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Body force $\mathbf{f}$ in the momentum equation typically represents:
A{'text': 'Surface forces only', 'label': 'A'}
B{'text': 'Forces acting on every fluid PARTICLE — most commonly GRAVITY ($\\mathbf{f} = \\mathbf{g}$), Coriolis, electromagnetic, etc.', 'label': 'B'}
C{'text': 'Pressure', 'label': 'C'}
D{'text': 'Viscosity', 'label': 'D'}
Answer & Solution
Correct answer: B. {'text': 'Forces acting on every fluid PARTICLE — most commonly GRAVITY ($\\mathbf{f} = \\mathbf{g}$), Coriolis, electromagnetic, etc.', 'label': 'B'}
Body forces act per unit mass on every particle (gravity, electromagnetic, Coriolis in rotating frames). They appear separately from surface forces (pressure + viscous stress). For most engineering flows, $\mathbf{f} = \mathbf{g}$ — gravity.
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