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