Forces, density & pressure
In this AS-level lesson we meet the different kinds of force that act on objects — weight in a gravitational field, the electric force on a charge, the normal contact force, friction and drag, and upthrust — and learn that a body's weight acts at its centre of gravity. We then study the turning effect of a force: the moment, defined as force times perpendicular distance, the couple, and the torque of a couple. From there we build up equilibrium precisely: the principle of moments, the two conditions for equilibrium (no resultant force AND no resultant moment), and the closed vector triangle for three coplanar forces. Finally we define density ρ = m/V and pressure p = F/A, derive the hydrostatic pressure equation p = ρgΔh from first principles, and explain the origin of upthrust as a difference in pressure — Archimedes' principle.
By the end you should be able to (NSSCAS Physics (AS) 1.5):
- Describe the force on a mass in a uniform gravitational field (W = mg) and on a charge in a uniform electric field (F = EQ)
- Identify the normal contact force, and explain frictional and viscous (drag) forces including air resistance
- Explain the origin of the upthrust on a body in a fluid as a difference in hydrostatic pressure (Archimedes' principle)
- Apply the concept that the weight of a body acts at a single point, its centre of gravity
- Define the moment of a force as the product of force and perpendicular distance from the pivot to the line of action, and apply it to everyday tools
- State that a couple is a pair of equal, opposite, parallel forces, and define and use the torque of a couple as one force times the separation
- State and apply the principle of moments
- Recall and apply the conditions for equilibrium — no resultant force AND no resultant moment — and use a closed vector triangle for three coplanar forces in equilibrium
- Define and use density ρ = m/V and pressure p = F/A
- Derive, from the definitions of pressure and density, and use the hydrostatic pressure equation p = ρgΔh
Miss Elizabeth and Mike talk through the whole topic — with the figure and working drawn live.