learning objectives : 1.to know the names of some special case moments.... (couple/torque) 2.to...
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Learning Objectives :Learning Objectives :1.1. To know the names of some special case To know the names of some special case
moments.... (Couple/Torque)moments.... (Couple/Torque)
2.2. To understand stable/unstable To understand stable/unstable equilibrium, tilting, & topplingequilibrium, tilting, & toppling
Book Reference : Pages 100 & 101-103Book Reference : Pages 100 & 101-103
Definitions : Couples & TorquesDefinitions : Couples & Torques““A couple, (sometimes called a torque) is a A couple, (sometimes called a torque) is a pair of equal and opposite forces acting on pair of equal and opposite forces acting on a body but not on the same line”a body but not on the same line”
Force F
Taking moments about a random point p.....
Force F
d - xx
d
p
Force F
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
Force F
d - xx
d
p
So regardless of where you take the moments the total moment is always Fd
Couple = Fd
(or torque)...... In squiggly Greek letters
τ = Fd (The units will be NM)
(τ is pronounced tau)
Stable & Unstable Equilibrium:Stable & Unstable Equilibrium:
An object in An object in Stable Equilibrium Stable Equilibrium will will return to the equilibrium position once return to the equilibrium position once releasedreleased
Centre of Mass
Weight
Point of Support
Stable because Centre of Mass is directly below the point of support and raises as it is tilted
Stable & Unstable Equilibrium:Stable & Unstable Equilibrium:
An object in An object in Unstable Equilibrium Unstable Equilibrium will will notnot return to the equilibrium position return to the equilibrium position once releasedonce released Centre of Mass
Point of Support
Unstable because Centre of Mass is directly above the point of support and lowers as the object is tilted
Neutral Equilibrium:Neutral Equilibrium:
An object in An object in Neutral Equilibrium Neutral Equilibrium will will rollroll
Object will roll, centre of mass neither raised or lowered.
Centre of Mass
Force
Which is which?Which is which?
Which is which?Which is which?
Unstable
stable
Neutral
Tilting :Tilting :
p
Tilting :Tilting : b
d
F
W p
Taking moments about p...Taking moments about p...
Clockwise : FdClockwise : Fd
Anticlockwise : Wb/2Anticlockwise : Wb/2
At the point of tilting Fd = Wb/2At the point of tilting Fd = Wb/2
What force is required to start to tilt a What force is required to start to tilt a bookcase weighing 800N with a base of bookcase weighing 800N with a base of 80cm x 80cm if the force is applied at 80cm x 80cm if the force is applied at the top of the 1.8M bookcase?the top of the 1.8M bookcase?