Understanding Forces: Types, Measurement, and Newton's Laws

Force Interactions

Strength: Action of one body on another, or interaction. Net force (resultant) is the vector sum of all forces acting simultaneously on the same body.

Like acceleration and speed, force is a vector quantity, defined by its magnitude and direction.

Force Measurement

To combine forces on a body, add them vectorially. The net force is equivalent to a single force producing the same effect.

Special Forces

Weight

Weight is the gravitational force a planet or star exerts on a body.

Normal Force

The normal force is a contact force preventing an object from passing through a surface, always perpendicular to it.

Friction Force

Friction force is exerted between two surfaces in contact. Its magnitude on solids depends on surface roughness.

F_r = μN

• F_r = friction force
• μ = Coefficient of friction (between 0 and 1)
• N = normal force

Static Friction Force

Acts between surfaces without relative motion.

F_r = μ_sN

• F_r = friction force
• μ_s = static friction coefficient
• N = normal force

Kinetic (Dynamic) Friction Force

Acts when surfaces move relative to each other.

F_r = μ_kN

• F_r = friction force
• μ_k = Kinetic friction coefficient
• N = normal force

Newton's Laws

Action-Reaction Law (Third Law)

For every action (force) on a body, there's an equal and opposite reaction force. These forces have the same magnitude, opposite directions, and act on different bodies.

Common Errors:

• Weight and normal force are not an action-reaction pair because they act on the same body.
• A force's magnitude isn't determined by its visual effects; action and reaction forces are always equal.

Law of Inertia (First Law)

When the net force on a body is zero, it maintains constant velocity or stays at rest. A non-zero net force is needed to change a body's motion state.