Overview
This lecture explains Newton's First Law (the Law of Inertia) and uses everyday examples—like standing on a bus—to illustrate how objects behave at rest and in motion when acted on by forces.
Everyday Examples: The Bus Scenario
- When a bus starts moving suddenly, you fall back due to your body's tendency to stay at rest (inertia).
- When a moving bus stops suddenly, you fall forward because your body tends to keep moving (also inertia).
Objects at Rest
- Objects at rest stay at rest unless acted on by an unbalanced force.
- Forces like gravity and the normal force can be balanced, keeping the object stationary.
- Applying an unbalanced force (like a push) causes motion.
Objects in Motion
- Objects in motion continue with the same velocity unless acted on by an unbalanced force.
- After a kick, a chair keeps moving, even after contact is lost, showing motion doesn’t need a continual force.
- Friction and air resistance are unbalanced forces that slow and stop moving objects.
Friction and Its Effects
- Friction opposes motion and is the reason moving things eventually come to a stop on Earth.
- Less friction (like on ice) allows objects to move farther before stopping.
Newton’s First Law (Law of Inertia)
- Objects at rest remain at rest; objects in motion stay in motion at constant velocity unless acted on by an unbalanced force.
- Inertia is an object’s resistance to changes in its state of motion.
- Newton’s First Law is also called the Law of Inertia.
Key Terms & Definitions
- Unbalanced Force — a force that is not canceled out by other forces, causing a change in motion.
- Friction — a force that opposes motion between two surfaces in contact.
- Inertia — the tendency of an object to resist changes to its state of motion.
- Newton’s First Law (Law of Inertia) — the principle that objects remain in their current state of rest or motion unless acted on by an unbalanced force.
Action Items / Next Steps
- Review Newton’s laws and observe examples of inertia in daily life.
- Consider how friction affects motion in different environments (e.g., compare sliding objects on ice vs. grass).