PHYSICS Turning Effects of Forces Mr R Gopie
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PHYSICS Turning Effects of Forces
Mr R Gopie
Mr R Gopie PHYSICS
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TURNING EFFECTS OF FORCES : MOMENTS
Moment of a force and principle of moments.
The moment of a force is the turning effect of that force about the same axis or
turning point (called a pivot or fulcrum-‐ such as a hinge) and its magnitude is given by
the product of the magnitude of the force and the perpendicular distance between the
line of action of the force and the axis or turning point.
The moment of a force will tend to turn the body on which it acts in either a
clockwise sense or an anti-‐clockwise sense.
If a body is acted upon by a system of forces and that body is in equilibrium
under the action of that system of forces then the principle of moment applies. The
principle of moments states that if a body is in equilibrium under the action of a system
of forces then the sum of the clockwise moments of the forces about any given point on
the body is equal to the sum of the anticlockwise moments of forces about the same
point.
EQUILIBRIUM;
In fact, if a body is in equilibrium under the action of a system of forces then the
following conditions apply:
1) the principle of moments applies
2) There is no resultant force, i.e. the resultant force is zero. So the sum of the
forces acting in any one direction is equal in magnitude to the sum of the forces
acting in the opposite direction. For instance, the sum of the upward forces is
equal to the sum of the downward forces.
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Diag 6
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COUPLE AND TORQUE:
A couple is a pair of forces which are equal in magnitude but opposite in
direction and whose lines of action are displaced from one another (i.e. they do not act
along the same line)
A couple sets up a moment called a torque which tends to rotate the body on
which it acts. The moment of a couple, or torque, is given by the product of any one of
the forces in the couple and the perpendicular distance between the lines of action of the
forces.
APPLICATIONS OF THE PRINCIPLE OF MOMENTS – LEVERS
Many machines employ the principle of moments in their operation. In fact, the
simplest type of machine, the lever, operates on this principle.
Any lever is associated with three (3) characteristic features;
1) a fulcrum (F)
2) an applied force called the effort (E)
3) a force to be overcome called the load (L)
Depending on the relative positions of these three features there are the three
(3) classes of levers:
i) First class levers – which have F located between E and L. Examples
include a pair of scissors, or pliers, a crowbar, and a claw hammer
(being used to extract a nail, for example)
ii) Second class levers-‐ which have L located between F and E. an
example is a wheel barrow.
iii) Third class levers-‐ which have E located between F and L. examples
include a pair of tweezers and a pair of ice tongs.
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TUTORIAL
June 1995 paper 3 #1
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June 1997 paper 3 #3
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June 1999 paper 2 #2
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June 2000 paper 3 #4
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June 2003 paper 2 # 3
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June 2005 paper 3 #4
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June 2006 paper 2 #4
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June 2008 paper 2 #4
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