When
two or more forces acting on an object produce zero resultant force, i.e. the
object
has
no acceleration; we say that the forces are balanced. The forces that produce
this
balanced
condition are called balanced forces.
It
is seen in figure 3.1 that an object is suspended
with
a thread. The force of attraction of earth on the
object
i.e. the weight of the object W
is acting
vertically
downward. The tension of the thread T
is
acting
vertically upward. Here the two forces are
equal
in magnitude but opposite in direction thus
canceling
each other’s action and producing a
balanced
condition.
Only
the attractive force of the earth i.e. the force of
gravity
will act on the object if the thread is cut.
Then
the object will fall down with acceleration due
to
gravity. Here, the force of gravity or weight of the
object
is the unbalanced force. If the body is
displaced
slightly along one side, the tension of the
thread
T and the weight W
will not be in a straight
line.
Then a resultant force will act on the body
without
creating balanced condition. Due to this, the
body
will oscillate. This is an example of unbalanced
force.
Another
example of balanced and unbalanced forces
can
be seen in the game of tug-of-war competition.
In
this game, a handkerchief is tied to the center of the rope. In this
competition equal
numbers
of competitors pull on the rope in two sides and try to move the handkerchief
to
their
side. If the handkerchief does not move then it is understood that either the
team are
applying
equal amount of force and the rope or the handkerchief is in balanced
condition.
Here, forces applied by the two teams are balanced forces.
If
one team applies more force than the other, then the resultant force will act
along them
thus
creating unbalanced force and the handkerchief will move to their side. Then
this
team
will be declared as the winning team in the competition.
Fig.
3.4: Tug-of-war (unbalanced force)
Fig.
3.1
Fig.
3.2
Fig.
3.3
54
Physics
3.4 Momentum
The
physical quantity which is produced by the combination of mass and velocity of
a
moving
body is the momentum. Momentum depends on the mass and velocity of the
body.
Think about a loaded truck and a private car. You have to stop both the cars
within
the
same distance. Which car needs hard brake to stop? The answer is truck. Though
the
truck
and the car are moving with the same speed, the truck possesses that physical
quantity
which is more is its momentum.
Momentum
is a measure of how difficult it is to stop something that is moving.
Momentum
is related to the force. This relation is obtained quantitatively in Newton’s
second
law of motion.
Momentum
is the product of the mass and velocity of a moving body.
Let,
the mass of a body = m
Velocity
= v
Momentum
is a vector quantity. Its direction is in the direction of velocity. It is
observed
from
equation (3.1) that the momentum of a body will be large if the body has a
large
mass
and moving faster.
Unit: the unit of momentum is, unit of mass × unit of velocity,
i.e. kg×ms-1
or kg
ms-1 .
If
a body of 1 kg moves with a velocity of 1ms-1, its momentum will be 1kg ms-1.
Dimension: The dimension of momentum: [p] = MLT-1.
3.5 Effect of force on motion
A force can cause a stationary
object to start moving.
What
happens when a stationary football is kicked by a player? It is seen that the
ball
moves
off in the direction it is kicked from its stationary state. In this case the
ball
accelerates
from rest. Here acceleration is positive, and in the same direction as the
force
exerted
by the kick.
A force can cause a moving object
to increase its velocity.
What
would be the velocity when a cricket ball is hit by the batsman in the same
direction
as the ball moves? It is observed that the ball moves faster after the hit. In
this
case
the acceleration is positive and its velocity increases. The velocity of a
moving
marble
increases if it is stroked in same direction as it moves.
A force can cause a moving object
to decrease speed.
Think
you are riding a bicycle in the village road. After a while you observed that
the
road
is slanting downwards. Now what will you do? Will you continue the pedaling as
Physics
55
before
or give a brake? You have to apply brake to cross this sloping road safely. Due
to
this,
the cycle decelerates.
A force can cause a moving object
to change its velocity or direction of motion.
A
batsman hits the cricket ball coming from opposite direction during a game of
cricket.
Due
to the hit, the magnitude and direction of the velocity of the ball change. The
ball
moves
in a different direction due to this hit by the bat. In this case too, the ball
has
acceleration.
