A system
of parallel forces can be so arranged that they can only succeed in causing a
body to rotate and not move linearly e.g. when you turn on a tap you are
applying parallel but oppositely directed forces on the top with your
fingers. Two such parallel forces which
are not in a straight line can constitute a couple if the
of parallel forces can be so arranged that they can only succeed in causing a
body to rotate and not move linearly e.g. when you turn on a tap you are
applying parallel but oppositely directed forces on the top with your
fingers. Two such parallel forces which
are not in a straight line can constitute a couple if the
forces are equal in
magnitude but oppositely directed.
Note: Two equal but oppositely directed parallel
forces, not acting in a straight line constitute a couple.
forces, not acting in a straight line constitute a couple.
The two
equal but opposite parallel forces have a zero resultant force but they have a
turning effect or moment on the body on which they act. They can produce only a rotation on the body.
equal but opposite parallel forces have a zero resultant force but they have a
turning effect or moment on the body on which they act. They can produce only a rotation on the body.
Also note
that the moment of a couple is the product of one of the forces and the
perpendicular distance between the lines of action of the two forces. Moment of the couple in the diagram above
(fig a) is given by moment > p x d = p x 2r where r is the radius of the
wheel and d is it’s diameter.
that the moment of a couple is the product of one of the forces and the
perpendicular distance between the lines of action of the two forces. Moment of the couple in the diagram above
(fig a) is given by moment > p x d = p x 2r where r is the radius of the
wheel and d is it’s diameter.
In the
diagram (fig b), moment = f x a
diagram (fig b), moment = f x a
The
distance between the two equal forces is called the arm of the couple. The moment of a couple is also called a
torque.
distance between the two equal forces is called the arm of the couple. The moment of a couple is also called a
torque.
Application of the effect of couples in everyday
life
life
1.
It is
easier to turn a tap on or off by applying two equal and parallel forces in
opposite directions.
It is
easier to turn a tap on or off by applying two equal and parallel forces in
opposite directions.
2.
It is
also easier to turn a steering wheel of a vehicle by applying a couple with our
two hands instead of a single force with one arm.
It is
also easier to turn a steering wheel of a vehicle by applying a couple with our
two hands instead of a single force with one arm.
Note: that the effect of a couple can be cancelled
by applying an equally similar couple of opposite moment to the body experiencing
the couple. No single force however
large its magnitude can cancel the effect of a couple.
by applying an equally similar couple of opposite moment to the body experiencing
the couple. No single force however
large its magnitude can cancel the effect of a couple.