Understanding Forces: Types, Measurement, and Newton's Laws
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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.
Fr = μN
- Fr = friction force
- μ = Coefficient of friction (between 0 and 1)
- N = normal force
Static Friction Force
Acts between surfaces without relative motion.
Fr = μsN
- Fr = friction force
- μs = static friction coefficient
- N = normal force
Kinetic (Dynamic) Friction Force
Acts when surfaces move relative to each other.
Fr = μkN
- Fr = friction force
- μk = 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.