1. Chapter 10 Energy, Work, and Simple Machines
Quiz 10

2. Chapter 10 Objectives
Describe the relationship between work and energy
Display an ability to calculate work done by a force
Identify the force that does work

3. Chapter 10 Objectives
Differentiate between work and power and correctly calculate power used
Demonstrate knowledge of why simply machines are useful
Communicate an understanding of mechanical advantage in ideal and real machines

4. Chapter 10 Objectives
Analyze compound machines and describe them in terms of simple machines
Calculate efficiencies for simple and compound machines

5. Work Work = Change in Energy. No work done means no change in energy.
Energy is conserved, but given from one object to the other
Work is the transfer of energy by means of forces.
The work done on the system can be positive (energy taken) or negative (energy lost) and is equal to the change in energy of the system.

6. Work Work = Force times distance
Work is the product of the forces exerted on an object and the distance the object moves in the direction of force.
Positive work is done when the motion of force is in the direction of the movement.
Negative work is done when the motion of force is in the opposite direction of the movement.

7. Work An object slides down a surface which has friction.
What force does positive work?
What force does negative work?
Work as is energy, is conserved, so if a positive work is done, there has to be negative work or a net work (increase in kinetic energy)

8. Power Power is how fast work is done Work divided by time
Measured in Watts
1 Watt = 1 Joule / second
1 Horsepower is approx 746 Watts

9. Simple Machines Trade Force for Distance
MA of 1 means both the Load and the Force move the same distance, and force = load
MA of 2 means the Force moves 2x as far but is half as large as the load
MA of ½ means the Load moves 2x as far and a force 2x the load is needed

10. Efficiency Useful Energy Out / Energy put in
If you put 200 J of energy into a machine, but it only puts out 180 J of work, it is 90 % efficient
Energy losses (energy not being used to accomplish purpose) include
Sound
Heat
Friction

11. Mechanical Advantage
Lever: Ratio of Effort to Resistance distance from fulcrum
This is true for all 3 types of levers
Pulley: MA of 1 for fixed, MA > 1 for movable pulleys (determine by number of support ropes)
Wheel and Axle: Ratio of Wheel to Axle radius

13. Mechanical Advantage Screw: Ratio of circumference to pitch
Inclined Plane: Ratio of slope to height
Flat plane = MA of infinity
Vertical plane = MA of 1
Wedge: Ratio of slope to base
Same as inclined plane, but serves purpose of separation/change in direction of forces

15. Human Body Many levers in human body
Biceps are good example of type 3 lever (poor MA)
How strong is the human bicep?
Your body uses 2,000 Calories (1 C = 4,184 J)
How big of a light bulb are you?

16. Work Questions
When lifting weights, a student bench presses 135 lbs. The student lifts the weights 40 cm off their chest. If the human body is only 25% efficient at converting chemical energy to mechanical energy, how many calories will the student burn lifting the weight?
1 food calorie = 4,186 J

17. Work Questions
A box of mass 15 kg is at rest on a flat surface. If the value of Uk between the box and the surface is 0.25, how much energy will be required to push the box a total distance of 12 m?

18. Work Questions
A box of mass 18 kg is at rest on an inclined plane of 20 degrees. If the value of Uk between the box and the plane is 0.43, how much work is required to push the box a total distance of 6.0 m up the hill?