2. Work, Power, & Simple Machines Assistance from Derek Hinrichs

3. 15-1 WORK Work is the product of the force applied to an object and the distance through which the force is applied. Work is the amount of force applied to an object times the distance the object moves in the direction of the force while the force is applied. NOTE! A unbalanced force must be applied. Remember: Unbalanced forces make objects accelerate in the direction of the force!

4. If you push against a door and it doesn’t move, is this work? No, the object does not move, therefore it is not work. If you carry a box across the room in your hands, is work done by your arms? No, the direction of the force (up) is not the same as the direction of movement (across).

5. Work does NOT involve time If you hold a soup can with extended arms, you are not doing any work as long as that can does not move up and down. Why? The direction of the force to hold the can in the air is up; as long as that is not the direction of movement, no work is done. One can exert energy without doing work.

6. WORK = FORCE x DISTANCE W = F x d

7. The unit for work is Newtonmeter (Nm) or a joule (J) Remember: Mass is not a force! Mass (Kg) must be converted to newtons (N). F = ma [Kg m/s 2 ] {weight = mass x 9.8 m/s 2 }

8. 15-2POWER

9. In English, many people use the word “work” when they want to use the “power.” Power and work are related, but they are not the same thing. Power is the rate at which work is done, or the amount of work done per unit time. Remember from your math class, any time you see the term rate it means something is divided by time. So...

10. POWER = WORK / TIME P = W / t

11. Think about what this really means. A car has more power than a bike; a bulldozer has more power than a person; 2 horses have more power than one. WHY? Why does it take more power to take an elevator up 3 flights of stairs rather than walk? Think about it; the same work is done- the same amount of weight (force) goes the same distance in the direction of the force. How can it take more power? More work in the same time & the same work in less time both mean MORE POWER.

12. The rate at which the work is accomplished is much different. An elevator, for example, takes much shorter time, thus increasing the power. P = (F x d) / t (this is the same formula as before, just with the formula for work substituted in for “w.” The unit for power is joules/second (J/s) or the watt (W). One horsepower is equal to 750 watts.

13. 15-3 Simple Machines Machine - a device that makes work easier by changing the size or direction of an applied force.

14. How do machines make work easier? They multiply the applied force. –i.e. It is easier to pull a nail with a crowbar than with fingers. They change the direction of the applied force. –i.e. It is easier to lift a box up with a pulley than simply lifting it straight up.

15. Forces in Machines There are ALWAYS two forces involved in using a machine. Effort force - the force applied TO a machine. –i.e. The force that a person exerts on a crowbar. Resistance force - the force applied BY a machine. –i.e. The force that the crowbar exerts on the nail.

16. Forces of Machines Effort ForceResistance Force Also notice how the downward effort force will move the object up. The required effort force is less than the resistance force.

17. Forces of Machines Also with a pulley the downward effort force will move the object up. Effort Force Resistance Force

18. WORK INPUT work inputWork must be done ON a machine before the machine can do any work, this is called work input. Input Output

19. Work Input Recall the formula W = F x d This same formula applies to calculating the work put into a machine. W I = F E x d E

20. Work Output work outputWork done BY a machine is called work output. The formula also applies to calculating the work put out by a machine. W O = F R x d R

21. Work by machines Although machines make work easier, they do NOT multiply work!

22. Same Work Someone loading a truck will perform the same work lifting the boxes directly into the truck as a person that walks up a ramp carry the same boxes. The end result is the truck is loaded.

23. Work by machines It is the same amount of work, only easier. In other words, they do not make it possible to do more work than the work put into a machine.

24. Work output is always less than work input!!! < Output < Input

25. Mechanical Advantage mechanical advantageThe number of times a machine multiplies the effort force is called mechanical advantage (MA). i.e.If a person uses a 20 N force to lift a 100 N object, the MA is 5. There are no units for mechanical advantage.

26. Mechanical Advantage AMA = F R / F E This calculates the actual mechanical advantage.

27. What if the mechanical advantage is less than 1? The resistance force is smaller than the effort force. It seems harder to do the work, but the effort distance is shorter.

28. Mechanical Advantage Think of a broom. The effort force is greater than the resistance force. You apply more effort to the end of the broom than the broom has friction with the floor. BUT how far you move the broom handle is much less than how far the broom moves across the floor.

29. What good would a machine be if the mechanical advantage was equal to one? It changes the direction of the force. Remember: Work output can NEVER be greater than work input.

30. Leverage Something is always sacrificed for something to be gained. If there is less force applied, to get the same work, what must happen? Hint: W = F x d If the effort applied goes down, the effort distance must go up to equal the same amount of work.

31. What help is it if the resistance force is less? If the work done is the same and less effort is applied, a greater distance must be used. What help is it if the resistance force is greater? If the work done is the same and more effort must be applied, less distance must be used. Think about when each of these could be used.

32. EFFICIENCY The comparison of work output to work input is EFFICIENCY EFFICIENCY. Efficiency is expressed as a percent. There are no units. Efficiency = (W O / W I ) x 100

33. Friction in Machines If a machine is efficient, a machine changes most of the work put into the machine into work output. Friction is involved in any machine! Some of the work put into the machine, then, must be used to overcome friction before it can convert this to work output.

34. Efficiency A machine can NEVER be more than 100% efficient. (In fact, no machine is even 100% efficient.) Less friction increases efficiency. More friction decreases efficiency. Automobile engines are only about 20% efficient. Where does all the work go that is put into the engine?