CBSE Notes Class 9 Science Chapter 8 - Force and Laws of Motion

A force is a push or pull exerted on an object due to its interaction with another object. As a vector quantity, force has both magnitude and direction. Forces can cause objects to accelerate, decelerate, alter their direction, or change their shape. Sir Isaac Newton developed three fundamental laws of motion that define the relationship between a body, the forces acting on it, and its resulting motion.

1.0Force and Motion

• Force is an external influence that alters a body's state of rest or its uniform straight-line motion.
• Motion-Motion refers to the change in an object's position relative to a chosen reference point or frame of reference. It is a core concept that describes how objects move through space over time.

2.0Effects of Force

1. It can transform the shape and size of an object.
2. It can move a stationary object.
3. It can bring a moving object to a stop.
4. It can either speed up or slow down a moving object.
5. It can transform the direction of a moving object..
   

3.0Balanced and Unbalanced Forces

Resultant Force-The resultant force is the single force that, when applied to a body, produces the same effect as the combined effect of multiple forces acting on that body.

Balanced Forces- If the vector sum of all the forces acting on a body is zero.

• In such cases the acceleration of the object is zero,and its velocity remains constant.
• If the total  force acting on the object is zero,the object either remains at rest or continues to move with constant velocity.
• FNet=0

Unbalanced Forces-If the net force exerted on an object is not zero,then the force acting on it is said to be unbalanced.

• Acceleration of the object is not zero and its velocity changes.
• If the total  force acting on an object is not zero then such a force changes state of rest or the state of uniform motion.
• FNet≠0

4.0Inertia and Mass

Inertia-The property of matter that causes it to resist changes to its state of rest or uniform straight-line motion is known as inertia. Inertia can be categorized into three types:

• Inertia of Rest
• Inertia of Motion
• Inertia of Direction

Mass-Mass is a measure of the quantity of matter in an object or substance. It is a key property that indicates how much an object resists changes in its motion and how it interacts with gravitational forces. Mass is calculated in  grams (g) or kilograms (kg).

• Mass is a quantitative measure of Inertia ,more the mass ,more will be the inertia and vice-versa.

5.0Newton’s First Laws of Motion

• An object will remain at pause or continue to move in a straight line path at a constant speed unless external force act upon him. 
• Every object has a natural tendency to resist changes to its state of rest or motion, and this resistance is known as inertia.