This topic accounts for approximately 14% of your exam marks.
stable
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Stable14%
F = ma, resultant forces and Hooke's Law calculations are high-frequency multi-mark questions.
What a force is
A force is a push or a pull that one object exerts on another whenever the two objects interact
The SI unit of force is the newton (N)
Force is a vector quantity: it has both a magnitude and a direction (see section 2)
The named forces in the syllabus
Weight (gravitational force): the pull of a planet on every object that has mass. A bigger mass feels a bigger weight when placed in the same gravitational field
Reaction force (normal contact force): the push a surface exerts on an object that rests on it. It acts perpendicular to the surface
Friction: a force that opposes the motion of an object as it slides over another surface. Friction arises because no two surfaces are perfectly smooth at the molecular scale; tiny irregularities catch against one another
Drag force: friction in a fluid (a liquid or a gas). The fluid's particles bump into the moving object and slow it down
Air resistance: a special case of drag where the fluid is air. The force grows as the object moves faster, because more air particles hit it per second
Thrust: a forward driving force produced by an engine, jet or rocket
Upthrust: the upward push that a fluid exerts on any object floating in or submerged in it
Electrostatic force: the attraction between opposite charges and repulsion between like charges
Magnetic force: the attraction between unlike poles and repulsion between like poles
Tension: the pull that runs along a stretched rope, wire, cable or spring when something is pulling on each end
Three things forces can do
A force acting on an object can:
change its speed (cause it to accelerate or decelerate along the same direction)
change its direction of motion
change its shape (stretch, squash, bend or twist it)
Vocabulary cautions
Refer to the gravitational pull on an object as its weight in exam answers, because the word "gravity" can ambiguously mean either the force or the field strength, and mark schemes often reject it
For the upward push that a falling object feels, use air resistance (or drag). "Wind resistance" is not a physics term, and "air pressure" is a different concept altogether