Learning Objectives
- Understand the concept of balanced and unbalanced forces
- Learn Newton's three laws of motion
- Understand inertia and its types
- Define momentum and understand the law of conservation of momentum
- Solve numerical problems using Newton's laws
Key Concepts
Force
A force is a push or pull that can change the state of motion, shape, or direction of an object. SI unit: Newton (N).
Balanced forces: Net force is zero. No change in state of motion. Example: a book on a table.
Unbalanced forces: Net force is not zero. Causes acceleration (change in velocity). Example: pushing a box to make it move.
Newton's First Law of Motion (Law of Inertia)
An object at rest stays at rest, and an object in motion continues in motion with the same speed and in the same direction, unless acted upon by an unbalanced external force.
Inertia: The tendency of an object to resist any change in its state of motion. Mass is the measure of inertia — heavier objects have more inertia.
Types of inertia: Inertia of rest (e.g., dust flies off when carpet is beaten), inertia of motion (e.g., passenger falls forward when bus stops suddenly), inertia of direction (e.g., sparks fly tangentially from a grinding wheel).
Momentum
Momentum (p) = mass × velocity = mv
SI unit: kg·m/s. Momentum is a vector quantity. An object at rest has zero momentum.
Newton's Second Law of Motion
The rate of change of momentum of an object is directly proportional to the unbalanced force applied and takes place in the direction of the force.
F = ma (Force = mass × acceleration)
Also: F = (mv - mu) / t = m(v - u) / t
1 Newton is the force required to give a mass of 1 kg an acceleration of 1 m/s².
Newton's Third Law of Motion
To every action, there is an equal and opposite reaction. Action and reaction forces act on different objects simultaneously.
Examples: Walking (foot pushes ground backward, ground pushes foot forward), swimming (hand pushes water backward, water pushes body forward), recoil of gun, rocket propulsion.
Law of Conservation of Momentum
The total momentum of a system of objects remains constant if no external force acts on it.
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
This law is a direct consequence of Newton's third law. Applications: recoil of gun, rocket propulsion, collision of objects.
Summary
Force causes a change in the state of motion of objects. Newton's first law defines inertia, the second law relates force to acceleration (F = ma), and the third law states that forces always occur in equal and opposite pairs. The total momentum of an isolated system is conserved.
Important Terms
- Force: A push or pull acting on an object; SI unit: Newton (N)
- Inertia: The tendency of an object to resist changes in its state of motion
- Momentum: Product of mass and velocity of an object (p = mv)
- Newton: Force needed to accelerate 1 kg by 1 m/s²
- Conservation of Momentum: Total momentum remains unchanged in the absence of external force
Quick Revision
- First Law: objects resist change in motion (inertia)
- Second Law: F = ma; 1 N = 1 kg × 1 m/s²
- Third Law: every action has an equal and opposite reaction
- Momentum p = mv (kg·m/s)
- Conservation of momentum: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂