GenPhy1_Newtons Law

Newton's Laws of Motion and Applications

Introduction

Presented by Engr. Darren L. Valenzuela, ECE, ECT

Table of Contents

• Law of Inertia

• Law of Acceleration

• Law of Reaction

• Problem Solving

• Reading Questions

Law of Inertia

DefinitionAn object at rest stays at rest and an object in motion continues in motion in a straight line at constant speed unless acted upon by an unbalanced external force.

Key Concepts

• Inertia: The tendency of an object to resist changes in its state of motion. This property is directly related to the mass of the object; higher mass means greater inertia.

• Mass: Measure of an object's resistance to changes in motion due to a force. Heavier objects require a larger force to change their motion compared to lighter objects.

Applications

1. Seatbelts: They protect occupants of vehicles by preventing them from continuing to move forward when a car suddenly stops.

2. Effects of Slipping on Ice: When a person walks on ice, the lack of sufficient friction can result in a slip, as the inertia of their body causes them to continue moving while their feet may stop suddenly.

3. Opening and Closing Doors: When you abruptly push or pull on a door, inertia causes it to resist the change in motion until enough force is applied.

Law of Acceleration

DefinitionThe acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.Formula: F_net = m * a

Key Concepts

• Acceleration: The rate of change of velocity of an object. It increases as more force is applied and decreases as mass increases.

• Momentum (p): The quantity of motion possessed by an object, expressed as p = m * v, where v is the velocity of the object. Momentum is a vector quantity, having both magnitude and direction.

Applications

• Examples include practical scenarios such as throwing a ball, where the force applied determines how fast it is thrown, riding a bicycle, where acceleration varies based on pedaling force, and driving a car, where acceleration depends on engine power and resistance faced (e.g., air and road friction).

• Impulse-Momentum Theorem: Describes how the impulse (force applied over time) is equal to the change in momentum:J = ΔpJ = F * tThis is critical in understanding how impacts can be managed in sports or vehicle collisions.

Law of Reaction

DefinitionFor every action, there is an equal and opposite reaction.Formula: F_A = -F_B

Understanding Newton's Laws

1. 1st Law: Every body remains at rest or in uniform motion unless acted upon by a net external force.

2. 2nd Law: The amount of acceleration is proportional to the acting force and inversely proportional to the mass.

3. 3rd Law: For every action, there is an equal and opposite reaction, which explains why rockets propel themselves by expelling gas backwards.

Force and Motion Concepts

Types of Forces

• Gravitational Force: The force exerted by gravity on an object (Weight: W = m * g, where g is the acceleration due to gravity).

• Normal Force: The perpendicular force exerted by a surface against an object; it acts to support the weight of the object and prevent it from falling through the surface.

• Friction Force: Resists motion; acts parallel to the surface in opposition to the applied forces. It is crucial in everyday activities, like walking and driving.

Friction Types

1. Static Friction: Resists the initiation of motion between two surfaces in contact, preventing them from starting to move.

2. Kinetic Friction: Resists motion of moving objects; typically lower than static friction.

3. Coefficient of Friction: A value that describes the frictional force between surfaces in contact (f = μN, where μ is the coefficient and N is the normal force). This value varies depending on the materials in contact.

Problem Solving Section

Example Problems

• Football Kicker: Mass = 0.42kg, Launch Speed = 30m/s, Impact Time = 8ms. Calculation of Average Force = 354 lb-f is used to analyze the force exerted by the kicker on the ball.

• Constant Velocity Car: With a Horizontal Net Force of 0N and Acceleration of 0m/s², the car maintains a constant velocity when the applied force equals the frictional force of 1500N.

• Box on Frictionless Surface: When a Force of 200N acts on a 10kg box, we find Acceleration = 20m/s² and subsequent speed calculations show Speed after 8s = 160m/s, providing insight into motion principles.

• Box with Friction: For a force applied of 300N on a 20kg box facing 200N of friction, we calculate the Net Force = 100N leading to an Acceleration = 5m/s² and Distance traveled after 12s = 360m.

• Skaters Interaction: A 120kg skater pushing an 80kg skater results in an Acceleration = -1m/s²; and the Force exerted = 120N demonstrates Newton's third law of equal and opposite reactions.

• Static Friction Problem: Using a 100kg mass that requires 460N to move, we calculate the coefficient of static friction as μ_s = 0.47 which is crucial for determining the force needed to initiate motion.

• Tension Force Problem: For an 800N box held by a rope at 45° to vertical, calculations yield Tension = 1131.37N essential for understanding forces in systems.

• Friction Coefficient Problem: A 10 kg block pulled with a 50N force at a 25° angle leads to a calculation of μ_k = 0.59, informing frictional dynamics in different scenarios.

Conclusion

A comprehensive understanding of Newton's Laws of Motion, the various types of forces, and problem-solving techniques is essential for mastering physics principles that govern motion and forces, crucial for applications in mechanics, engineering, and various scientific fields.