1. Bell Work: Work Intro

2. Work and Power

3. Work  Work is a force causing something to move a distance.  Work = force x distance  W=f x d  Units for work: Newton-meter (N-m) or Joule  Direction of motion and force must be the same.

4. Work is NOT  Work does not involve time.  Work is not motionless.

5. Examples  Example of work: weight lifter pushing barbells from shoulder to over head Two men moving 20 boxes to another room.  Example of NO work: weight lifter holding barbells over his head Two men taking a break from moving boxes

6. Power  Power - how fast work is done.  Power is the rate at which work is done, or amount of work done per unit of time.  Power = work divided by time P = W/t

7. Power Units  Units for power: Newton-meter/second (N- m/s) or Joule/second (J/sec) or Watt (W)  Kilowatt (kW) – used to measure large quantities of power like electrical power.  1000 W in a kW  Horsepower (hp) – also a unit of power. 1 hp = 750 watts

8. Examples  Lifting barbell in 2 seconds  Two men moving 20 boxes to another room in 10 minutes.

9. Machines  A machine is a device that makes work easier.  Examples: hammer, bicycle ramp, scissors, shovel, and doorknob

10. Machines  Machines can do a variety of jobs: pump water from a well, hoist a sail, plow a field, catch a fish.  Machines make work easier by changing the size or direction of the applied force.

11. Two forces involved in using a machine 1. The force applied to a machine is the effort force (FE)  Example: when you pull down on the handle of a crowbar, you are applying an effort force.

12. Two forces… 2. The force applied by the machine is the resistance force (FR)  Example: Motor turns fan

13. Input  Work input (W I ) – work done on a machine  W I = F E x d E  Example: work input of the crowbar (W I ) is equal to the force you apply to the handle (F E ) times the distance the handle moves (d E )

14. Output  Work Output –Work done by a machine.  W O = F R x d R  Example: The work output of the crowbar (W O ) is equal to the force the crowbar exerts on the nail (F R ) times the distance the nail moves (d R ).

15. What do machines do??  Machines make work easier, but do not multiply work.  So, work output can never be greater than work input

16. Mechanical Advantage  The number of times a machine multiplies the effort force is called the mechanical advantage (MA)  MA = F R /F E  Example: if a crowbar allows you to exert only 20 newtons of force to raise a 200 newton object, its mechanical advantage is 10.

17. Efficiency  The comparison of work output to work input is called the efficiency of a machine.  Efficiency = W O /W I x 100  High efficiency means that much of the work input is changed to useful work output.  Low efficiency means that much of the work input is lost and a great deal of useful work output does not result.

18. Simple Machines I. Inclined Plane – a slanted surface used to raise an object. Example: a ramp  Wedges and Screws – wedge is an inclined plane that moves. Have an edge that is thinner at one end. Examples: knife, ax, wedge  A screw is also an inclined plane. It is an inclined plane wrapped around a cylinder to form a spiral.

19. Simple Machines II. Levers – a bar that is free to pivot a fixed point when an effort force is applied. The fixed point of the pivot is called the fulcrum.

20. Levers ****Three classes of levers:  Fulcrum is between the effort force and the resistance force. Example: crowbar, pliers, scissors and seesaws  Resistance force is between fulcrum and effort force. Examples: wheelbarrows, doors  Effort force is greater than the resistance force and the fulcrum

21. Simple machines III. Pulleys – a chain, belt, or rope wrapped around a wheel. A pulley can change either the direction or the amount of an effort force. IV. Wheel and Axle – a lever that rotates in a circle. Made of 2 wheels of different sizes. Examples: bicycles, Ferris wheels, water wheels and gears.