alleryNewton's second law is often written as F = m a. This form is strictly valid when:
AThe body is moving at relativistic speeds
BThe mass of the body is constant
CFriction is the only external force
DThe force is impulsive in nature
Answer & Solution
Correct answer: B. The mass of the body is constant
1. The general form is F = dp/dt where p = m v.
2. Expanding: dp/dt = m (dv/dt) + v (dm/dt).
3. The familiar form F = m a follows only when dm/dt = 0, i.e. mass is constant.
4. Variable-mass systems (rockets) need the full F = dp/dt form.
_Source: syllabus Physics Ch 4 "Laws of Motion", §4.5 Eq. (4.3)–(4.5), p. 54_
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