HES307 Set03A

Announcements and Overview

• Introduction to the second quarter of the course focusing on linear kinetics.

• Transition from previous exam to new materials.

• Assignment of lecture quizzes:

  • Quiz 4 availability: Thursday afternoon or Friday morning, due next week by noon.

  • Focus on Sets 3a, 3b (Linear Kinetics and Free Body Diagrams), Set 4 (Impulse-Momentum), Set 5 (Work-Power-Energy).

Exam Updates

• One student remains who hasn't taken the exam yet; date pending.

• Adjustments made to the exam scoring due to true/false question errors and ambiguous wording on some questions, dropping problematic items.

• Class average increased to 75 via grading curve.

• If a student scores better on exam two than exam one, the exam one score will be replaced with the average of the two (with conditions).

Importance of the Foundation

• Understanding of kinematics is essential for grasping kinetics.

• Continuous reference back to earlier course material, including concepts of mass and force.

Chapter Focus and Structure

• Skipping Chapter 10 for now; moving directly to linear kinematics (Chapter 11) and then progressing to angular kinematics.

• Development of frameworks for working with forces, including typical notations and equations.

Force: Definition and Concept

• Forces cannot be seen but have visible effects on objects.

• Definition: A mechanical action or effect that produces acceleration on a body.

• Forces represented by vector arrows typifying pushes or pulls in specific directions.

• SI unit of force is the Newton (N), equivalent to kg·m/s²;

  • Clarification of mass vs. force: Mass in kilograms (kg), force in Newtons or pounds.

Free Body Diagrams (FBDs)

• FBD as a pictorial tool to illustrate an isolated object subject to various external forces.

• Essential for analyzing the net effect of forces.

• Fundamental understanding of free body diagrams and their applications to both linear kinetics and angular principles.

Newton's Laws of Motion

• Emergency of fundamental concepts in classical mechanics, established in 1687.

• Newton's First Law (Law of Inertia):

  • Objects in motion stay in motion; objects at rest stay at rest unless acted upon by an external force.

  • Inertia defined as resistance to change in state of motion, quantified by mass.

  • Important consideration of uniform motion: Defined as constant momentum, not merely constant velocity.

• Newton's Second Law (Law of Acceleration):

  • Mathematical formulation: F = ma (where F is the force, m is mass, a is acceleration).

  • Details surrounding conditions of constant mass, directionality of forces, and impact of non-zero forces on motion.

• Newton's Third Law (Action-Reaction):

  • For every action, there is an equal and opposite reaction.

  • Example application: Person pushing against the ground and the ground pushing back against the person.

  • Depth of analysis into mutual actions involving two bodies and how they can be represented in FBDs.

Law of Universal Gravitation

• All objects exert gravitational forces of attraction towards each other.

• Equation of gravity: F = G(m1*m2)/r², where G is the gravitational constant.

• Weight of an object on Earth expressed as W = mg, where g is the local gravitational constant (≈9.81 m/s²).

Mass vs. Weight

• Mass (m): Quantification of matter, a scalar quantity measured in kilograms.

• Weight (W): A vector quantity representing gravitational force, derived as W = mg, expressed in Newtons.

Center of Mass (COM) vs Center of Gravity (CG)

• Center of Mass (COM):

  • The average location of mass distribution in an object.

  • Exists irrespective of gravitational force.

• Center of Gravity (CG):

  • Point through which the gravitational force acts.

  • Exists only in the presence of gravity.

  • Under most practical conditions, COM and CG can be treated as the same point given similar conditions.

Ground Reaction Forces (GRFs)

• Defined as interaction forces between the body and the ground, typically observed during motion.

• Composed of vertical and horizontal components: referred to as the Y (up/down) and X (forward/backward) components respectively.