CBSE Class 9 Science Revision Notes Chapter 9: Force And Laws Of Motion

Key Concepts and Formulas to Remember

• Force is an external agent that can change or tend to change the state of rest or uniform motion of an object.
• Newton's First Law (Law of Inertia): An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
• Inertia is the natural tendency of an object to resist changes in its state of motion or rest. Mass is a measure of inertia.
• Newton's Second Law: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force. Mathematically, F = ma (where F is force, m is mass, a is acceleration).
• Momentum (p) is the product of an object's mass and its velocity. p = mv. Its SI unit is kg m/s.
• Newton's Third Law: To every action, there is always an equal and opposite reaction. Action and reaction forces always act on different bodies.
• Law of Conservation of Momentum: In an isolated system (no external force), the total momentum before and after an interaction (like a collision) remains constant. m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂.

Key Terms and Definitions

Force

A push or pull that can change the state of motion or rest of an object.

Inertia

The inherent property of a body by virtue of which it resists any change in its state of rest or uniform motion.

Momentum

A measure of the quantity of motion contained in a body, defined as the product of its mass and velocity (p=mv).

Balanced Forces

When two or more forces acting on an object cancel each other out, resulting in zero net force and no change in the object's state of motion.

Unbalanced Forces

When the net force acting on an object is not zero, causing a change in the object's state of motion (acceleration).

Impulse

The product of force and the time for which it acts. It represents the change in momentum (Impulse = FΔt = Δp).

Friction

A force that opposes the relative motion or tendency of motion between two surfaces in contact.

Understanding Newton's Laws of Motion

Sir Isaac Newton's three laws of motion are fundamental principles describing the relationship between a body and the forces acting upon it, and its motion in response to those forces. These laws are crucial for understanding almost all aspects of classical mechanics.

Newton's First Law of Motion, often called the Law of Inertia, states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force. This means objects tend to resist changes in their state of motion. For example, if a bus suddenly brakes, passengers lurch forward due to their inertia of motion. Conversely, if a bus starts suddenly, passengers are pushed backward due to their inertia of rest. The mass of an object is a direct measure of its inertia; more massive objects have greater inertia and are harder to move or stop.

Newton's Second Law of Motion provides a quantitative relationship between force, mass, and acceleration. It states that the rate of change of momentum of an object is directly proportional to the applied unbalanced force and takes place in the direction of the force. Mathematically, this is expressed as F = ma, where 'F' is the net force acting on the object, 'm' is its mass, and 'a' is the acceleration produced. This law explains why a heavier object requires a greater force to achieve the same acceleration as a lighter object, or why applying a larger force results in greater acceleration. The SI unit of force is the newton (N), defined as the force required to accelerate a 1 kg mass by 1 m/s².

Newton's Third Law of Motion is famously stated as: "To every action, there is always an equal and opposite reaction." This means that whenever one object exerts a force on a second object, the second object simultaneously exerts a force of equal magnitude and opposite direction on the first object. These action-reaction forces always act on different bodies, never on the same body. For instance, when you walk, your foot pushes backward on the ground (action), and the ground pushes forward on your foot (reaction), propelling you forward. Similarly, a rocket expels gases downward (action), and the gases push the rocket upward (reaction). Understanding these laws is key to solving problems involving motion and forces.

Worked Examples

• {"title":"Calculating Momentum","description":"Q: A car of mass 1200 kg is moving with a velocity of 15 m/s. Calculate its momentum.\nA: Given, mass (m) = 1200 kg, velocity (v) = 15 m/s.\nMomentum (p) = m × v\np = 1200 kg × 15 m/s\np = 18000 kg m/s"}
• {"title":"Calculating Force","description":"Q: A force acts on an object of mass 5 kg and accelerates it from rest to a velocity of 10 m/s in 2 seconds. Calculate the magnitude of the force.\nA: Given, mass (m) = 5 kg, initial velocity (u) = 0 m/s, final velocity (v) = 10 m/s, time (t) = 2 s.\nFirst, calculate acceleration (a) = (v - u) / t = (10 - 0) / 2 = 5 m/s².\nNow, Force (F) = m × a\nF = 5 kg × 5 m/s²\nF = 25 N"}
• {"title":"Conservation of Momentum","description":"Q: A bullet of mass 20 g is horizontally fired with a velocity of 150 m/s from a pistol of mass 2 kg. What is the recoil velocity of the pistol?\nA: Given, mass of bullet (m₁) = 20 g = 0.02 kg, velocity of bullet (v₁) = 150 m/s.\nMass of pistol (m₂) = 2 kg, initial velocity of pistol (u₂) = 0 m/s, initial velocity of bullet (u₁) = 0 m/s.\nUsing conservation of momentum: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂\n(0.02 × 0) + (2 × 0) = (0.02 × 150) + (2 × v₂)\n0 = 3 + 2v₂\n2v₂ = -3\nv₂ = -1.5 m/s (The negative sign indicates recoil, opposite to bullet's direction)."}

Balanced vs. Unbalanced Forces

Exam Tip: Avoiding Common Pitfalls

Students often confuse action and reaction forces acting on the same body. Remember, according to Newton's Third Law, action and reaction forces always act on two different bodies. For instance, when you push a wall, you exert force on the wall, and the wall exerts an equal and opposite force on you. They do not cancel each other out because they are acting on different objects. Also, pay close attention to units in numerical problems (e.g., converting grams to kilograms, km/h to m/s) and vector directions for momentum and force calculations. Always state the direction of force or velocity when applicable.

Quick Revision Checks

• Q: What is inertia? Give an example of inertia of rest. A: Inertia is the tendency of an object to resist changes in its state of motion or rest. Example: When a stationary bus starts suddenly, passengers fall backward due to their inertia of rest.
• Q: State Newton's Second Law of Motion and write its mathematical form. A: Newton's Second Law states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force. Its mathematical form is F = ma.
• Q: Why do cricketers pull their hands back while catching a fast-moving ball? A: Cricketers pull their hands back to increase the time duration over which the ball's momentum changes. According to F = Δp/Δt, increasing Δt reduces the force (F) exerted on their hands, preventing injury.
• Q: What is the Law of Conservation of Momentum? A: The Law of Conservation of Momentum states that in an isolated system (where no external force acts), the total momentum of interacting objects remains constant before and after the interaction.

Frequently Asked Questions