Fixing Exam problems (2 of n)

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Fixing Exam problems (2 of n)
Forces in Equilibrium (Tension) (Coefficient of
Cock-up 86 90)
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The Ball is in Equilibrium.... What do we always
know about forces in Equilibrium?
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0.51N _at_ 100mm mark, 0.69N _at_ 800mm mark122N _at_
1.0 end and 108N at the other (both vertically
upwards)620kN, 640kN
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Learning Objectives

1. To know the names of some special case
   moments.... (Couple/Torque)
2. To understand stable/unstable equilibrium,
   tilting, topling

Book Reference Pages 100 101-103
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Definitions Couples Torques
A couple, (sometimes called a torque) is a pair
of equal and opposite forces acting on a body but
not on the same line
Force F
x
d - x
p
Force F
d
Taking moments about a random point p.....
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Force F
x
d - x
p
Force F
d
The moment at the end nearest p Fx
(anticlockwise) The moment at the other end
F(d-x) (anticlockwise) Total moment is the sum
Fx f(d-x) Multiplying out.... Fx Fd -Fx
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So regardless of where you take the moments the
total moment is always Fd Couple Fd (or
torque)...... In squiggly Greek letters t Fd
(The units will be NM) (t is pronounced tau)
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Stable Unstable Equilibrium An object in
Stable Equilibrium will return to the equilibrium
position once released
Point of Support
Centre of Mass
Weight
Stable because Centre of Mass is directly below
the point of support
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Stable Unstable Equilibrium An object in
Unstable Equilibrium will not return to the
equilibrium position once released
Centre of Mass
Point of Support
Unstable because Centre of Mass is directly above
the point of support
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Tilting
p
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Tilting
b
F
d
p
W
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Taking moments about p... Clockwise
Fd Anticlockwise Wb/2 At the point of tilting
Fd Wb/2 What force is required to start to
tilt a bookcase weighing 800N with a base of 80cm
x 80cm if the force is applied at the top of the
1.8M bookcase?