Key Equations

Forces in Action - OCR A-Level Physics

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Moment of a force
$$\text{moment} = F \times d$$
  • Where:
    • $F$ = N
    • $d$ = m
    • $moment$ = N m
d must be the perpendicular distance from the pivot to the line of action of the force.
Torque of a couple
$$\tau = F \times d$$
  • Where:
    • $F$ = N
    • $d$ = m
    • $\tau$ = N m
d is the perpendicular distance between the lines of action of the two equal and opposite forces.
Density
$$\rho = \frac{m}{V}$$
  • Where:
    • $\rho$ = kg \(m^{-3}\)
    • $m$ = kg
    • $V$ = \(m^{3}\)
Density is a material property. Convert cm^3 to \(m^{3}\) by multiplying by \(10^{-6}\).
Pressure
$$p = \frac{F}{A}$$
  • Where:
    • $p$ = Pa (N \(m^{-2}\))
    • $F$ = N
    • $A$ = \(m^{2}\)
F must be the force acting perpendicular to the surface of area A.
Pressure in a fluid column
$$p = \rho g h$$
  • Where:
    • $p$ = Pa
    • $\rho$ = kg \(m^{-3}\)
    • $g$ = m \(s^{-2}\)
    • $h$ = m
h is the vertical depth below the surface. This gives only the pressure due to the fluid; add atmospheric pressure for total pressure.
Weight
$$W = mg$$
  • Where:
    • $W$ = N
    • $m$ = kg
    • $g$ = m \(s^{-2}\)
Weight is the gravitational force on a mass. Use g = 9.81 m \(s^{-2}\) unless stated otherwise.
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