wednesday PS 10-24

Homework

Stairs

Simple machines

Today

• Go over homework

• Finish bleacher exercise

• Introduce Simple machines

Answers to homework

• Homework answers• Pg 379 Pg 3811) 130000 J 1) 66.0 W2) 1J 2) 9x108 W3) 1750 J 3) 107 W4) 3960 J 4) a. 36W b. 100W5) 6000J 5) a. 146 W

b. 175 W

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 weight

Force of stairs

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)

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

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)

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

Answer

• Both methods require the same amount of work

• Using the Jack and Lever is easier

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

Changing direction, size of Force

work in = work out

Small downward force

Large distance covered

Big upward force

Small distance covered

Fidi = fodo

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

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

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

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

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

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? __________________________

Homework

• Pg 384

• Practice 1-5

• Section 1 review problems 1-8

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

Inclined plane family

• The inclined plane family includes:

• Inclined planes• Screws• Wedges

• The object to be moved moves along the ramp

Wedge

Screw

The lever family

• All levers have a fulcrum (pivot)

• Placement of the input force

output force

fulcrum

Determine class of lever

• 1st class lever

2nd class lever

• Fulcrum at one end• Input force at other

end• Output force is in

middle

3rd class lever

• Fulcrum at one end• Output force at other

• Input force in the middle

• Use more force to get more distance