work -- using an unbalanced force to move an object over a distance

- the object must move for work to be done on it

exs.     1.)  leaning on a wall -- no work is done  (Why?)
           2.)  lifting a bin full of newspapers -- work is done  (Why?)
           3.)  holding a bin full of newspapers waist high -- no work is done  (Why?)
           4.)  pulling a sled -- some work is done (Why?)

Formula

Work (Joules) = Force (Newtons) X Distance (meters)

OR   W = F  X  d

Problems:

1.)  To help rearrange the furniture in your classroom, you exert a force of 20 N to push a desk 10 m.
       How much work do you do?

F = 20 N
d = 10 m
W = ?

W = F X d

W = 20 N  X  10 m

W =  200 N-m or 200 J (joules)
 

2.)  A hydraulic lift raises a 12,000 N car 2 m.   How much work is done on the car?

F = 12,000 N
d = 2 m.
W = F X d
W = 12,000 N X 2 m
W = 24,000 N-m or 24,000 J (joules)

Power -- the rate at which work is being done
           -- another way of thinking of it is the amount of work done is a certain amount of time

Power ( Watts) = Work (Joules)          OR   P = W
                            Time (seconds)                        t

-- it can also be written as

Power = Force x Distance (since force x distance is the same as work)
                Time
 

P = F X d
          t

     - the units for power is the Watt (joules/sec)

Problems:

1.)  A car performs 40,000 J of work in 20 s.   What is the power of the car?

W = 40,000 J
 t = 20 s
P = ?

P = W
        t

P = 40,000 J
          20 s

P = 2,000 J  OR 2,000 W. (watts)
                s

2.)  A crane lifts an 8,000 N beam 75 m. to the top of a building in 30 s.   How much power does the crane use?

F = 8,000 N
d = 75 m
t = 30 s

P = ?

P = F X d
          t

P = 8,000 N X 75 m
                30 s

P = 20,000 N-m  OR 20,000 W.
                    s
 
 

Machines  -- are devices to make work easier or more effective

How do Machines make Work Easier?

A machine makes work easier by changing the amount of force you exert, the distance over which you exert your force,
or the direction in which you exert your force.

Mechanical Advantage (MA) -- the numerical value that indicates how many times a machine will
                                                 multiply the user's effort force.

• MA = force (load) the larger the number the easier it is to move the load force (effort)
• the MA will be equal to 1 when the machine's function is to just change direction
• the MA will be > 1 when the machine's function is to multiply effort
• the MA will be <1 when the machine's function is to multiply speed

 Work Input -- is the amount of work (Effort force x effort distance) that is put into the machine by the user

 Work Output --  is the amount of work ( Load force x Load distance) done by the machine

- if we discount friction and its effects for a moment, the amount of work done with the machine is equal to the
  amount of work done without the machine

- for example a box raised to a shelf is the same box raised to the same shelf with or without a pulley, ramp or any
  other machine, so the work is the same

- Machines have three possible functions that they can perform

1.  they can multiply the effort exerted by the user, this makes it seems as though the user is stronger than they
      really are ( they put in 5 N of force and move 10N of load)  ex. using a prybar to move a rock

2. they can multiply the speed of the load, but at the expense of having to put in more effort ( they put in 10 N
    and only move 5N load, but twice as fast)   ex.  the kicker on a kicker haybaler

3.  they can change the direction of the user's effort
    (ex. they can pull down but the load moves up / useful at flag poles)

Note:  A machine can never do both increase effort and speed at the same time, only one or the other.

Determining How Helpful a Machine Is

     - as mentioned above work in is the same as work out if friction is ignored
     - friction however always exists in any machine and therefore the user must put in more effort than is needed
       because the user needs to overcome the friction preventing the movement of the load
     - efficiency is the comparison of the work output to the work input and is stated as a percent - remember it is
       impossible to get work out of something if no work is put in
     - therefore the work output must always be less than the input and so the efficiency can never be 100%

Efficiency % = work output X 100  or
                             work input

Simple Machines

1.)  Inclined Plane -- a flat slanted surface with no moving parts  (ex. a slanted ramp or a farm plow)

     - a ramp is an inclined plane and it is used to multiply the user's effort, and change the direction of the user's force
     - it has a MA of >1, and a change in direction
     - the accomplishes this because the ramp will support the load against gravity so the user doesn't have to
     - the user can push horizontally and the load will move vertically
     - if the user puts rollers on the load to decrease the friction on the ramp the MA and efficiency both increase
     - the ramp will allow the user to move a large load (100N) with a small effort (25N) but it will have to done over
       a longer distance

2.)  Wedge -- an inclined planes that moves   ex.  knife, ax, zipper

     - the longer the incline on the wedge the easier it will be to use (less input force) but the longer it will take
 

3.    Screw -- an inclined plane wrapped around a cylinder in spiral rotation

     - the more wraps there are the longer the plane and the easier it is to move the load along the threads
     - of course when it becomes easier it must always take a little longer as well
     - the closer the threads the more input force is needed
     - when the user twists the head of the screw sideways it moves down into the wood
     - the screw multiplies the user's effort and changes the direction of the force just like the incline plane
     - the MA is >1 and there is a change of direction

4.   Lever

     - the lever is just a rigid bar with a swivel point called a fulcrum

fulcrum -- fixed point that a lever pivots around

     - the location of the fulcrum determines what class of lever it is and what functions and MA it will have

[3 classes of levers]
 

1st class  (like the teeter totter and scissors) - this class has the lever in between the load and the effort - if the fulcrum is nearer the effort, the lever will multiply speed ( at the cost of working harder) and change   direction of the effort force - it will have a MA of <1 - if the fulcrum is nearer the load, the lever will multiply effort ( at the cost of speed) and will also change direction   of the effort force - it will have a MA of >1   2nd class ( the wheelbarrow and door)      - this class has the fulcrum at the end and the load in the middle     - this class will always multiply effort ( at the cost of speed) and there is no change in direction ( you lift up, the load goes up) - the MA of this type is always >1 - the longer the handles or the closer to the fulcrum the load is placed the more it will multiply the user's effort 3rd class (bat, shovel, fishing pole) - this class has the fulcrum at the end and the effort in the middle - this class will always multiply speed ( at the cost of working harder) and will not change direction - a small amount of very intense effort will move a light load a very big distance - the MA of this type is always <1

5.)  Pulley ( a grooved wheel with a rope)

-- pulleys allow you to change either the direction or amount of the input force

  Types of Pulleys

fixed pulley -- attached to a permanent spot and do not move even though a rope or
                      string will be able to turn around it

ex. raising a sail

movable pulley -- will rise or fall as the string or rope turns around it  ex. construction cranes

6.) Wheel and Axle (rope on wishing well) -- is really a lever that rotates in a circle

The larger object is the wheel and the smaller is the axle.

ex.  door knob, steering wheel

Compound Machines -- most machines are combinations of one or more simple machines

ex. bicycles have many simple machines: numerous wheel and axles, levers, screws and inclined planes

REMEMBER machines can either multiply effort or speed but not both ...  a machine may or may not change direction!