Forces, density and pressure

You will be able to

• Apply the principle of moments, and the conditions for equilibrium, to a beam on a pivot or two supports.
• Define a couple and calculate its torque as one force × the perpendicular distance between the forces.
• Use a closed triangle of forces, and resolved components, for a body in equilibrium under three forces.

You will be able to

• Use Δp = ρgΔh to find the pressure difference between the top and bottom faces of a submerged object.
• Explain that upthrust arises from this pressure difference and calculate it as F = ρgV (Archimedes).
• Predict the reading of a spring balance, and whether an object floats, from its weight and the upthrust.