Bedienungsanleitung Knex 78610 - Education Intro to Levers and Pulleys Teachers Guide (Seite 54 von 62) (Englisch)

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Teacher’s Notes

The correct answer is 4 times.

The multiplication can be found by

counting the number of lengths of rope

supporting the movable pulleys. In

this example there are 2 movable

pulleys, each with two supporting

ropes = 4. (The three pulleys at the

top of the structure are fixed pulleys.)

See: A Quick Guide to Pulleys.

Whole class

 • Introducetheactivitiesbyexplaininghowthecranes

use ‘block and tackle’ mechanisms to lift very heavy

loads and how a block and tackle can make the job

of lifting heavy objects easier.

 • BeforebuildingtheirK’NEXBlockandTacklemodel

ask the children to look at the plans for their model on

Page 15 of the Building Instructions booklet.

 • Explainhowtheblockandtackleisconstructedby

winding the rope around a number of fixed and

movable pulley wheels.

 • Howmanytimesdotheythinktheliftingforcewillbe

increased by this pulley system?

Working in Groups of 2-3

Let’s Investigate

 • AskthechildrentoconstructtheirK’NEXBlockand

Tackle model and allow them some time to investigate

how it works.

 • Askeachgrouptoidentifyandlabelthefixedand 

movable pulleys in their model.

 • Howdothechildrenthinkusinganumberoffixedand

movable pulleys in a lifting mechanism will affect its

ability to lift a load?

 • Askeachchildinturntoplaceapapercupcontaining

one or more 200g masses on the palm of his or her

hand and to raise and lower it. This will allow them to

gain some impression of the load being lifted by their

model and the forces needed to raise it.

 • Suggestthechildrentryliftingthepapercupusingthe

K’NEX Block and Tackle model.

  •Askwhathappenedtotheirmodelwhentheyfirst

tried to lift the weighted paper cup?

  •Askthechildrentoexplaintheirobservations.

  •Whatotherscienceconceptisworkinghere?

  •Howmighttheysolvetheproblem?

Force meters could be used to

record the actual forces needed to

lift the weighted paper cup vertically.

These can then be compared with

those needed when using the block

and tackle.

The K’NEX Block and Tackle model

becomes unstable and will topple

forwards as the weighted paper cup

is lifted. When the load is lifted off the

desktop the structure acts like a lever

that pivots around its front edge. The

front edge becomes the fulcrum and

if the weight of the load is greater than

the weight of the model, the model

will topple in the direction of the load.

The forces in the system have

become unbalanced.

A counterweight is needed to balance

the weight of the load being lifted.

The folder shown in the photograph

on Page 15 of the Building Instructions

booklet is acting as the counterweight.

Lesson 9: The Block and Tackle

levers and pulleys

website: www.knexeducation.co.uk

Education

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