A.2.5—Contact forces
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Classify Contact Forces
Contact-force family
Contact forces arise when bodies or a body and fluid interact: normal force, friction, tension, elastic restoring force, viscous drag and buoyancy.
Use the interaction geometry
Normal force is perpendicular to the surface; friction acts along the surface opposing relative motion or attempted motion; tension acts along a taut string; drag opposes motion through a fluid; buoyancy acts upward due to fluid pressure differences.
Check the condition
Friction can be static or kinetic, drag depends on speed and shape, and buoyancy depends on displaced fluid. The magnitudes are determined by the interaction and constraints, not by a memorized universal value.
Common trap
Do not include every possible contact force. Include only interactions actually present in the described situation.
A.2.5 Exam Analysis
Assessment in practice
1–2 marksHow it is assessed
The evidence tests a terminal-velocity free-body diagram and asks for a physical explanation of a discrepancy in an experiment.
Command terms
Identify / Explain
What earns marks
Identify every contact interaction present and draw its direction on the selected body. At terminal velocity, use zero resultant force but retain weight and drag; in an experiment, connect differences from accepted values to friction, air resistance or release conditions.
Watch for
Removing weight or drag because acceleration is zero at terminal velocity; zero resultant force does not mean zero individual forces.
Representative question
Question 1
[Maximum number: 1]
A ball is thrown from an aircraft in flight.
Which of the following shows the correct free-body diagram for the forces acting on the ball when terminal velocity is reached?
A
B
C
E
D