09 Newton's second law and the equation of motion
Statement
- The rate of change of momentum of a body is proportional to the resultant force acting on it and takes place in the direction of that force. With suitable units the constant of proportionality is 1: F = d(mv)/dt.
- If the mass is constant, the equation of motion becomes F = m dv/dt = m a: the resultant force produces an acceleration in its own direction, directly proportional to the force and inversely proportional to the mass.
- Mass is a constant scalar measuring inertia; weight is the force of the Earth's attraction and changes with position: W = m g, with g ≈ 9.8 m/s².
Units of force
1newton (N)
Definition
- force giving 1 kg an acceleration of 1 m/s²
Value
- 1 kg·m/s²
2dyne
Definition
- force giving 1 g an acceleration of 1 cm/s²
Value
- 1 g·cm/s²
3kg-wt (kgf)
Definition
- weight of a mass of 1 kg
Value
- 9.8 N
4g-wt (gf)
Definition
- weight of a mass of 1 g
Value
- 980 dyne
- Between the two absolute units: 1 N = 10⁵ dyne.
- In F = ma only absolute units are used, so a force given in kg-wt or g-wt must first be converted.
Worked examples
- A constant force of 12 N acts on a 3 kg body at rest. a = 12/3 = 4 m/s²; v = v₀ + at = 0 + 4(5) = 20 m/s after 5 s.
- A force of 2 kg-wt acts on a 5 kg body. F = 2 × 9.8 = 19.6 N, so a = 19.6/5 = 3.92 m/s².