Short Notes – Chapter 8- Force and Law of Motoin-Ncert – Class 9 Science

Short Notes – Chapter 8- Force and Law of Motoin-Ncert – Class 9 Science

8.1 Balanced and Unbalanced Forces

Explanation of Balanced Forces:

◆When equal forces are applied in opposite directions (e.g., pulling on opposite sides of the block with equal forces), they cancel each other out.

◆This results in a balanced force situation where the forces do not cause the object to move. The block remains stationary because the forces are balanced.

Understanding Unbalanced Forces:

◆If unequal forces are applied (e.g., pulling on opposite sides with different magnitudes of force), the forces are unbalanced.

◆The block moves in the direction of the greater force. This indicates that an unbalanced force is necessary to cause motion.

Role of Friction:

◆When pushing a box with a small force, the box doesn’t move due to friction opposing the push. Friction arises between surfaces in contact, such as the bottom of the box and the rough surface of the floor.

◆Even with increased force (children pushing harder), if the force is still balanced by friction, the box won’t move.

◆However, if the pushing force exceeds the frictional force, an unbalanced force is present, causing the box to move.

Application to Riding a Bicycle:

◆When pedalling stops on a bicycle, it slows down due to friction forces opposing its motion.

◆To maintain motion, pedalling must continue to overcome the frictional forces.

◆This illustrates the necessity of an unbalanced force (pedalling force exceeding friction) to sustain motion.

Clarification on Object Motion:

◆An object maintains uniform motion when balanced forces (pushing force and frictional force) cancel each other out, resulting in no net external force.

◆To accelerate an object’s motion, an unbalanced force is required, causing either a change in speed or direction.

◆The object continues to change speed or direction as long as the unbalanced force acts on it.

◆If the unbalanced force is removed, the object maintains its velocity acquired until that point.

8.2 First Law of Motion

Galileo’s Observations on Inclined Planes:

◆Galileo observed that objects move with constant speed when no force acts on them.

◆When a marble rolls down an inclined plane, its velocity increases due to the unbalanced force of gravity. Conversely, its velocity decreases when it rolls up the slope.

◆Galileo proposed an experiment with an ideal frictionless double inclined plane, suggesting that a marble released on one side would roll down, climb up the opposite side, and return to its initial height if the inclinations were equal.

◆If one side’s inclination decreases gradually, the marble travels further distances until reaching the original height.

◆When the right-side plane becomes horizontal (slope reduced to zero), the marble continues to roll indefinitely, as there is no net external force acting on it.

Newton’s Laws of Motion:

◆Newton’s first law states that an object remains at rest or in uniform motion in a straight line unless acted upon by an external force.

◆This law is also known as the law of inertia, describing the tendency of objects to resist changes in their state of motion.

◆Inertia explains various experiences, such as remaining at rest until acted upon by an external force (e.g., braking in a car), or continuing in motion despite changes in the vehicle’s motion (e.g., sudden starts or sharp turns in a car or bus).

Applications of the Law of Inertia:

◆Examples include the use of safety belts in cars to prevent injuries due to sudden stops (slowing down the body’s forward motion), and the tendency to fall backward when a bus accelerates suddenly (opposing the sudden motion of the body due to inertia).

8.3 Inertia and Mass

Definition of Inertia:

◆Inertia refers to the resistance offered by an object to change its state of motion. If an object is at rest, it tends to remain at rest; if it is moving, it tends to keep moving.

Variation in Inertia:

◆Not all bodies have the same inertia. For example, it is easier to push an empty box than a box filled with books. Similarly, kicking a football results in it flying away, but kicking a stone of the same size with equal force may hardly move the stone and may even cause injury.

◆The amount of force needed to change the motion of an object varies depending on its mass and its tendency to resist changes in motion.

Relation between Inertia and Mass:

◆Heavier or more massive objects offer larger inertia. The inertia of an object is measured by its mass.

◆Quantitatively, the relationship between inertia and mass is that the mass of an object is a measure of its inertia. In other words, inertia is the natural tendency of an object to resist a change in its state of motion or rest, and this resistance is quantified by the object’s mass.

8.4 Second Law of Motion

Introduction to Newton’s Second Law:

◆The first law of motion indicates that when an unbalanced external force acts on an object, its velocity changes, resulting in acceleration.

◆Newton’s second law of motion extends this concept by quantifying how the acceleration of an object depends on the force applied to it.

Observations from Everyday Life:

◆Everyday experiences, such as the impact of a table tennis ball versus a cricket ball, or the difference in consequences between a stationary truck and a moving truck hitting a person, suggest that the impact produced by objects depends on their mass and velocity.

◆Similarly, to accelerate an object, a greater force is required to achieve a greater velocity.

Introduction of Momentum:

◆Newton introduced the concept of momentum (p) as the product of an object’s mass (m) and velocity (v). Mathematically, momentum is defined as:

p=mv

◆Momentum has both magnitude and direction, with its direction aligned with the velocity.

◆The SI unit of momentum is kilogram-meter per second (kg·m/s).

Relation between Force and Momentum:

◆Applying an unbalanced force results in a change in the velocity (and thus momentum) of the object.

◆The force necessary to change the momentum of an object depends not only on the magnitude of the force but also on the time over which the force is exerted.

◆The rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of the force.

8.5 Third Law of Motion

Explanation of Newton’s Third Law:

◆Newton’s third law states that for every action, there is an equal and opposite reaction.

◆When one object applies a force on another object, the second object responds by exerting a force back on the first object. These action and reaction forces are always equal in magnitude but opposite in direction.

◆The forces act on different objects and never on the same object.

Examples of Action and Reaction Forces:

◆In sports like football, when players collide while trying to kick the ball with greater force, both players feel hurt because each applies a force to the other. This illustrates the presence of a pair of opposing action and reaction forces.

◆An experiment with two spring balances connected together demonstrates the equal and opposite nature of action and reaction forces.

Alternative Statement of the Third Law:

◆An alternative statement of the third law is “to every action, there is an equal and opposite reaction.”

◆Action and reaction forces always act on two different objects simultaneously.

Illustrative Examples:

◆When a person intends to start walking, they exert a force on the ground backward, and in response, the ground exerts an equal and opposite force on their feet forward, propelling them forward.

◆Although action and reaction forces are always equal in magnitude, they may not produce accelerations of equal magnitudes due to differences in the masses of the objects involved.

◆Examples such as firing a gun (resulting in recoil) and a sailor jumping out of a rowing boat (causing the boat to move backward) demonstrate the application of Newton’s third law in real-life scenarios.