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Knex 78610 - Education Intro to Levers and Pulleys Teachers Guide Manual

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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
®
… 54 …


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