Class 9 Chapter 9

Force and Laws of Motion

Force is a fundamental concept in physics that describes the influence that can cause an object to undergo a change in its state of motion.

Force is defined as any action or influence that can cause an object to accelerate, decelerate, change direction.

The most common unit of force in the International System of Units (SI) is the Newton (N). One Newton is defined as the force required to accelerate a one-kilogram mass at a rate of one meter per second squared (1 N = 1 kg⋅m/s²).

Forces are vector quantities, which means they have both magnitude and direction.

Forces can be classified into two main categories:

1. Balanced forces

2. Unbalanced forces

1. Balanced Forces: Balanced forces occur when the forces acting on an object are equal in magnitude and opposite in direction. When balanced forces act on an object, the net force is zero. As a result, the object remains at rest if it was initially at rest, or it continues to move at a constant velocity if it was already in motion. In other words, there is no change in the object’s state of motion due to the balance between the forces.

For example, imagine a person pushing a stationary car with a force of 1000 Newtons towards the east, while another person pushes it with an equal force of 1000 Newtons towards the west. In this case, the forces are balanced, and the car will not move because the net force is zero.

1. Unbalanced Forces: Unbalanced forces occur when the forces acting on an object are not equal in magnitude and/or not opposite in direction. When unbalanced forces act on an object, there is a net force that causes the object to accelerate in the direction of the stronger force. This acceleration can be a change in speed or a change in direction.

For example, if you push an object with a force of 20 Newtons towards the east, and another person pushes it with a force of 10 Newtons towards the east as well, the net force on the object is 10 Newtons towards the east. As a result, the object will accelerate in the eastward direction.

In summary, balanced forces result in no change in an object’s motion, while unbalanced forces cause an object to accelerate or decelerate in a particular direction.

Newton’s Laws of Motion

1. First Law of Motion –

Newton’s first law of motion, also known as the law of inertia, states that:

“An object at rest will remain at rest, and an object in motion will continue in its motion with a constant velocity, unless acted upon by an external force.”

In simpler terms, this law suggests that objects have a natural tendency to maintain their state of motion (either at rest or moving in a straight line at a constant speed) unless something else interferes and causes a change in their motion. For example –

1. If a book is sitting on a table and no one touches it, it will remain at rest.
2. If a ball is rolling on a frictionless surface with no forces acting upon it, it will keep moving in a straight line at a constant speed.
3. A coin tossed on a table: When you toss a coin onto a flat surface, it eventually comes to rest due to friction and air resistance. In the absence of these forces, it would continue to move indefinitely with the same speed and direction.

Inertia –

Objects have a natural tendency to maintain their current state of motion or rest unless any external force is not applied.

The amount of inertia an object possesses depends on its mass. Objects with more mass have greater inertia, and it requires more force to accelerate or decelerate them compared to objects with less mass.

More mass more inertia, Less mass less inertia.

Example – Seatbelts in cars: Seatbelts are essential for safety because they help prevent passengers from continuing to move forward at their previous speed in the event of a sudden stop or collision, as per their inertia.

Second law of Motion –

Momentum – momentum is a fundamental concept that describes the quantity of motion an object possesses. It is a vector quantity, meaning it has both magnitude and direction. The momentum of an object is determined by its mass and velocity.

Momentum(p) = mass (m) x velocity (v)

p = m x v

For example, when a heavy truck and a small car are moving at the same speed, the truck will have more momentum due to its greater mass.

Second law of motion states that the rate of change of momentum of a body is directly proportional to the force applied on it.

Change in momentum = p2 – p1

Rate of change in momentum = (p2 – p1)/t

F  ∝   (p2 – p1)/t          here p2 = mv    p1 = mu

F  ∝   (mv – mu)/t

∝   m(v – u)/t

F ∝    ma         [ a = (v-u)/t ]

F = kma      here k = constant

If k = 1 then

F = ma

SI unit of Force is Newton (N). and Formula unit is kgm/s².

the second law states that the force acting on an object is directly proportional to the mass of the object and its acceleration.

Third Law of Motion –

The third law of motion states that –

For every action, there is an equal and opposite reaction.

In simpler terms, this law explains that whenever one object exerts a force on another object, the second object exerts an equal force in the opposite direction on the first object.

Mathematically, if we have two objects labeled as Object A and Object B, and Object A exerts a force on Object B (F_AB), then Object B exerts an equal force back on Object A (F_BA). The magnitudes of the forces are the same, but they act in opposite directions.

For example, when you push a book across a table, the book exerts an equal and opposite force back on your hand.

1. Recoil of a Gun: When a gun is fired, the bullet is propelled forward with an explosive force (action). Simultaneously, the gun experiences a recoil in the opposite direction (reaction) due to the equal force pushing backward.
2. Walking: When you walk, your foot exerts a backward force on the ground (action). The ground then exerts an equal and opposite force on your foot forward, propelling you forward (reaction).
3. Jumping off a Diving Board: When you jump off a diving board, you apply a downward force on the board (action). As a result, the board exerts an equal and opposite force upward, propelling you into the air (reaction).