Chapter-How Forces Affect Motion Science Exploration class 9 in english Medium CBSE Notes

Chapter 6. How Forces Affect Motion

In everyday life, more than one force often acts on an object at the same time. Whether the object moves or remains at rest depends on the combined effect of all the forces acting on it. This section explains balanced forces, unbalanced forces, net force and the addition of forces with classroom-oriented examples. 

11. Multiple Forces Acting on an Object

An object may experience two or more forces simultaneously. The final effect depends on the magnitude and direction of all the forces acting together. 

Key Points

• Single Force – Only one force acts on the object.
• Multiple Forces – Two or more forces act at the same time.
• Result Depends on Net Force – The object's motion depends on the combined effect of all forces.

12. Net Force (Resultant Force)

The single force that produces the same effect as all the forces acting together is called the net force or resultant force. 

Definition

Net force is the overall force acting on an object after combining all the individual forces.

Rules for Finding Net Force

• Same Direction – Add all the forces.
• Opposite Directions – Subtract the smaller force from the larger force.
• Direction – The net force acts in the direction of the larger force.

Example

• 10 N east + 5 N east = 15 N east.
• 20 N east and 8 N west = 12 N east.

13. Balanced Forces

When equal forces act on an object in opposite directions, they cancel each other. The net force becomes zero, so there is no change in the state of motion. 

Definition

Balanced forces are equal and opposite forces whose resultant is zero.

Characteristics

• Net Force is Zero
• No Change in Motion
• Object Remains at Rest or Continues Moving with Constant Velocity

Examples

• A book resting on a table.
• A tug of war where both teams pull with equal force.
• A person standing still on the ground.

14. Unbalanced Forces

When the forces acting on an object are unequal, the net force is not zero. The object accelerates in the direction of the net force. 

Definition

Unbalanced forces are unequal forces whose resultant is non-zero.

Characteristics

• Net Force is Non-zero
• Changes the State of Motion
• Produces Acceleration

Examples

• Kicking a stationary football.
• Pushing a shopping trolley.
• A car accelerating on a road.

15. Difference Between Balanced and Unbalanced Forces

Balanced Forces	Unbalanced Forces
Net force is zero.	Net force is not zero.
No change in the state of motion.	Changes the state of motion.
Do not produce acceleration.	Produce acceleration.
Equal and opposite forces.	Unequal forces.

16. Addition of Forces

The effect of multiple forces can be determined by adding or subtracting them according to their directions.

Cases

• Forces in the Same Direction – Add the magnitudes.
• Forces in Opposite Directions – Subtract the smaller force from the larger one.
• Resultant Direction – Always along the larger force.

17. Everyday Examples of Balanced and Unbalanced Forces

Situation	Type of Force
Book lying on a table	Balanced Forces
Person standing on the ground	Balanced Forces
Football kicked by a player	Unbalanced Forces
Car speeding up	Unbalanced Forces
Object slowing down due to a greater opposing force	Unbalanced Forces

18. ATP Education Concept Builder

• Balanced Forces → Net Force = 0.
• Unbalanced Forces → Net Force ≠ 0.
• Net Force → Combined effect of all forces.
• Same Direction → Add the forces.
• Opposite Direction → Subtract the forces.
• Motion Changes → Only when the net force is non-zero.

ATP Education Exam Booster

Remember the golden rule: Balanced forces do not change the state of motion because the net force is zero, whereas unbalanced forces always change the motion of an object because the net force is non-zero. Most CBSE competency-based questions ask students to identify whether the forces acting in a given situation are balanced or unbalanced and to calculate the net force.

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