BALANCED
AND UNBALANCED FORCES
1. The forces that we come across in day to day life can be
divided broadly into two types : (i) Balanced Forces, (ii) Unbalanced Forces.
1. Balanced Forces
If the resultant of all the forces acting on a body is zero,
the forces are called balanced forces.
The balanced forces cannot change the state of rest or state
of uniform motion or the direction of motion of the body. It means that a body
at rest will continue to be at rest under the action of balanced forces. Similarly,
a body moving uniformly along a straight line will continue to move along the
same straight line with the same speed even when balanced forces are applied on
the body. This is as if no force is acting on the body.
2. Unbalanced Forces
If the resultant of all the forces acting on a body is not
zero, the forces are called unbalanced forces.
The unbalanced forces can change the state of rest or the
state of uniform motion or the direction of motion of the body.
It should be clearly understood that when we talk of a force
acting on a body, we usually mean an unbalanced force capable of changing the
state of the body.
Further, note that balanced forces cannot change the state of
rest or state of uniform motion or the direction of motion of the body.
However, balanced forces can change the shape and size of the body. For
example, when we press a rubber ball between our two hands by
applying equal and opposite forces on the ball, the shape of the ball changes
from spherical to oblong.
Examples of balanced and unbalanced forces
1. Let us consider a wooden block placed on a
horizontal table. A string X is attached to the right face of the block as
shown in Fig. 2.3.
When we apply a small force P to the string X,
the block does not move. This is because two pairs of balanced forces are
acting on the block :
(i) weight mg of the block acting vertically
downwards on the table.
(ii) force of reaction R of the horizontal table
acting on the block vertically upwards.
As Ra=mg, therefore, this is one pair of
balanced forces, which fails to move the block in the vertical direction. The
other pair of forces is :
(i)
applied force P tending to move the block to the
right,
(ii)
force of
friction F between the rough table top and rough bottom surface of the block,
acting to the left and opposing the motion.
As F = P,
therefore, this is the second pair of balanced forces which fails to move the
block in the horizontal direction.
If we go on increasing the applied force to the
string X, a stage reaches when the block begins to move on the table. At this
stage, the applied force P becomes greater than the maximum value of opposing
force of friction F. Thus the second pair of forces becomes unbalanced. That is
why motion is produced in the block.
2. When we hold a suitcase steady at some
height, the suitcase is under the action of two forces :
(i) weight mg of the suitcase due to a gravitational
pull of Earth, acting vertically downwards,
(ii) force applied P by our hand in the
vertically upward direction.
As mg=P, therefore, the forces are balanced, and
the suitcase is steady. If we were to drop the suitcase, applied force P
becomes zero. The suitcase would move vertically downwards under the action of
unbalanced force mg.
3. In a tug of war, i.e., rope-pulling by two
teams, the rope does not move in any direction, because the forces applied by
the two teams are equal and opposite, i.e., the forces applied are balanced
forces. However, when one team releases the rope, an unbalanced force acts on
the other team due to which the other team falls backwards.
4. When we stop pedaling a bicycle we are
riding, the bicycle begins to slow down. This is because of unbalanced force of
friction between the tyres and the road, which opposes the motion. To keep the
bicycle moving, we have to start pedalling again. It appears as if an object
maintains its motion under the continuous application of an unbalanced force.
However, this conclusion is wrong. In fact, an unbalanced force is needed only
for starting motion in a body at rest or for changing the speed or for changing
the direction of motion. These changes in speed or direction will continue so
long as the unbalanced force is acting. The moment the unbalanced force is
removed, the object would continue to move with the velocity it has acquired
till then. There will be no further change in velocity in the absence of an
unbalanced force. It should be clearly understood that an unbalanced force is
needed to move a body from state of rest. But no such force is required to
maintain uniform motion of the body. An unbalanced force is also required for
changing speed of the body or for changing direction of motion of the body.
5. A ball set rolling on the ground stops after
sometime. This is due to unbalanced force of friction between the ball and the
ground.
Points which must be learn before go to exams
6. If the resultant of all the forces
acting on a body is not zero, the forces are called unbalanced forces.
The unbalanced forces can change the
state of rest or state of uniform motion or the direction of motion of the body
or size and shape of the body.
7. An unbalanced force is needed to
move a body from state of rest. But no such force is required to maintain
uniform motion of the body. An unbalanced force is also required for changing
speed of the body or for changing the direction of motion of the body.
Bye Bye
Thank You!!
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