Physics · Unit 4: Forces, density and pressure · 15 min read · Updated 2026-05-11
Common force types — CIE A-Level Physics
CIE A-Level Physics · Unit 4: Forces, density and pressure · 15 min read
1. Non-Contact Force Types★★☆☆☆⏱ 3 min
The most common non-contact force at A-Level is weight, the gravitational force on an object's mass. Other non-contact forces you will encounter are electric force (between charged objects) and magnetic force (between magnetic materials/moving charges).
Weight is always calculated with the formula $W = mg$, where $m$ is mass and $g$ is acceleration due to gravity.
Exam tip: Always use the value for g given in the question paper, do not assume 9.81 without checking
2. Contact Forces: Normal Reaction and Tension★★☆☆☆⏱ 4 min
Normal reaction is the force a surface exerts on an object in contact with it. It always acts **perpendicular (normal) to the contact surface**, pointing outward from the surface.
Tension is the pulling force exerted by a stretched string, rope or rod on an object attached to it. It always acts along the line of the string, pulling away from the object.
3. Contact Forces: Friction and Drag★★★☆☆⏱ 4 min
Static friction acts to prevent motion, and has a maximum value given by $f_{\text{max}} = \mu_s N$, where $\mu_s$ is the coefficient of static friction and $N$ is normal reaction. Kinetic friction acts when the object is moving, given by $f_k = \mu_k N$.
Drag (air resistance for objects moving through air) is a fluid friction that opposes motion of an object through a fluid. Its magnitude increases with the speed of the object, and it always acts opposite to the direction of motion relative to the fluid.
Exam tip: Friction always acts parallel to the contact surface, never perpendicular to it
4. Upthrust (Buoyancy Force)★★★☆☆⏱ 4 min
By Archimedes' principle, the magnitude of upthrust is equal to the weight of the fluid displaced by the object. This gives the formula $U = \rho_{\text{fluid}} V_{\text{displaced}} g$, where $\rho$ is fluid density and $V$ is the volume of displaced fluid.
Common Pitfalls
Why: This is only true for horizontal surfaces with no other vertical forces acting
Why: Tension is always a pulling force along a string; ideal strings cannot push
Why: Drag direction depends on motion direction, not vertical position
Why: Weight is a non-contact force that acts on all objects near Earth's surface, regardless of contact
Why: Only the submerged part of the object displaces fluid