Dynamics and Forces Overview

Overview

This lecture covers the fundamentals of dynamics, focusing on Newton's laws of motion, the concept of force, distinctions between mass and weight, and common problem-solving strategies in physics.

Introduction to Dynamics and Forces

• Dynamics is the study of the causes of motion, primarily focusing on forces.
• A force is any push or pull on an object, and it is a vector (has both magnitude and direction).
• Forces are categorized as contact forces (require physical contact) or field forces (act through space, e.g., gravity).
• The four fundamental forces of nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces (all field forces).

Newton's First Law (Law of Inertia)

• Newton's First Law: Objects at rest stay at rest, and objects in motion stay in motion unless acted on by a net force.
• Inertia is an object's resistance to changes in motion and is measured by its mass.
• The law only applies in inertial reference frames (not accelerating or rotating).
• Real-world motion is influenced by friction and resisting forces.

Mass vs. Weight

• Mass is the measure of an object's resistance to change in velocity (inertia) and is measured in kilograms (kg).
• Weight is the force due to gravity, calculated as weight = mass × gravitational acceleration (W = mg).
• Mass is intrinsic and constant; weight depends on local gravity (e.g., less on the moon).

Newton's Second Law

• Newton's Second Law: The net force acting on an object equals mass times acceleration (F = ma).
• Force and acceleration are vectors; F = m × a applies separately for x, y, and z components.
• The SI unit of force is the Newton (N), where 1 N = 1 kg·m/s².
• Larger mass requires more force for the same acceleration.

Problem-Solving Examples

• To accelerate a 1000 kg car at 4.9 m/s² requires about 4900 N of force.
• To bring a 1500 kg car from 100 km/h to rest in 55 meters, the required net force is about -11,000 N.

Gravitational Force and Normal Force

• Gravitational force is the attractive force between masses, near Earth given as F_gravity = mg.
• If two objects are at the same earth location, their weight ratios equal their mass ratios.
• Gravitational mass (from weight) and inertial mass (from motion) are equivalent.
• The normal force is the support force exerted by a surface, balancing the object's weight unless other forces act.

Application Problems with Normal Force

• If a box sits on a table, the normal force equals its weight.
• If an extra downward force is applied, normal force increases; if an upward force acts, normal force decreases.
• If the upward force exceeds weight, the box accelerates upward with net force calculated as F_net = F_upward - weight.

Key Terms & Definitions

• Force — a push or pull on an object; a vector quantity.
• Contact Force — force requiring physical contact.
• Field Force — force acting through space without contact.
• Inertia — resistance to change in an object's motion.
• Mass — measure of inertia; measured in kilograms.
• Weight — force of gravity on an object; W = mg.
• Normal Force — support force from a surface.
• Newton (N) — SI unit of force; 1 N = 1 kg·m/s².
• Inertial Reference Frame — a non-accelerating frame of reference.

Action Items / Next Steps

• Practice solving problems using Newton's laws.
• Review examples calculating force, acceleration, and normal force.
• Prepare for homework on identifying and calculating forces in various scenarios.