Motion: Balanced and Unbalanced Forces

Key Concepts

• Balanced forces and their effects
• Unbalanced forces and their effects
• Unbalanced net force and acceleration
• Difference between balanced and unbalanced forces

Introduction: 

We have already learned about the force and its effects and combining different forces acting on a body to find the net force and its direction. We have also concluded that forces cause motion. However, we are yet to find out the answer to the question, ‘what sort of forces cause each type of motion that we see around?’ There are different states in which a body can exist, such as rest, uniform motion, accelerated motion, decelerated motion, etc. We shall find out the forces that cause each of these types of motion in this section. 

Explanation: 

Let us consider two cases as follows: 

Let us consider two cases as follows: 

Case 1	Case 2
When a book is lying on a table, it has two forces acting upon it, i.e., upward normal force exerted by the table on the book and downward force due to its weight on the table, as shown in the figure below.	Suppose two teams are playing a game of tug of war with each team exerting forces on the rope, as shown in the figure below.

Here, two forces are equal in size, i.e., 16 N, and in opposite directions. The net force here is given by:  Net force = 16 + (- 16) = 0  [The reference direction is towards the right]  This means that the two forces balance out each other. Such forces which balance out each other to give a zero net force are called balanced forces.

Here, the forces exerted by the teams on the rope are 121 N towards the left and 110 N towards the right. So, the net force on the rope is given by:  Net force = 110 + (-121) = -11 N  [The reference direction is towards right]  The net force is 11 N, towards the left. This means that the two forces do not balance out each other.  Such forces which do not balance out each other result in a non-zero net force and are called unbalanced forces.

Balanced Forces 

‘When the net force on an object is zero, the forces acting on the object are called balanced forces.’ 

When the forces acting on an object are balanced, the state of rest/uniform motion of the object does not change. In other words, the velocity of the object does not change when it is under the action of balanced forces. 

Balanced forces are responsible for two states in which an object can exist, i.e., rest and uniform motion. 

Object at rest: 

When balanced forces act on an object at rest, it stays at rest. This is because there is no net force for the body to move. 

Examples: 

1. The figure below shows two persons pushing a heavy wooden dresser from opposite sides with 20 N force each. It means that they are applying the same sized forces in opposite directions on the dresser, hence, balancing out each other, producing 0 net force. Thus, the dresser does not move. 

2. The figure above shows two forces of pull (35 N each) exerted by two persons on the middle person are equal and opposite in direction, giving rise to a zero net force. As a result, the person in the middle does not move. 

Object in uniform motion: 

When the forces acting on a moving body balance out, it starts moving with a uniform velocity. In other words, it exhibits uniform motion in the same direction. 

Example: 

When a skydiver opens his parachute, the forces acting on him, i.e., the gravitational force and the air resistance, balance out, making the net force 0. Now, the skydiver falls with a uniform velocity. 

Unbalanced Forces 

The forces acting on an object are said to be unbalanced when the net force acting on it is not zero. An object which is subjected to unbalanced forces changes its velocity (speed and/or direction). In other words, it either accelerates or decelerates while moving in the same direction or changing it. The object moves in the direction of the net force.  

For example, a heavy wooden dresser pushed from opposite sides with differently sized forces (30 N and 65 N), as shown in figure 6.5 below, is under the action of unbalanced forces. The net force is not zero, but – 35 N. 

Unbalanced forces lead to a change in the velocity of the object. This change in velocity can take place in the following ways: 

Accelerated motion: 

Unbalanced forces can make a moving object speed up or an object at rest to start moving, i.e., increase the speed of an object. For example, a skydiver in the air is subjected to mainly two forces, i.e., a downward gravitational force and an upward air resistance. When she dives into the air, initially, her parachute is closed. As long as her parachute is closed, the air resistance is lesser than that of the downward gravitational force. That creates an unbalance in the forces, producing a non-zero net force in the downward direction. She accelerates with an increasing velocity in the downward direction as a result. 

Here, her net force = gravitational force – air resistance 

Thus, an unbalanced force speeds up (accelerates) the motion of a moving body. 

Decelerated motion: 

Unbalanced forces can make a moving object slow down or bring a moving object to rest, i.e., decrease the speed of an object. For example, a ball thrown upwards is subjected to two forces, namely, the downward gravitational force and the upward applied force. As long as the ball moves upwards, the applied force is greater than that of the gravitational force the ball experiences. This leads to an upward non-zero unbalanced force and makes to ball move upwards while gradually decreasing its velocity to zero when it reaches a certain height. Thus, a ball thrown in the air experiences an upward unbalanced force which makes it decelerate until it reaches the maximum height. 

Here, the net force = applied force – gravitational force 

Thus, an unbalanced force slows down (decelerates) the motion of a moving body. 

Change in the direction of motion: 

Unbalanced forces can make a moving object change its direction of motion, causing a change in the velocity of the object. For example, when a billiard ball hits a rail, it changes its direction of motion because of an unbalanced force applied by the rail. Here, the net force is along the new direction of motion of the ball, and the velocity (speed + direction) of the ball changes. Thus, an unbalanced force can cause a change in the direction of a moving body. 

[Fig 6.7: (a) Change in direction and (b) Circular motion due to unbalanced forces

Figure 6.7 (b) shows a boy is swirling a ball tied to a string around himself, causing the ball to move along a horizontal circle. In a circular motion, the ball’s velocity keeps on changing at every point of the trajectory due to the change in the ball’s direction of motion. Here an unbalanced force is caused due to the tension force exerted by the string, called centripetal force. The net force is directed along the direction of motion of the ball, which also keeps on changing at every point. 

Thus, in a circular motion, an unbalanced force makes the object keep changing its direction of motion. 

Unbuutalanced Net Force And Acceleration 

It can be concluded now that the unbalanced forces cause acceleration (positive/negative). In fact, the greater the net force on an object, the larger its acceleration will be in the direction of the net force. 

For example, a train with one engine experiences less net unbalanced force and thereby less acceleration. The other with two engines experiences a greater net unbalanced force and greater acceleration. 

Differences Between Balanced And Unbalanced Forces 

The points of differentiation between balanced and unbalanced forces are as follows: 

Parameters	Balanced Forces	Unbalanced Forces
Stationary Body	They do not change the state of a stationary body.	They make a stationary body move.
Uniformly Moving Body	They do not change the state of uniform motion of a body.	They make a uniformly moving body move non-uniformly.
Acceleration	They do not produce any acceleration.	They produce both positive and negative acceleration.
Direction	They cannot change the direction of a moving body.	They can change the direction of a moving body.
Net Force	The net force is always zero.	The net force is always non-zero.
Example	A book lying on a table is under the action of balanced forces.	When the book is pushed, there is an unbalance in forces experienced by it, and as a result, it moves.

Question 1:  

The figure below shows a ball held by a hand and then being released. It also shows arrows indicating the forces acting on the ball. 

1. Identify the forces shown by the arrows.  

2. What is the nature of forces (balanced/unbalanced) acting in each case? 

3. Give reasons for your answer. 

4. In the first picture, if the downward force is 124 N, what is the value of the upward force? 

 

2. Balanced forces in the first case and unbalanced forces in the second. 

3. In the first case, the normal force and the gravitational force balance each other. Hence, the ball is at rest. In the second case, the normal force is taken away, and the body only experiences the force of gravity, which makes it accelerate downwards. Hence, it experiences unbalanced forces here. 

4. Let the upward normal force be x. 

                The net force = 0 (balanced forces) 

                Therefore, x + 124 = 0 

                              Or, x = -124 N 

                Thus, the upward normal force is – 124 N. 

Question 2: 

A car moving on a road is subjected to three forces, i.e., the forward force due to the engine (547 N), backward frictional force (248 N), and a small backward force (34 N) due to air resistance.  

1. Is the car under unbalanced forces?  

2. If yes, what is the net force on it, and in which direction? 

Answer: 

Let our reference positive direction be towards the right. 

Given that, 

Forward force due to the car’s engine = 547 N 

Backward frictional force = -248 N  

Backward force due to air resistance = -34 N 

Net force = 547 + (-248) + (-34) 

Net force = 265 N (towards right) 

1. Yes. 

2. Net force = 265 N towards right. 

Frequently asked questions 

1. Give examples for balanced and unbalanced forces? 

Ans: Balanced force examples: A car that continues movies at the same speed, an object floating on water, a person standing still. Unbalanced force examples: A vehicle accelerating, a fruit falling from a tree, a moving train slowing down before stopping. 

2. What is the difference between balanced and unbalanced forces? 

Ans: A balanced force doesn’t change the state of rest or motion of a body. But an unbalanced force affects the balance and creates a change in rest or motion. 

3. What is accelerated motion? 

Ans: Unbalanced forces can make a moving object speed up or an object at rest to start moving, i.e., increase the speed of an object. Eg: A moving car upon applying accelerator (unbalanced force), increases its speed. 

4. What is decelerated motion? 

Ans: Unbalanced forces can make a moving object slow down or bring a moving object to rest, i.e., decrease the speed of an object.