Forces, Newton's laws and equilibrium

Learning objectives

Identify the forces acting on a body — weight, normal reaction, tension, friction, buoyancy, drag and elastic restoring force — and represent them on a free-body diagram as arrows drawn from a single point

State and apply Newton's three laws of motion, using ΣF = ma for single bodies and for connected systems, and identify an action–reaction pair as two forces of equal magnitude acting on different bodies

Find the resultant of coplanar forces by resolving into perpendicular components, and apply the condition for translational equilibrium ΣF = 0 to bodies at rest or moving at constant velocity, including bodies on an inclined plane

Distinguish static from dynamic friction, apply Ff ≤ μs R and Ff = μd R, and explain why the coefficient of static friction is generally the larger of the two

Apply Hooke's law F = kx to a loaded spring, and calculate the buoyancy force on a submerged or floating body from Fb = ρVg using Archimedes' principle

Apply Stokes' law Fd = 6πηrv to a small sphere moving slowly through a viscous fluid, stating the conditions under which the relationship holds