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Work and Simple Machines
In science, the word work has a different meaning than you may be familiar with.
The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)
What is work?
Work or Not?According to the
scientific definition, what is work and what is not?a teacher lecturing to
his/her classa mouse pushing a piece
of cheese with its nose across the floor
WORK
Work = Force x Distance
The unit of force is NewtonsThe unit of distance is metersThe unit of work is Newton-metersOne Newton-meter is equal to one JouleSo, the unit of work is a Joule
Formula for work
W=FD
Work = Force x Distance
Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?
W=F * DWork = Force x
Distance
Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? Work = F X D = 20N*10m
200Nm=200 joules
History of Work
Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.
Simple MachinesAncient people invented simple machines
that would help them overcome resistive forces and allow them to do the desired work against those forces.
The six simple machines are: Lever Wheel and Axle Pulley Inclined Plane Wedge Screw
Simple Machines
A machine is a device that helps make work easier by accomplishing one or more of the following functions: Increasing the magnitude of a forceIncreasing the distance of a forceIncreasing the speed of a force Changing the direction of a forceTransferring a force from one place
to another
Simple Machines
Input force (the force you apply)
Output force (force which the machine applies to the task).
When a machine takes a small input force and increases the magnitude of the output force, a
Mechanical Advantage has been produced.
Mechanical Advantage
Friction is ignored when calculating IMA.
IMA > 1 means it increases force Each machine calculates IMA differently
As we cover each machine, put the IMA formula on the grid sheet for that machine
Ideal Mechanical Advantage
AMA is the ratio of output force/input force (R/E).
If an input force of 20 newtons and the output force of 100 newtons, the machine has an Actual Mechanical Advantage (AMA) of 5.
AMA = R / E Formula is the SAME for all machinesFriction decreases the AMA.
Actual Mechanical Advantage
No machine can increase both the magnitude and the distance of a force at the same time.
There is no such thing as a free lunch
The Lever A lever is a rigid bar that
rotates around a fixed point called the Fulcrum.
Effort Force supplied by you Resistance Force: force
supplied by the machine to move something
There are 3 Classes of levers
The 3 Classes of LeversThe class of a lever is
determined by the location of the effort, resistance and fulcrum.
Types of Levers
• Note the location of the F, E and R for each type of lever
• For all levers the following formulas are correct
• IMA = LE/LR = divide the length of the Effort arm by the length of the Resistance arm (both measured from force to the fulcrum)
• AMA = Resistance / Effort .
The fulcrum is located at some point between the effort and resistance forces.
Common examples of first-class levers include see-saw, crowbar, scissors, pliers, tin snips
A first-class lever ALWAYS changes the direction of force .
First Class Lever
FIRST CLASS LEVERS The Fulcrum is between E and R . If F is closer to R the Effort moves farther than Resistance, multiplies E and changes its direction
the Resistance is located between the fulcrum and the Effort.
Common examples of second-class levers include nut crackers, wheel barrows and bottle openers.
Advantage: Always increases the forceNever changes the direction of force.
SECOND CLASS LEVERS
R is between fulcrum and E. Effort moves farther than Resistance. Multiplies force, but NEVER changes its direction
Effort is between Fulcrum and Resistance Cannot increase the force Resistance moves farther than Effort. Multiplies the distance or speed that the effort force travels
Effort is applied between the Fulcrum and the Resistance.
Examples of third-class lever: tweezers, a rake and your arm
Never changes the direction of the force
Always produces a gain in speed and distance
Always DECREASES the force
Third Class Lever
Wheel and Axle
The wheel and axle is a large wheel rigidly secured to a smaller wheel or shaft, called an axle.
When the wheel or axle is turned, the other part also turns. One full revolution of either part causes one full revolution of the other .
IMA = Radius(wheel)
_____________ Radius(axle)
AMA = R/E
Wheel and Axle
Pulleys
Can change the direction of a force
or Can gain a Mechanical
Advantage depending on how the pulley(s) is(are) arranged.
Fixed PulleyFixed pulley :if it does
not rise or fall with the load being moved. A fixed pulley changes the direction of a force; however, it does not increase the force.
Moveable PulleyA Moveable pulley
rises and falls with the load that is being moved. A single moveable pulley creates an IMA of 2. It does not change the direction of a force.
The IMA of a moveable pulley is equal to the number of ropes that support the moveable pulley. Pulling down strand does not count.
Compound Pulley systems
Composed of at least 1 fixed and 1 moveable pulley linked
IMA = # of supporting strands.
PULLING DOWN DOES NOT COUNT AS A SUPPORTING STRAND.
Inclined PlaneAn inclined plane is an even
sloping surface. A Ramp. The inclined plane makes it
easier to move a weight from a lower to higher elevation.
IMA = Run/RiseIMA = Effort Distance/
Resistance Distance
Inclined PlaneThe IMA of an inclined
plane is equal to the length of the slope divided by the height of the inclined plane.
IMA(Slope) = run/riseMechanical Advantage,
is derived by increasing the effort distance through which the force must move.
Wedge
ScrewThe screw is also a
modified version of the inclined plane.
While this may be somewhat difficult to visualize, it may help to think of the threads of the screw as a type of circular ramp (or inclined plane).
IMA of a SCREW
INPUT WORK = Effort X Distance the Effort moved
IW = E x DE
DE is not the same as LE. LE is measured from fulcrum to the E. DE is measured along the direction of movement.
OUTPUT WORK = Resistance X Distance the Resistance moved
OW = R x DR
DR is not the same as LR. LR is measured from fulcrum to the R. DR is measured along the direction of movement.
Some output force is always lost due to friction.Efficiency=(Output Work/Input Work)*100
Formula for Efficiency is the same for ALL machines No machine has 100% efficiency due to friction.
Efficiency
Theoretical Effort (TE)
Machine Input Work(IW) Output Work(OW)
IMA AMAR/E
EfficiencyOutput workInput workX 100
Theoretical Effort (TE)R/IMA
Lever E x DE
DE = Distance
the Effort moved
R x DR
DR = Distance
that the Resistance moved
LE/LR
LE = Length of
the effort armLR = Length of
the resistance arm
R/E Output workInput workX 100
R/IMA
Pulley E x DE R x DR # supporting strands
R/E Output workInput workX 100
R/IMA
Wheel & Axel E x DE R x DR Radius (wheel)--------------------Radius (axel)
R/E Output workInput workX 100
R/IMA
Inclined Plane E x Run R x Rise Run/Rise R/E Output workInput workX 100
R/IMA
Wedge E x Run(length) R x Rise(width) Run/Rise orLength/Width
R/E Output workInput workX 100
R/IMA
Screw E x DE R x DR # threads--------------------2.5xLength(cm)
R/E Output workInput workX 100
R/IMA
Calculations related to Simple Machines KEY
1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the progress of the workers from an observation booth.
2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position?
3. Using a single fixed pulley, how heavy a load could you lift?
Practice Questions
4. Give an example of a machine in which friction is both an advantage and a disadvantage.
5. Why is it not possible to have a machine with 100% efficiency?
6. What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input.
Practice Questions
1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the progress of the workers from an observation booth.
Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work.
2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec?
Work = 7 m X 50 N X 2 = 700 N-m or J
3. Using a single fixed pulley, how heavy a load could you lift?Since a fixed pulley has a mechanical advantage of one, it will only change
the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction.
Practice Question answers
4. Give an example of a machine in which friction is both an advantage and a disadvantage.
One answer might be the use of a car jack. Advantage of friction: It allows a car to be raised to a desired height without slipping. Disadvantage of friction: It reduces efficiency.
5. Why is it not possible to have a machine with 100% efficiency? Friction lowers the efficiency of a machine. Work output is always less than
work input, so an actual machine cannot be 100% efficient.
6. What is effort force? What is input work? Explain the relationship between effort force, effort distance, and input work.
The effort force (E) is the force applied to a machine. Input work (IW) is the work done on a machine. The input work (IW) of a machine is equal to the effort force (E) times the distance (DE) over which the effort force is exerted.
Practice Question answers