IGCSE Physics (2026-2028) - C3/25: Force, Weight, Momentum, Impulse, Scalar & Vector Quantity

Types of Forces and Resultant Force
📌 Forces are defined as pushes or pulls that affect objects in motion, measured in Newtons (N).
⚖️ Key types of forces include Weight (gravitational force exerted by a planet), Contact Forces (like friction and air resistance/drag), and Upthrust (upward push from a fluid).
💥 An unbalanced (resultant) force causes an object to change speed (accelerate) or change direction.
➕ Resultant force is the sum of multiple forces; forces in the same direction are added, while forces in opposite directions are subtracted (e.g., Engine Thrust - Friction).

Newton's First Law and Gravity
📜 Newton's First Law states an object remains at rest or moves at a constant speed in a straight line unless acted upon by a resultant force.
⬇️ Weight ($W$) is the gravitational force exerted by the Earth, calculated using the formula $W = mg$, where $m$ is mass (kg) and $g$ is the gravitational field strength (approx. $9.8 \text{ N/kg}$ on Earth).
📏 Mass is the amount of matter (constant everywhere), while Weight changes depending on the local gravitational field strength ($g$). Gravitational field strength ($g$) is the force per kilogram ($\text{N/kg}$).

Acceleration, Terminal Velocity, and Circular Motion
🏃 Objects in free fall accelerate at the same rate, called acceleration of free fall or acceleration due to gravity ($g$), unless opposed by air resistance.
💨 Terminal velocity is reached when the resultant force is zero, meaning the downward force (Weight) equals the upward resistive force (like air resistance).
🔄 For an object to move in a circular path, a resultant force, known as centripetal force, must be directed towards the center of the circle. This force depends on mass, speed, and the radius of the circle.

Newton's Second Law, Momentum, and Impulse
🔗 Newton's Second Law describes the relationship $F = ma$ (Force = mass $\times$ acceleration). Greater mass requires a proportionally greater force to achieve the same acceleration.
📈 Momentum ($p$) is a measure of an object's resistance to changes in motion, calculated as $p = mv$ (mass $\times$ velocity), unit $\text{kg m/s}$.
⏱️ Impulse is the change in momentum, calculated as Impulse = Force $\times$ Time ($\text{N s}$). Conversely, Force can be calculated as the rate of change of momentum: $F = \frac{\Delta p}{\Delta t}$.

Scalar and Vector Quantities
🔢 Scalar quantities have only magnitude (e.g., time, speed, mass, energy).
➡️ Vector quantities have both magnitude and direction (e.g., force, acceleration, momentum).

Key Points & Insights
➡️ The intuitive belief that a heavier object falls faster is incorrect; if air resistance is negligible, both balls ($1 \text{ kg}$ and $5 \text{ kg}$) will hit the ground at the same time because gravity acts on every kilogram equally.
➡️ Safety mechanisms like airbags work by increasing the contact time ($\Delta t$) during a collision, which, according to $F = \frac{\Delta p}{\Delta t}$, results in a significantly reduced impact force experienced by the passenger.
➡️ When an object is moving at terminal velocity, its acceleration is zero, meaning the resultant force acting upon it is zero (Weight = Air Resistance).

📸 Video summarized with SummaryTube.com on Feb 03, 2026, 11:58 UTC