1. Chapter Three : work and simple machines

2. Lesson 1: Work and Power What is work?
Work is the transfer of energy to an object by a force that makes an object move in the direction of the force.
Work is only being done while the force is applied to the object
Your push makes your bike move= work is done
You push on the bike and it does not move= no work is done

3. Calculating Work Work equation
Work (in joules) = force (N) x distance (m)
W = Fd
A joule unit is newton x meter (N x m)
The joule is the SI unit of work and energy
The distance is the distance the object moves while the force is acting on it

4. Factors That Affect Work
The work done on an object depends on the direction of the force applied and the direction of motion
A force that acts in the direction of the motion does work
Force and motion are
in the same direction
Multiply the force and
the distance

5. Lifting Objects Lifting your backpack requires you to do work
Backpack has weight because of the gravity
To lift, you must pull with an upward force equal to or greater than the weight
The work done to lift any object is equal to the weight of the object multiplied by the distance it is lifted

6. Work and Energy Doing work on an object transfers energy to the object
Kinetic energy- energy of motion
Figure 4 on page 90
Boy applies a force on the tray to make it move; he does work
This work transfers energy to the tray
The added energy is the kinetic energy of the tray

7. Work and Energy Lifting an object also increases the object’s energy
Gravitational potential energy (GPE)
Energy of the object increases as its height increases
Girl lifts the tray: did work on the tray
GPE increases and it gains kinetic energy as it moves upward
Transferred energy to the tray which increases the tray’s potential and kinetic energy

8. What is power? Power is the rate at which work is done
More quickly=more power
Power (in watts) = work (in joules) / time (seconds)
P= W/t
The SI unit of power is the watt (W).
1 J/s = 1 W
Work transfers energy
Power is the rate at which energy is transferred to an object

9. Find Power
A boy does 18 J of work in 2.0s on his backpack as he lifts it from a table. How much power did the boy use on the backpack?
_______________________________________
A child pulls a wagon, doing 360 J of work in 8.0s. How much power is exerted?

10. http://science360.gov/obj/video/c5be5456- 2e39-49a7-8118-218868df89eb/work-energy- power

11. Lesson 2: Using Machines
What is a machine?
A machine is a device that makes doing something easier.
Snow shovel, scissors, watch
There are simple and complex machines
All machines make tasks easier but they do not decrease the amount of work required
Machines change the way in which the work is done

12. Input Force to Output Force
In order to use a machine, you must apply a force.
Input force- force you apply to the object
Figure 6 on page 96- apply a force on the hammer
The machine changes the input force into an output force
Output force- the force the machine applies
For example, the hammer changes the input force to an output force that pulls the nail out of the board

13. Input Work to Output Work
Machines convert, or change, input work to output work
input work, Win (input force x distance the machine moves in the direction of the input force)
The output work, Wout (output force x the distance the machine moves in the direction of the output force)
Work is done by applying an output force on something and making it move

14. Input Work to Output Work
For example: a pair of scissors
IDENTIFY THE INPUT FORCE AND OUTPUT FORCE:
You apply an input force that moves the machine and does work
You have to squeeze the handles to make the blades move
The scissors cut the paper.
Output force

15. How do machines make work easier to do ?
Three ways: It can change…
The size of the force
The distance the force acts
The direction of a force
General Rule: True for all machines
You apply a large force over a short distance
You apply a small force over a long distance

16. Change the Size of a Force
A crowbar is a machine
Make it easier by changing the input force into a larger output force
When the output force is greater than the input force, the output force acts over a shorter distance
Example: Figure 7

17. Change the Distance a Force Acts
Using a rake to gather leaves is an example of a machine that increases the distance over which a force acts.
The person’s hands move the top of the rake a short distance
What force is person applying?
The other end sweeps through a greater distance
What force is the rake applying?
Makes it easier to rake leaves

18. Continued…
The force applied by the rake decreases as the distance over the force acts (the output distance) increases
When the output force acts over a longer distance than the input force, the output force is less than the input force.

19. Change the Direction of a Force
Example : a pulley in Figure 7 on page 97
As the free end of the rope is pulled down, the object tied to the other end of the rope is lifted up.
The machine changes the direction of the applied force.
Equal output and input forces act over equal distances.

20. What is mechanical advantage?
Most machines change the size of the force applied to them.
A machine’s mechanical advantage is the ratio of a machine’s output force produced to the input force applied.
MA = Fout / Fin
Mechanical advantage = output force (in N) divided by input force (in N)

21. Continued… MA can be less than, equal to, or greater than 1
Greater than 1: output force > input force
Crowbar
Less than 1: output force < input force
Rake
Equal to 1 : output = input

22. Continued…
The ideal mechanical advantage (IMA) is the mechanical advantage if no friction existed.
Can machines operate at IMA?
No, because friction always exists.
Page 98: solve the practice problem

23. What is efficiency?
The output WORK done by a machine never exceeds the input WORK of the machine because of friction.
Friction converts some of the input work to thermal energy.
The converted energy cannot be used to do work.

24. Continued…
The efficiency of a machine is the ratio of the output WORK to the input WORK.
Efficiency (%) = output work (in J) x 100%
input work (in J)
Output work = Wout
Input work = Win

25. Continued… Try practice problem on page 99!
Because output work is always less than input work, a machine’s efficiency is always less than 100 percent.
Figure 8 on page 99
Lubricating a machine’s moving parts increases efficiency
Try practice problem on page 99!

26. Lesson 3: Simple Machines
What is a simple machine?
A simple machines do work using only one movement
Six types:
Lever
Wheel and axle
Inclined plane
Wedge
Screw
Pulley

27. Levers
A lever is a simple machine made up of a bar that pivots, or rotates, about a fixed point.
The fixed point about which a lever pivots is called a fulcrum.
Figure 10 (soda can)
Fulcrum: finger tab attaches to the can
Input and output forces act on opposite ends of fulcrum

28. Levers Input arm: distance from the fulcrum to the input force
Output arm: distance from the fulcrum to the output force
There are three types of levers
They differ in where the input and output forces are relative to the fulcrum

29. First-class Lever The fulcrum in between the input and output force
Direction of input and output are always different

30. Second-class lever
Output force is in between the input force and the fulcrum
The output and input act in the same direction
Makes the output force > input force
Wheelbarrow
Nut cracker

31. Third-class Lever
The input force is between the output force and the fulcrum
Output and input act in the same direction
Output force < input force
Tweezers, rake, Broom

32. First-class Lever The location of the fulcrum determines the MA
MA of a first class lever depends
Greater than 1: input arm is longer than the output arm
Output force is greater than the input force
Less than 1: input arm is shorter than the output
Output force is less than the input force
Equal to 1: input and output arms & forces are equal

33. Second-class Lever Output arm is shorter than the input arm
Output force is greater than the input force
The MA is always greater than 1
For all second-class levers

34. Third-class Lever Output arm is always longer than the input arm
Output force is less than the input force
MA is always less than 1
For all third-class levers

35. Levers in the Human Body
Body uses all three classes of levers to move
Muscles provide the input force
Neck
Foot
Arm
Figure 12 on page 106

36. Neck First-class Fulcrum: joint connecting spine and skull
Neck muscles provide the input force
Output force is applied to the head and helps support your head’s weight

37. Foot
When standing on your toes, the foot acts as a second- class lever
Ball of foot: fulcrum
Input force: comes from the muscles on the back of the leg

38. Arm Third-class lever Elbow: fulcrum
Input force: muscles located near the elbow (forearm/ bicep muscle)

39. Wheel and Axle
Is an axle attached to the center of a wheel and both rotate together
Axle=shaft
Example: screwdriver
Handle- wheel because it has the larger diameter
shaft=- axle attached to the handle
Both the handle and the shaft rotate when the handle turns

40. Using a Wheel and Axle When you turn the screwdriver
Input force applied to the handle (wheel)
Output force applied to the screw by the screwdriver’s axle
The wheel is larger than the axle= MA is greater than 1
This makes the screw easier to turn

41. Inclined Planes Is a flat, sloped surface. Also known as a ramp
Takes less force to move an object upward along a ramp than it does to lift the object straight up
Useful for moving heavy loads
Because of friction, no ramp operates at its IMA

42. Wedge A sloped surface that moves
A type of inclined plane with one or two sloping sides
Doorstop- has one sloped side
The shape of the wedge gives the output forces a different direction than the input force
Example: teeth
How?

43. Screws Is an inclined plane wrapped around a cylinder
The screw threads change the input force to an output force
What is the input force?
The output force pulls the screw into the material
Figure 16 on page 108

44. Pulleys
A simple machine that is a grooved wheel with a rope or a cable wrapped around it

45. Fixed Pulleys A fixed pulley only changes the direction of the force
Example: window blind
Cord passes through the fixed pulley (of the window frame)
MA= equal to 1

46. Movable Pulleys & Pulley Systems
Can be attached to the object being lifted
Called a movable pulley
Movable pulleys decrease the force needed to lift an object; distance the force acts over increases
Pulley system
Combination of fixed and movable pulleys that work together

47. MA of Pulleys
The IMA is equal to the number of sections of rope pulling up on the object

48. Find the MA

49. What is a compound machine?
Two or more simple machines that operate together form a compound machine
Figure 18
Can opener
second-class lever to move the handle
Wheel and axle to turn the blade
Wedge to puncture the lid

50. Gears A gear is a wheel and axle with teeth around the wheel
Two or more gears form a compound machine
When they interlock, one gear causes the other to turn
The direction of the gears change
Different sizes= different speeds

51. Gears Continued… Smaller gears rotate faster than larger gears
Amount of force transmitted through the gears is affected by the size
Input force applied to a larger gear is reduced when it is applied to a smaller gear