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Introduction to Newton's Laws

Review of Newton's Second Law (Postulate)

• Fundamental Principle: Newton's Second Law asserts that the force (F) acting on an object is equivalent to the time rate of change of its momentum, mathematically expressed as:F = dp/dt

• Simplified for Constant Mass: When the mass (m) of an object remains constant, the law simplifies to the familiar form:F = mawhere 'a' represents acceleration.

One-Dimensional Motion

• Single Particle Analysis: Focuses on analyzing the behavior of a single particle moving along the x-axis in one-dimensional space.

• Factors Affecting Force: The force acting on the particle is influenced by various factors, including:

  • Position (x): The location of the particle in space.

  • Velocity (v): The speed and direction of the particle’s movement.

  • Time (t): The duration for which the motion is analyzed.

• Common Simulation Scenario: In many physics simulations, it is common to model force as a function of the particle's position alone rather than its velocity or time.

Potential Energy Function

• Definition: The potential energy function U(x) is articulated in relation to force (F) by:

  • F = -dU/dxThis equation indicates that the force is the negative gradient of the potential energy with respect to position.

• Interdependence of Forces: Knowing either the force function or the potential energy function allows for the derivation of the other.

• Additive Constant: The potential energy is defined up to an additive constant (C), indicating that it can take different values while still describing the same physical system.

Conservation of Energy

• Principle of Conservation: The total energy (E) of a closed system remains constant over time and is the sum of kinetic energy (T) and potential energy (U):E = T + U

• Kinetic Energy Equation: Kinetic energy is given by:T = 1/2 mv²where 'm' is mass and 'v' is the velocity of the particle.

• Implication for Systems: Systems that are governed by forces derived from a potential exhibit conservation of total energy.

Example: Particle Under Gravity

• Gravitational Force: For a particle subject to the force of gravity, the force can be expressed as:F = -mgwhere 'm' is the mass of the particle and 'g' is the acceleration due to gravity (approximately 9.81 m/s²).

• Equations of Motion: This results in classical equations describing its motion:

  • Velocity: v = v₀ - gt

  • Displacement: x = x₀ + v₀t - (1/2)gt²where 'v₀' and 'x₀' are the initial velocity and position, respectively.

General Case for Conservative Forces

• Energy Conservation: The principle that total energy remains constant can be applied generally:T + U = constant

• Energy Equations Development: The derivation for motion under gravity can be mirrored through similar integration techniques applied to force and potential energy relationships.

Alternative Approach to Velocity Function

• Velocity from Energy: To express velocity in terms of total energy E, we derive:v = ±√((2/m)(E - U(x)))This formulation allows for determining motion based on total energy considerations.

Specific Case of Motion Under Gravity

• Upward Motion Analysis: When analyzing a particle thrown upwards against the gravitational force, its trajectory and maximum height can be determined using initial conditions:h_max = v₀²/(2g)

• Integration for Motion: Further derivation connects position and velocity over time utilizing integral calculus principles.

Case of Resistive Forces

• Introduction of Resistance: In real-world scenarios, resistive forces, such as air resistance, are also considered, leading to a modified force expression:Total Force (F) = -mg - cvwhere 'c' is a constant of proportionality linked to the resistive force proportional to velocity.

• Derivation of Velocity: By utilizing logarithmic and exponential functions, the time-dependent velocity can be expressed as:v(t) = -mg/c + (mg/c + v₀)e^(-ct/m)which leads to determining the terminal velocity, a constant value that a falling object approaches under the influence of drag forces.

Summary

• Insights from Conservation Principles: The conservation principles reformulate the equations of motion, providing valuable insights into various dynamic systems encountered in real-world scenarios, including projectile motion, the influences of resistive forces, and energy exchanges.

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