Forces

Overview

This lecture covers key concepts in forces, motion, and related physics principles for GCSE, including types of forces, Newton's laws, motion graphs, momentum, and practical applications.

Types of Forces

• A force is any push or pull on an object.
• Contact forces require physical touch (e.g., normal force, friction, air resistance, tension).
• Non-contact forces act at a distance (e.g., gravity, magnetism, electrostatics).
• All forces can be represented as vectors, showing both magnitude and direction.

Vectors and Scalars

• Vectors have both magnitude and direction (e.g., displacement, velocity, force, momentum).
• Scalars have magnitude only (e.g., distance, speed, mass, energy).

Resultant Force and Vector Addition

• Resultant force is the sum of all forces acting on an object, considering direction.
• Add vectors directly or use Pythagoras’ theorem/trigonometry if at angles.

Newton's Laws of Motion

• Newton’s First Law: An object’s velocity stays constant unless acted on by a resultant force (inertia).
• Newton’s Second Law: Resultant force equals mass times acceleration (F = ma).
• Newton’s Third Law: Every action force has an equal and opposite reaction force.

Balanced and Unbalanced Forces

• Balanced forces (sum to zero) mean no change in velocity (object at rest or constant speed).
• If unbalanced, the object accelerates in the direction of the resultant force.

Work, Energy, and Weight

• Weight is the gravitational force on mass (Weight = mass × g; g = 9.8 or 10 N/kg).
• Work done (energy transferred) = force × distance moved.
• Gravitational potential energy gained = mass × g × height.

Hooke’s Law and Springs

• Hooke’s Law: Force = spring constant × extension (F = kx), valid for elastic deformations.
• Spring constant unit is N/m; force and extension are directly proportional.
• Energy stored in a stretched spring = ½ kx².

Moments and Equilibrium

• Moment (turning force) = force × perpendicular distance from pivot (Nm).
• Principle of moments: Clockwise and anticlockwise moments balance for equilibrium.

Pressure

• Pressure = force / area (Pa or N/m²).
• Liquid pressure = height × density × g (P = hρg).
• Gas pressure depends on particle collisions, volume, and temperature.

Motion Graphs and Equations

• Speed = distance / time; velocity includes direction.
• Gradient on a distance-time graph shows speed; gradient on a velocity-time graph gives acceleration.
• Area under a velocity-time graph gives distance/displacement.
• A falling object accelerates at 9.8 m/s² due to gravity.
• Use motion equations (SUVAT) to solve acceleration problems.

Stopping Distances

• Stopping distance = thinking distance + braking distance.
• Thinking distance doubles with speed; braking distance quadruples (kinetic energy ∝ speed²).

Momentum and Collisions

• Momentum = mass × velocity; it's a vector with units kg·m/s.
• Total momentum is conserved in collisions (before = after).
• For coupled objects after collision, combined mass × velocity is used.
• Recoil: Equal and opposite momentum after events, e.g., firing a cannon.

Force and Momentum Change

• Force = rate of change of momentum (F = Δp / t).
• Increasing the time over which momentum changes reduces the force felt (e.g., seatbelts, air bags).

Key Terms & Definitions

• Force — A push or pull acting on an object.
• Vector — Quantity with both magnitude and direction.
• Scalar — Quantity with magnitude only.
• Resultant Force — Sum of all forces on an object considering direction.
• Inertia — Tendency to resist change in motion.
• Weight — Force of gravity on a mass.
• Hooke’s Law — Relationship between force and extension for springs.
• Moment — Turning effect of a force.
• Pressure — Force per unit area.
• Momentum — Mass times velocity, conserved in closed systems.

Action Items / Next Steps

• Revise the definitions and equations discussed.
• Practice vector addition, motion graph analysis, and Newton’s laws problems.
• Prepare for experiments on springs and momentum.