simple machines unit 2. simple machines s8p3. students will investigate the relationship between...

Simple Machines

Unit 2

Simple MachinesS8P3. Students will investigate the relationship between force, mass, and the motion of objects.

c. Demonstrate the effect of simple machines (lever, inclined plane, pulley, wedge, screw, and wheel and axle) on work.

Work In science, the word work has a different

meaning than you may be familiar with in your everyday life. The scientific definition of work is:

• The transfer of energy when a force moves an object over a distance in the same direction of the force. Energy: the ability to do work

• If no movement happens, no work is done.

• Work = force x distance (W = Fd) • Measured in newton-meters or joules (J)• Examples: pushing a shopping cart,

turning a door knob, kicking a soccer ball, lifting a box

Work or Not Work A scientist delivers a speech to an

audience of his peers. No

A bodybuilder lifts a dumbbell above his head. Yes

A student pushes against a wall that does not move. No

A father pushes a baby in a carriage. Yes

A woman carries a grocery bag to her car. No

Simple Machines Simple Machine

A device that makes work easier by changing the size and/or the direction of the force used to do the work.

A simple machine does not help you to do less work.

Work with a simple machine = Work without a simple machine

No machine can increase both the magnitude of the force and the distance an object travels at the same time. Therefore, there is a trade-

off between force and distance.

WorkWork Input (Win)

work done on a machine

Work Output (Wout)work done by a machine

Work Law of Conservation of Energy

Energy can never be created or destroyed. Energy can be transformed from one form to another. You can never get more work out than what you put in.

In an ideal machine...

In a real machine... some energy (output force) is given off (“lost”) as friction.

Win = Wout

Win > Wout

Mechanical Advantage (MA): number of times a machine multiplies the effort force

6 Kinds of Simple MachinesInclined Plane Family

Inclined PlaneWedge Screw

Lever FamilyLeverPulleyWheel and Axle

Inclined Plane Inclined Plane

A straight, slanted surface used to raise objects because it is higher on one end

Example: Ramps, stairs, ladders

hl

FdW

How does the Simple Machine Make Work Easier

for You?

The Effects on Work Mechanical Advantage

Inclined planes: You use less effort force over a longer distance.

The longer the inclined plane = smaller effort force needed = easier work

MA greater than 1 (MA > 1)

Wedge Wedge

A moving inclined plane with 1 or 2 sloping sides

Examples: knives, hatchets, ax blade, blades of scissors, nails, teeth, plow, and chisel

A wedge transfers force in one direction into force in two directions.

Wedges are used to split or cut things apart.

FdW

ScrewScrew A screw is an inclined plane

wrapped around a shaft or cylinder. Examples: a fastener (screw),

jar lid, top of jar, drill bit, light bulb, vise

The inclined plane allows the screw to move itself when rotated.

FdW

How does the Simple Machine Make Work

Easier for You?

The Effects on Work Mechanical Advantage

Screws: You use less effort force over a longer distance.

The closer the threads on the screw = longer the inclined plane = smaller effort force needed = easier work

MA greater than 1 (MA > 1)

Lever

Levera bar that pivots at a fixed

point called a fulcrum

“Give me a place to stand and I will move the Earth.”

– Archimedes

Engraving from Mechanics Magazine, London, 1824

Effort arm

Resistancearm

Fulcrum

FdW

The 3 Classes of LeversThe class of a lever is determined by the

location of the effort force, the load, and the fulcrum.

Effort force (input force): the force applied to the lever

Load (output/resistance force): the object being moved

Lever

Mechanical Advantage (MA)

r

e

L

LMA

Effort arm length(input force)

Resistancearm length(output force)

Le must be greater than Lr in order to multiply the force.

FdW

Lever First Class Lever

fulcrum is located between the effort force and resistance force (load)

can increase force, distance, or neither always changes the direction of force (i.e. a downward

effort force on the lever results in an upward movement of the resistance force)

Examples: crowbars, scissors, pliers, tin snips, shovels, and seesaws

r

e

L

LMA FdW

Lever Second Class Lever

the load (resistance) is located between the fulcrum and the effort force

always increases effort force does not change the direction of force effort force moves farther than resistance

When the load is located closer to the fulcrum than to the effort force, an increase in force (mechanical advantage) results.

Examples: nut crackers, wheel barrows, doors, and bottle openers

r

e

L

LMA FdW

Lever Third Class Lever

the effort force is applied between the fulcrum and the resistance force (load)

always increases the distance that the effort force travels does not change the direction of force always produce a gain in speed and distance and a

corresponding decrease in force Examples: arm, tweezers, hammers, baseball bats, brooms,

and rakes

r

e

L

LMA FdW

Pulley Pulley

grooved wheel with a rope or chain running along the groove

a “flexible first-class lever”

a load is attached to one end of the rope and a force is applied to the other end

Le

Lr

F

FdW

How does the Simple Machine Make Work Easier for You?

The Effects on Work Mechanical Advantage

Pulleys: Your effort force changes direction and/or you use less effort force over a longer distance.

Your effort force changes direction = easier workAND/ORMore pulleys = smaller effort force needed = easier work

MA equal to 1 (MA = 1)

MA greater than 1 (MA > 1)

PulleyMechanical Advantage

equal to the number of supporting ropes

MA = 0 MA = 1 MA = 2

FdW

Pulley

Fixed Pulley

MA = 1does not

increase force

changes direction of force

FdW

Pulley

Movable PulleyMA = 2 increases forcedoes not change direction

FdW

PulleyPulley System/Block & Tackle

MA = 4 combination of fixed and movable

pulleys increases force may or may not change direction

FdW

Wheel and Axle Wheel and Axle

two wheels of different sizes that rotate together the wheel is always larger than the axle a pair of “rotating levers”

Examples: door knob, gears, car axle, pencil sharpener, screw driver, faucet handles

Wheel

Axle

FdW

Wheel and Axle

When effort is applied to move the wheel, the axle turns a shorter distance, but moves with more force.

The larger the wheel is when compared to the axle, the larger the mechanical advantage.

FdW

How does the Simple Machine

Make Work Easier for You?

The Effects on Work

Mechanical Advantage

Wheel and Axles: You use less effort force over a longer distance when turning the wheel.

The larger the wheel = smaller effort force needed = easier work

MA greater than 1 (MA > 1)