How much work is done by an applied force to lift a 15-Newton block 3.0 meters?
How much work is done on the wall by a person pushing on the wall with 5600 N of force?

5. 4.1 - Work and Machines Machine – Types of machines Simple machines
Compound machines
Two or more simple machines

6. 4.1 - Work and Machines Efficiency – Efficiency(%)= output work (J)
input work (J)
Machines can be made more efficient by reducing friction
All machines are less than 100% efficient
X 100

7. Let’s calculate efficiency!!!
You do 222 J of work pushing a box up a ramp. If the ramp does 200 J of work, what is the efficiency of the ramp?
Find the efficiency of a machine that does 800J of work if the input work is 2,000J.
The input work on a pulley system is 75J. If the pulley system is 84% efficient, what is the output work from the pulley system?

8. 4.1 - Work and Machines
How are machines useful?
Increase speed
Change direction of force
Increase the force
Mechanical advantage –
MA = output force (N)
input force (N)

9. Calculating Mechanical Advantage!!!!
To pull a weed out of a garden, you can apply a force of 50 N to the shovel. The shovel applies a force of 600 N to the weed. What is the mechanical advantage of the shovel?
To lift a block on a movable pulley, you can apply a force of 50 N to a rope. The rope applies a force of 700 N to the block. What is the mechanical advantage of the rope?

10. Change Requires Energy Energy – Work Transfers Energy
4.2 – Describing Energy
Change Requires Energy
Energy –
Work Transfers Energy
Ability to do work
Uses the same units as work
System –
Energy is transferred between systems when interact

11. 4-2 – Describing Energy Forms of Energy - Mechanical Heat (radiant)
Potential and kinetic
Sound
Wind
Heat (radiant)
Particles in matter moving
Geothermal
Chemical
Food, fossil fuels
Electromagnetic
Light,microwaves, radio waves, electricity,etc
Nuclear
Fission – smashing atoms
Fusion – joining atoms ( like on the Sun)

12. 4.2 – Describing Energy Kinetic Energy
energy of motion
depends on mass and speed
Kinetic Energy = ½ mass x speed (KE = ½ m v2)
KE (J) = ½ m (kg) X v (m/s)2

13. Kinetic Energy Practice
What is the kinetic energy of a kg elephant moving at 34 m/s?
What is the kinetic energy of a 22.7 kg boy on his 45.5 kg bike rolling down a hill 25 meters high at 7.21 m/s?
Determine the kinetic energy of a 1000-kg roller coaster car that is moving with a speed of 20.0 m/s.
If the roller coaster car in the above problem were moving with twice the speed, then what would be its new kinetic energy?

14. 4.2 – Describing Energy Potential Energy
stored energy(elastic,chemical,gravitational)
Gravitational Potential Energy = mass x gravity x height
GPE (J) = m (kg) g (N/kg) h (m)

15. Potential Energy Practice
What is the potential energy of a hammer that weighs 25 N that is sitting on top of a ladder 5 meters high?
What is the potential energy of a golf ball that weighs 3.2 N that is resting on top of a lab table?
A 45 N boy is on his 15 N bicycle at the top of a large hill that is 65 meters high. What is the total potential energy?

16. 4-3 - Conservation of Energy
Law of Conservation of Energy
Energy cannot be created or destroyed; it may be transformed from one form into another or transferred from one object to another, but the total amount never changes

17. 4-3 - Conservation of Energy
Energy Transformations
Mechanical Energy Transformations

18. 4-3 - Conservation of Energy
Energy Transformations
Mechanical Energy Transformations

19. 4-3 - Conservation of Energy
Energy Transformations
One form of energy becomes another form
electrical heat, light

20. Conservation of Energy
PE = 15000
KE =
PE =
KE =
PE =
KE =7500
PE =
KE =
PE =
KE = 15000

21. Evaluating Energy Transformations

22. 4-3 - Conservation of Energy
Power is how fast the energy is changed from one form to another
Power = energy
time interval
P (watts) = E (J)
t (s)
Unit of power is joule per second, or watt

23. Power Practice
A light bulb uses 300 J of energy in 4 seconds. How much power does the light bulb require?
What is the power of an electric toothbrush if it can do joulles of work in 75 seconds?
An athlete is using the row machine in the gym.  She does 3245 joulles of work on the oars in 72 seconds.  What is her power output?