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CIE A-Level Maths Study Notes

3.1.4 Modeling Contact Forces

In the field of Mathematics, grasping the dynamics of contact forces is essential. This section explores the representation of contact forces, distinguishing between smooth and rough surfaces, and scrutinizes the limitations and physical implications of these models.

Introduction to Contact Forces

  • Key Idea: Contact forces are essential in physics for understanding how objects interact with surfaces.
  • Main Types: Normal reaction and frictional force.
Contact Forces

Images courtesy of Javatpoint

Normal Reaction

  • Definition: Force perpendicular to a surface, counteracting the object's weight.
  • Varies With: Other forces on the object, not just weight.

Examples of Normal Reaction

1. On a Horizontal Surface

  • An object on a flat surface; normal reaction equals the object's weight (mg).

2. On an Inclined Plane

  • On a slope, normal reaction balances the weight's perpendicular component.

Frictional Forces

  • Nature: Resists relative motion or its tendency between surfaces.
  • Types: Static (stationary objects) and kinetic (moving objects).
  • Surfaces:
    • Smooth: Often idealized as frictionless.
    • Rough: Both normal reaction and friction apply.

Limitations in Modeling Contact Forces

  • Simplifications: Theoretical models simplify real-world surfaces and interactions.
  • Consequence: Possible inaccuracies in practical applications.

Application of Contact Force Concepts

Example 1: Object on an Inclined Plane

  • Problem: Find frictional force for a 5 kg object on a 30° incline, friction coefficient 0.3.
Inclined Plane
  • Forces:
    • Weight: Downwards.
    • Normal Reaction: Perpendicular to plane.
    • Frictional Force: Up the plane, opposes motion.
  • Calculations:
    • Weight Perpendicular: W=5×9.81×cos(30°) W_{\perp} = 5 \times 9.81 \times \cos(30°)
    • Weight Parallel: W=5×9.81×sin(30°) W_{\parallel} = 5 \times 9.81 \times \sin(30°)
    • Normal Reaction R=W R = W_{\perp}
    • Frictional Force F=μR=0.3×R F = \mu R = 0.3 \times R
  • Result: Frictional force ~ 12.74N12.74 N.
  • Graph:
 Inclined Plane graph

Example 2: Object on a Horizontal Surface

  • Problem: Minimum force to move a 10 kg object, friction coefficient 0.5, on a horizontal surface.
Horizontal Surface
  • Calculations:
    • Normal Reaction R=10×9.81 R = 10 \times 9.81
    • Max Static Friction Fmax=μ×R=0.5×RF_{\text{max}} = \mu \times R = 0.5 \times R
  • Result: Minimum force ~ 49.05N49.05 N.
  • Graph:
Horizontal Surface graph
Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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