CHAPTER 15- TURNING FORCES Name:
Class/section:
Pivot is the central point, pin, or shaft on which a mechanism rotates or oscillates.
The turning effect or moment of a Force or Torque
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Moments occur when forces cause objects to rotate about some pivot.
The size of the moment depends upon:
➢ The size of the force.
➢ The perpendicular distance from the force to the pivot.
❖ The turning effect (or moment) of a force is equal to the force multiplied by the
perpendicular distance from the force to the pivot.
Moment of a force
pivot.
= Force × perpendicular distance from the force to the
❖ Its SI unit is newtons metres.
❖ Principle of moments
In equilibrium,
Total anticlockwise moment = total clockwise moment
❖ The principle (or law) of moments states that in equilibrium in total anticlockwise
moments are equal to the clockwise moments.
❖ Centre of Mass
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The centre of mass of an object (sometimes called the centre of gravity) is the point
through which the whole mass (weight) of that object acts.
For a symmetrical object of uniform density (such as a symmetrical cardboard
shape) the centre of mass is located at the point of axis of symmetry.
❖ Stability
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An object is stable when its centre of mass lies above its base
The object on the right will topple, as its centre of mass is no longer over its base
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If the centre of mass does not lie above its base, then an object will topple over.
The most stable objects have a low centre of mass and a wide base.
The most stable objects have wide bases and low centres of mass
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• The second car has a wider wheelbase
and lower centre of gravity than the first
car.
• It is more stable. It falls back onto its
base and does not topple over.
• The wheel acts as the pivot for the car.
• The weight has a turning effect or
moment, which causes the car to topple
over or fall back.
A double decker bus is stable as it has a:
• low centre of gravity because of its low, heavy engine and heavy bottom deck;wide
wheelbase.
A traffic cone is stable as it has a:
• low centre of gravity G because of its heavy base.
• wide base.
❖ To find the centre of mass of an irregular shape:
• The irregular shape (a plane laminar) is suspended from a pivot and allowed to settle.
• A plumb line (lead weight) is then held next to the pivot and a pencil is used to draw a
vertical line from the pivot (the centre of mass must be somewhere on this line).
• The process is then repeated, suspending the shape from two different points.
• The centre of mass is located at the point where all three lines cross.
❖ Types of equilibrium
• Objects can have three types of equilibrium depending upon the position of centre of gravity
Stable equilibrium: A plumbline is said to have stable equilibrium because when we pull the
plumbline to one side, the centre of gravity is raised. On releasing it, gravity will try to bring it back
to the original position where the centre of gravity is at the lowest.
Unstable equilibrium: a ruler balanced vertically is said to be at unstable equilibrium because
when slightly tilted, the line through the centre of gravity passes outside the base and keeps on
falling down . So, the object will never regain its original position.
Neutral equilibrium : a ball on a perfectly leveled ground is saif to be at neutral equilibrium
because when the ball is moved, the line through the centre of gravity always pass through the base.
The centre of gravity neither rises nor falls.
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