A pendulum is just a weight (the bob) hanging from a rod or string of length L. For small swings, its period depends on only two things:

T = 2π √(L / g)

Here L is the length from the pivot to the centre of the bob, in metres, and g is the strength of gravity, about 9.81 m/s² at Earth's surface. (This law comes straight from simple harmonic motion, which the Forces & Motion lab derives; here we just use it.)

Read the formula slowly, because three surprises are hidden in it:

• Mass is missing. A heavy bob and a light bob of the same length swing with the same period. Gravity pulls harder on the heavy bob, but it also has more mass to move, and the two effects cancel exactly.
• L is under a square root. Make the pendulum 4 times longer and T becomes only 2 times longer (√4 = 2). To double the period you need 4 times the length.
• g matters. Weaker gravity means a slower swing.