Table of Contents
What Is Force?
Force is a vector quantity that causes an object to accelerate, decelerate, or change direction. The SI unit is the newton (N), defined as the force needed to accelerate 1 kg at 1 m/s^2. Force is one of the most fundamental concepts in physics, underlying everything from planetary orbits to bridge design to car crashes.
Newton's laws of motion describe how forces affect objects. The first law (inertia) states objects maintain their state unless acted upon by a force. The second law (F=ma) quantifies the relationship. The third law states forces come in equal and opposite pairs.
Newton's Second Law
Where F is net force in newtons, m is mass in kg, a is acceleration in m/s^2. Weight is a special case where a = g (gravitational acceleration, 9.81 m/s^2 on Earth).
Force Types
| Force | Nature | Example |
|---|---|---|
| Gravity | Attraction between masses | Weight, orbits |
| Normal | Surface reaction | Floor supporting you |
| Friction | Opposes relative motion | Braking, walking |
| Tension | Pull through rope/cable | Towing, suspension |
| Spring | Restoring (Hooke's law) | Suspension, trampolines |
FAQ
What is the difference between mass and weight?
Mass is the amount of matter (kg), constant everywhere. Weight is the gravitational force on that mass (N), varying with location. A 70 kg person weighs 686 N on Earth but only 113 N on the Moon, while their mass remains 70 kg in both places.
What is net force?
Net force is the vector sum of all forces acting on an object. If multiple forces act, only the net (resultant) force determines acceleration. An object with balanced forces (net F = 0) moves at constant velocity (or stays still).
How is force measured?
Forces are measured with dynamometers (spring scales), load cells (strain gauges), and force plates (piezoelectric sensors). Newton's cradle demonstrates force transfer. Bathroom scales measure the normal force your body exerts, which equals your weight when stationary.