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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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