1. Homework Stairs Simple machines
wednesday PS 10-24
Homework
Stairs
Simple machines

2. Today Go over homework Finish bleacher exercise
Introduce Simple machines

3. Answers to homework Homework answers Pg 379 Pg 381 130000 J 1) 66.0 W
1J 2) 9x108 W
1750 J 3) 107 W
3960 J 4) a. 36W b. 100W
6000J 5) a. 146 W
b. 175 W

4. The bleachers
Determine the vertical work done as you moved up the stairs
The vertical displacement is about 2.8 m
The force that pushed you up the stairs is = to your weight
Find the amount of work done
Force of stairs
weight

5. How many series of steps would you need to burn off a 600 calorie meal?
For every 4180 J of work done = the work needed to burn 1 calorie
Good news, people burn calories all the time
Flights of steps = 600 / (your work/4180)

6. Calculate your power output
For the time your walked, how many 60 watt bulbs you have provided the power needed to light?
How valuable was your effort.
1000 watts provided for 1 hour is worth
7-10 cents
Value (in cents) =
(power) x (time (in hours)) x 8.5

7. Complete handout Name _________ Class Period ___
Your mass ____(kg) Your weight ____(N)
Work done ____(J) Power output ___(W)
Number of flights of stairs _______
Number of 60 W bulbs _______
Value of your effort _______ (cents)

8. Simple machines Which method to fix a tire requires more work?
Use a jack to lift the car and a lever to loosen the lug nuts OR
Lift the car yourself, and loosen the lug nuts by hand

10. Answer Both methods require the same amount of work
Using the Jack and Lever is easier

11. Purpose of simple machines
Makes the work easier to do
NOT to make less work
Simple machines can:
Change direction of the input force
Change the amount of input force needed, by changing the distance over which the force is applied

12. Changing direction, size of Force
work in = work out
Fidi = fodo
Small downward force
Large distance covered
Small distance covered
Big upward force

13. Mechanical advantage
Get more force out of the machine, by increase increasing the input distance
Example: long ramp compared to no ramp in lifting something
Multiplying force, or distance to the person’s advantage

14. MA = mechanical advantage
A number that represents either the force multiplier of a machine
MA = output force (Fo) = input distance
input force (Fi) output distance
No units (they cancel out)
The bigger the ratio the bigger force advantage

15. Ideal machine
In real machines, not all of the input force is used to move the object
Some is used to overcome internal friction within the machine or external friction where 2 surfaces meet
An ideal machine would lose no input force to friction

16. Calculating mechanical advantage
What is the mechanical advantage of a ramp 5.0 m long and 1.5 m high?
5.0m = input distance (di)
1.5m = output distance (do)
5.0 m
1.5
MA = di/do = 5/1.5 = 3.3

17. Answering question on MA
Identify which force, distance are being applied to the machine (di, Fi)
Identify which force, distance result from use of the machine (do, Fo)
No units for mechanical advantage
The two ratios can be set = to each other

18. Summary of last two days
Work is done when _______________
The standard unit for work = _______
How is the concept of power related to work? ________________________
How do machines make work easier for us? __________________________

19. Homework
Pg 384
Practice 1-5
Section 1 review problems 1-8

20. Simple machines
Most basic machines that make work easier for humans to do work
Other machines are either modifications or combinations of these machine types
Divided into 2 families of machines
The inclined plane family
The lever family

21. Inclined plane family The inclined plane family includes:
Inclined planes
Screws
Wedges
The object to be moved moves along the ramp

22. Wedge

23. Screw

24. The lever family All levers have a fulcrum (pivot)
Placement of the input force
output force
fulcrum
Determine class of lever
1st class lever

25. 2nd class lever Fulcrum at one end Input force at other end
Output force is in middle

26. 3rd class lever Fulcrum at one end Output force at other
Input force in the middle
Use more force to get more distance