Newton's First Law (Law of Inertia): Objects resist changes in their state of motion. If not moving, they tend to stay not moving, and if moving, they tend to keep moving in the same direction and speed unless a force acts upon them.
Inertia: Tendency of an object to resist changes in its motion.
Example: Car brakes suddenly -> body keeps moving forward.
Example: Running and tripping -> upper body keeps moving.
Example: Car accelerating suddenly -> body appears to be pushed back but actually stays while car moves forward.
Example: Roller coaster drop -> body seems to stay while coaster moves.
Demonstrations of Inertia
A board and nails example demonstrates how inertia works with mass and force.
A board on a head without books experiences more force than one with books between the head and board.
Force Measurement
Newton (N): Unit of force in the metric system, named after Sir Isaac Newton.
1 Newton ≈ 0.2 pounds.
Example: 200 pounds ≈ 900 Newtons (200 * 4.45).
Net Force and Equilibrium
Net Force: Sum of all forces acting on an object.
If moving, a net force acts on it; if stationary, the net force is zero.
Normal Force (Support Force): Force exerted by a surface to support the weight of an object placed on it.
Always perpendicular to the surface.
Balances gravity to achieve equilibrium.
Equilibrium Types
Static Equilibrium: Object is not moving, net force is zero.
Dynamic Equilibrium: Object moving at a constant velocity, net force is still zero.
Mechanical Equilibrium: General term for objects in equilibrium, either static or dynamic.
Expressed as sum of forces equals zero (ΣF = 0).
Additional Examples
Suspended Object: Gravity pulls down; tension force in the string pulls up. Both forces balance, net force is zero.