Forces

Introduction

• The discussion revolves around understanding forces acting on objects and solving physics problems related to motion.

• A visual aid (drawing a picture or free body diagram) is essential for analyzing forces.

Key Concepts in Forces

Free Body Diagrams

• Begin by drawing a free body diagram to visualize forces.

• Example force value mentioned: 40 newtons.

• Forces can cancel each other out; e.g., 20 newtons down and 40 newtons up.

• Importance: Understanding that net force equals zero does not mean the object is at rest.

  • An object can be stationary or moving at a constant speed, with zero acceleration.

Types of Forces

• Specific Forces labeled in the diagram:

  • Friction: Acts opposite to the direction of motion.

  • Gravity (Weight): The force acting downwards on the object due to its mass.

  • Normal Force: The support force exerted by a surface, opposing an object's weight when resting on it.

  • Drag / Wind Resistance: The force opposing motion through the air.

  • Thrust: Generally not discussed in detail; focuses on applied force in pushing vehicles forward.

Units of Measurement

• Unit of Force: Newton (N).

  • Example: 1 Newton = 1 kg·m/s² (one kilogram accelerates at one meter per second squared).

Dimensional Analysis

• Procedure for analyzing physical quantities.

• Requires understanding the relationship among different units.

• Example used: Convert units for mass from kilograms to newtons.

Calculation of Forces

• Essential equation: F = m a (Force = mass × acceleration).

• Example problem using values:

  • Mass: 1.12 kg

  • Acceleration: 1.11 m/s²

  • Resulting Force: 1.2 N.

Understanding Drag Force

• Three factors affect drag force:

  1. Speed: Faster motion experiences greater drag due to more interactions with air particles.

  2. Cross-Sectional Area: Larger area experiences more drag than a smaller area (e.g., a balloon vs an arrow).

  3. Density of Medium: Denser mediums (like water) produce more drag compared to less dense mediums (like air).

Terminal Velocity

• Defined as the constant velocity an object reaches when the drag force equals its weight.

  • Initial drop from an airplane example explained.

  • Vertical speed starts at zero; gravitational acceleration is approximately 9.8 m/s².

  • Weight calculated as: **Weight = mass × gravity.

  • At terminal velocity, acceleration stops as forces balance out.