Newton's First Law

Newton's first law: an object stays at rest, or keeps moving in a straight line at a constant speed, unless a net force acts on it. This tendency to keep its motion is called inertia.

"Net force" is the sum of all the forces on the object. When the forces cancel, the net force is zero and the motion does not change. The object is in equilibrium:

ΣF = 0

At rest: the two forces cancel, so the net force is zero.

The weight of an object of mass m is mg, with g = 9.8 N/kg: each kilogram weighs 9.8 N. A 2 kg lamp hangs at rest, so its cord pulls up with the same size as its weight: T = 2 · 9.8 = 19.6 N.

A puck sliding on ice at a constant 3 m/s also has zero net force on it: no force is needed to keep it moving.

On the board. A physics challenge goes the same way every time: set the axes, draw the forces, then write the sum of the forces along each axis in symbols, like ΣFy = T − mg = 0. Only then put the numbers in and solve; the answer carries its unit in brackets, like T = 29.4 [N].