3.2. Forces In Action

Introduction

PMT resources on tuition courses for OCR A Physics A-level Topic 3.2: Forces in action

Dynamics and Force

Resultant Force (F): Affects the acceleration of a body with mass (m).Formula: F = maSI Unit of Force: kg·m/s² (Newton, N)1 N is the force causing a 1 kg mass to accelerate by 1 m/s².

Weight: Gravitational force acting on an object.Formula: W = mg (g ≈ 9.81 m/s² on Earth).

Common Forces

• Weight: The gravitational force acting through the center of mass.

• Friction: Resistive force opposing motion, includes static and kinetic friction.

• Drag: Friction opposing motion of objects through fluids.

• Tension: Force transmitted through a string or rope when pulled.

• Upthrust (Buoyancy): Upward force in a fluid counteracting weight.

• Normal Contact Force: Support force acting perpendicular to a surface.

Free-Body Diagrams

Model forces acting on an object with vectors marked by names, helping identify individual and net forces.

Motion Under Constant Force

Constant net force results in constant acceleration (F = ma) illustrated by forces on inclined planes. Vector addition is crucial for analyzing net forces.

Drag Force

Opposes motion through fluids.Factors affecting drag:

• Speed: Increases with speed, significant at high velocities.

• Shape: Streamlined shapes reduce drag; blunt shapes increase it.

• Texture: Surface roughness can affect drag.

• Fluid Density: Higher density fluids exert greater drag.

Inequalities: Drag increases in proportion to speed².

Terminal Velocity

As an object in free fall accelerates, drag increases until it balances weight, resulting in constant speed (terminal velocity), demonstrating dynamic equilibrium.

Experimental Techniques

Terminal velocity experimentation involves viscous liquids and time measurements, commonly using a pulley system and ticker timer for accuracy.

Moment of a Force

Defined as the product of force and perpendicular distance from the pivot.Formula: M = Fd (Newton-meters, Nm).

Couples and Torque:A couple consists of two equal, opposite forces causing rotation without translation. Torque is the product of force and distance from the pivot.

Principle of Moments

In static equilibrium, the sum of clockwise moments equals the sum of anti-clockwise moments, useful in solving unknown forces or distances.

Centre of Mass and Centre of Gravity

Centre of mass: The point where total mass acts.Methods include the plumb line method to find the balance point.

Density and Pressure

Density (ρ): Mass per unit volume (ρ = m/V), SI unit: kg/m³. Varies by state.

Pressure

Defined as the normal force per unit area (p = F/A), SI unit: Pascal (Pa).Fluid Column Pressure: Derived from height, density, and gravity (p = hρg), essential for fluid dynamics.

Archimedes' Principle

Upthrust: Upward force from pressure difference on submerged objects, equal to the weight of displaced fluid.Sinking and Floating: An object sinks if upthrust < weight and floats if they are equal, illustrating the balance between gravitational and buoyant forces.