Force and Laws of Motion – Balanced and Unbalanced Force, Inertia, Types of Inertia

When the sum of forces becomes zero, the forces balance each-other and the forces are called balanced forces.

Activity

We take a box and tie two strings to it in opposite direction. One string is at end A and other string is at end B. When we pull string at A, the box moves towards that direction. When we pull string at B, the box moves towards that direction. But when both the strings are pulled and the forces acting on two strings balance each-other, the box does not move. This is the situation of balanced forces where the two forces have balanced each other.
But when the two forces do not balance each other, the box will move in the direction of greater force.
When two forces acting on a body are equal and opposite in direction, the forces are balanced and the resultant force is zero being vector sum of the two forces. The body remains at rest.
When one of the force becomes more, the resultant force becomes more than zero in the direction of the greater force. The body starts moving in the direction of the greater force. The force is called unbalanced force.

The balanced forces do not change the state of rest or state of motion of a body. The unbalanced forces change the state of rest or state of motion of the body. The body in rest starts moving. The moving body accelerates or the direction of motion changes.
In the game of tug of war, when two teams are applying same force, the rope does not move. The reason is balanced forces. When one team applies more force than the other, the forces become unbalanced and the rope gets pulled by the stronger team.

#Example – In this example, try to identify the number of times the velocity of the ball changes –
“A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team.”
Also identify the agent supplying the force in each case.
Solution
(i) When the football player kicks the football to another player, the velocity is changed. The football player applies the force to change the velocity.
(ii) When second player kicks the football towards the goal, the velocity is changed second time. The second player is the agent applying the force.
(iii) When the goalkeeper of the opposite team stops the football, the velocity is changed third time.  The goalkeeper applies the force to change the velocity.
(iv) The velocity changes again when the goalkeeper kicks the football towards a player of his own team. Here the goalkeeper applies the force on the football in the direction of the player of his own team.
Thus, the velocity of the football changes four times.

Force and Laws of Motion

INERTIA

The tendency of a body to remain at rest or if moving to continue moving in a straight line is called inertia.
If there is no external force acting on it, a body will continue to remain at rest or that which is moving will continue to move with the same velocity. Inertia can be understood by the following examples –

(a) Suppose we put a pile of books on a paperboard and then we pull out the
paperboard. We see that the pile of books comes down instead of moving with the paperboard.
(b) In fig.2, we take a glass tumbler and place a thick square card on its mouth. A coin is placed on the card.When we flick the card, the card moves away but the coin drops into the glass tumbler.

(c) When the bus starts suddenly, a person sitting inside it falls backward.

From the examples, it is clear that a body resists a change in its state of rest or of uniform motion due to the property of inertia. Greater is the inertia of a body, greater is the force required to change its state of rest or state of motion.

In the above fig., we look at a experiment in which a marble is set rolling on an inclined plane and another glass plane is kept near. We see that the marble reaches to the height from where it starts rolling. When the angle of second plane is changed, the marble reaches to the same height. If the second plane is made horizontal, the marble continues to roll.
In practice, the marble stops after travelling some distance due to friction. if there is no friction, the marble will continue rolling.
This is similar to a bicycle which continues to roll when the
bicyclist stops pedaling the cycle. The inertia of the rolling stone and the bicycle make those keep rolling. However, the unbalanced force of friction stops the rolling stone and the bicycle from keeping these rolling forever.

In fact, the more mass is, the higher is the inertia and therefore, more force is required to change its state of rest or state of motion. For example it is easier to stop a table-tennis ball than a cricket ball travelling with same speed. The property of body to resist change in state is described by Newton’s first law. Therefore, the law is called the law of inertia. There are three types of inertia.

(i) Inertia of rest

The inherent property of a body by virtue of that, it cannot change its position of rest by itself, is called inertia of rest. The body resists the change in its state of rest.
There are following examples-

(a) When a car starts suddenly, a passenger sitting inside it falls backward because the lower part of the passenger starts moving with the car but the upper part tries to be at rest due to inertia of rest.

(b) If the branches of a mango tree are shaken with force, the branches move but the mangoes try to be at rest due to inertia of rest. Therefore, mangoes fall from the trees.

(c) If a heap of carom coins put one on top of each other is hit with a striker, only the lower most coin moves, because others remain at rest due to inertia of rest.

(d) In the fig., the coin falls into the glass tumbler due to inertia of rest of the coin.

(e) When we dust a carpet by shaking it, the carpet is set into motion but dust remains at rest due to inertia of rest, as a result the dust falls off the carpet.

(ii) Inertia of motion

A body in the state of motion tries to be in that state and opposes the forces acting on it for the change in the state of motion. There are following examples –

(a) A person sitting in a moving bus falls forward when the bus is suddenly stopped. This happens because the lower part of the person comes at rest but the upper part of the person continues to be in motion due to inertia of motion.

(b) In the fig., the rolling marble continues to roll on the horizontal plane due to the inertia of motion.

(c) The ceiling fan continues to rotate even after switching it off due to its inertia of motion.

(d) A passenger who jumps from a moving train will fall since the feet touch the ground and comes to rest but upper portion of the body continues to be in state of motion. Therefore, the person falls in the direction of motion of the train. This is due to inertia of motion.

(iii) Inertia of direction

A body opposes the forces that tries to change its direction of motion. There are following
examples of the inertia of direction –

(a)When a car takes sudden turn, the passengers sitting in it are thrown outwards. This is due to inertia of direction.

(b)When a vehicle moves along a wet road, the water particles sticking to the wheel fly off tangentially due to inertia of direction.
(c)When we sharpen a knife on a grinding wheel, the sparks fly tangentially. This is due to inertia of direction.

Next –

Newton’s First Law of Motion