Overview
This lecture introduces Newton's three laws of motion, explores different forces like gravity, normal, and tension, and explains how they combine to determine acceleration in various scenarios.
Newton’s Laws of Motion
- Newton published three laws of motion in 1687, foundational for understanding the effects of forces.
- First law (inertia): Objects stay at rest or in motion unless acted on by a net force.
- Second law: Net force equals mass times acceleration (Fₙₑₜ = ma).
- Third law: For every action, there is an equal and opposite reaction.
Inertia, Mass, and Force
- Inertia is an object's resistance to changes in motion; greater mass means more inertia.
- Mass is the measurable property that quantifies inertia.
Net Force and Equilibrium
- Net force is the sum of all forces acting on an object, accounting for direction and cancellation.
- Equilibrium occurs when net force is zero; objects in equilibrium have constant velocity (which can be zero).
Gravity and Weight
- Gravitational force on an object is calculated as F(g) = mg, where g = 9.81 m/s².
- Weight is the force of gravity on an object and is measured in Newtons (N), not kilograms (kg).
Normal and Tension Forces
- The normal force acts perpendicular to a surface, balancing other forces such as gravity.
- The normal force changes in magnitude to match the applied force up to its breaking point.
- Tension is the pulling force exerted by a rope or cable, matching the load it supports.
Applying Newton’s Laws: Examples
- Free body diagrams visually represent all forces acting on an object with labeled arrows.
- For a box at rest, gravity pulls down and the normal force pushes up; net force is zero.
- When a box hangs by a rope, tension replaces the normal force as the upward force.
Solving Force Problems: The Elevator Example
- Use free body diagrams to track forces and directions.
- For a two-body system (lift and counterweight), set up equations for each and use algebra to eliminate unknowns.
- Net acceleration is the difference in weights divided by total mass; acceleration calculated as 0.795 m/s².
Key Terms & Definitions
- Inertia — Tendency of an object to resist changes in its motion.
- Net Force — Total force acting on an object after all forces are combined.
- Equilibrium — State where net force is zero and velocity is constant.
- Normal Force — Perpendicular support force from a surface.
- Tension — Pulling force transmitted by a string, rope, or cable.
- Weight — Force of gravity on an object; calculated as mass times gravity (mg).
Action Items / Next Steps
- Practice drawing free body diagrams for different objects and force scenarios.
- Solve sample problems using Fₙₑₜ = ma and F(g) = mg.
- Review key definitions and ensure you can distinguish mass from weight.