1 of 32 Boardworks Ltd 2009 Boardworks Ltd 2009 2 of 323 of 32
Boardworks Ltd 2009Balanced and unbalanced forcesWhen forces in
opposite directions have the same value, they cancel each other out
!he forces are balanced !he o"#ect is either stationary or moves at
a constant speed $f the forces on an o"#ect are not "alanced, there
is a resultant force, and the o"#ect accelerates in the direction
of this force% of 32 Boardworks Ltd 2009Turning effects from
balanced forces& of 32 Boardworks Ltd 2009Examples of turning
effects!hese are some e'amples of everyday turnin( effects)*an you
think of any more+ Boardworks Ltd 2009 , of 32- of 32 Boardworks
Ltd 2009What is a moment?. moment is a turnin( effect caused "y a
force!he si/e of the turnin( effect caused "y a force depends on
two factors What are they+!he "i((er the force, the lar(er the
turnin( effect!he lar(er the perpendicular distance from the pivot
to the point where the force is applied, the lar(er the turnin(
effect0 of 32 Boardworks Ltd 2009$n this e'ample, the forces are
constant, "ut the lar(er the perpendicular distance, the faster the
rounda"out accelerates)The moment equationmoment (Nm)=force (N)
perpendicular distance from piot (m)!he si/e of a moment o"eys the
moment equation)perpendicular distanceforce9 of 32 Boardworks Ltd
2009!erpendicular distances10 of 32 Boardworks Ltd 2009When 1ark
and !im lift their feet off the (round, what will happen to the
see2saw+"loc#$ise and anticloc#$ise moments!he force of (ravity
creates a turnin( effect on each person 3n the left2hand side, it
creates an anticloc#$ise moment3n the ri(ht2hand side, it creates a
cloc#$ise moment!he clockwise moment is larger than the
anticlockwise moment, so the see2saw will turn in a cloc#$ise
directionTim %ar#11 of 32 Boardworks Ltd 2009The principle of
moments$f an o"#ect is "alanced a"out a pivot, then we can say
that)sum of anticloc#$ise moments!his is the principle of
moments4o, the forces on an o"#ect must o"ey two conditions to keep
the o"#ect completely stationary)sum of cloc#$ise moments=1 !he
forces must "e "alanced2 !he moments must "e "alanced12 of 32
Boardworks Ltd 2009&sing the principle of moments Boardworks
Ltd 2009 13 of 321% of 32 Boardworks Ltd 2009What is a centre of
mass?*an you "alance a ruler on the flat end of a pencil+ $f so,
where do you put the pencil to make it "alance+!he point at which
all the mass of an o"#ect seems to "e concentrated is called its
centre of mass!he ruler is stationary when the moments from the
wei(hts of its entire len(th "alance out!his is e5uivalent to one
wei(ht force actin( throu(h the centre of the ruler1& of 32
Boardworks Ltd 2009'orces and centres of massWhen forces on an
o"#ect are "alanced, its centre of mass is stationary, or moves at
a constant speed(When the forces on an ob)ect are balanced* it is
either stationar+ or moes at a constant speed,-6ow can the idea of
centre of mass help us to understand this statement+ What does
7stationary8 mean if an o"#ect is turnin(+1, of 32 Boardworks Ltd
2009"entre of mass or centre of grait+?$ma(ine that the
(ravitational field on the left of this picture suddenly "ecomes
much stron(er than the (ravitational field on the ri(ht Would the
see2saw still "alance+!he see2saw8s centre of mass is still in the
centre of the picture But its centre of grait+ 9 the point where
all its $eight appears to act 9 has moved to the left$n a uniform
(ravitational field, such as on the :arth8s surface, centre of
(ravity is e5uivalent to centre of mass1- of 32 Boardworks Ltd
2009'inding a centre of mass10 of 32 Boardworks Ltd 2009'inding a
centre of mass19 of 32 Boardworks Ltd 2009&nderstanding centres
of mass Boardworks Ltd 2009 20 of 3221 of 32 Boardworks Ltd
2009When is an ob)ect stable?.n o"#ect is said to "e in stable
equilibrium if it returns to its ori(inal position when it is
distur"ed!his happens "ecause its centre of mass moves up$ards when
pushed, so (ravity tends to pull the o"#ect "ack down towards its
e5uili"rium position.n o"#ect is in sta"le e5uili"rium if its
centre of mass is suspended "elow a pivot, or "alanced a"ove a wide
"ase",.,%,22 of 32 Boardworks Ltd 2009When is an ob)ect unstable?.n
o"#ect is said to "e in unstable equilibrium if it falls or topples
when it is distur"ed!his happens "ecause its centre of mass moves
do$n$ards when pushed, so (ravity tends to pull the o"#ect further
away from its e5uili"rium position.n o"#ect is in unsta"le
e5uili"rium if its centre of mass is "alanced a"ove a pivot point
or a narrow "ase",.,%,23 of 32 Boardworks Ltd 2009/table or
unstable?2% of 32 Boardworks Ltd 2009Neutral equilibrium.n o"#ect
is said to "e in neutral equilibrium if it moves sideways when it
is distur"ed!his happens "ecause its centre of mass sta+s at the
same height when pushed, so (ravity does not tend to pull the
o"#ect towards or away from its ori(inal position4pheres and
cylinders are e'amples of o"#ects that can "e in neutral
e5uili"rium when restin( on flat (round",.,%,2& of 32
Boardworks Ltd 20090ecogni1ing states of equilibrium2, of 32
Boardworks Ltd 20092osing equilibrium2- of 32 Boardworks Ltd
2009%a#ing things more stableLook at the picture to the ri(ht *an
you e'plain why this situation is dan(erous, usin( ideas a"out
centre of mass and sta"le and unsta"le e5uili"rium+ 6ow could you
make the "us more sta"le+ !hink of t$o solutions20 of 32 Boardworks
Ltd 2009&nderstanding stabilit+ Boardworks Ltd 2009 29 of 3230
of 32 Boardworks Ltd 20093lossar+31 of 32 Boardworks Ltd
20094nagrams slide32 of 32 Boardworks Ltd 2009%ultiple5choice
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