Force
A push or pull that changes (or tends to change) an object's state of motion, measured in newtons (N). A vector quantity (has magnitude and direction). Newton's second law: F = ma, where m is mass and a is acceleration. Forces can be contact (friction, normal force) or non-contact (gravity, electromagnetic).
Formula
F = ma
Real World
During a SpaceX Falcon 9 launch, the rocket engines produce a thrust force greater than the gravitational force on the rocket's mass, creating a net upward force and therefore upward acceleration.
Exam Focus
Always draw a free-body diagram first; 'resultant force' questions require you to subtract opposing forces before applying F = ma.
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