Six Metal Pendulum Bob Set

Quick Answer: The Six Metal Pendulum Bob Set is a group of six metal bobs that can be swapped on a simple pendulum, used to investigate which factors change the period of a swing. Because the bobs differ in metal, and so in mass, students can show that the period depends on length, not on mass.

Testing the Variables of a Simple Pendulum

The simple pendulum is one of the first experiments in which students control variables. Length, amplitude and bob mass can each be changed while the others are held fixed. A pendulum bob set built from six metals gives a range of masses without changing the design of the bob, so the mass test is fair: students time, say, twenty swings with each bob at the same length and find the period stays almost the same.

That result, predicted by the relation T = 2 pi times the square root of L over g, surprises many students, who expect a heavier bob to swing faster. The set then moves easily into the classic measurement of g. Keeping one bob and changing the length, students plot T squared against L; the straight-line graph has a slope of 4 pi squared over g, and a careful class often lands within a few per cent of 9.8 metres per second squared.

The lighter bobs also show something the heavy ones hide. Over a long run their swings die away sooner, because air resistance takes a larger share of their energy. It is a useful way to discuss why real experiments use dense bobs and count only the first swings.

Applications

• Showing that the period of a simple pendulum does not depend on bob mass
• Determining g from a graph of T squared against length
• Comparing damping of light and heavy bobs over many swings
• Density comparison of different metals using the bobs as specimens

Specifications

Care & Handling

• Store the pendulum bob set in its box or tray so bobs do not roll off the bench and dent.
• Check that threads or hooks are secure before each swing; a bob released mid-swing can travel some distance.
• Wipe the bobs dry after handling to prevent tarnish on reactive metals.

Why Choose LabEquip

Physics teachers running the simple pendulum practical need one bob set per bench group, often alongside a complete Simple pendulum kit. LabEquip lists the set in its General Items for mechanics lab equipment. Send quantities through the contact page.

Frequently Asked Questions

Does a heavier bob make a pendulum swing faster?

No. For small swings the period depends only on the length of the pendulum and the gravitational field strength. Timing each bob at the same length gives nearly identical periods, which is the point the experiment is designed to show.

Where should the pendulum length be measured to?

From the point of suspension to the centre of the bob, not to its top or bottom. Measuring to the wrong point is the most common source of error when calculating g.

How large should the swing be?

Keep the amplitude small, under about 10 degrees from vertical. The simple formula assumes small swings; larger ones take slightly longer and give a period that is too high.

How can the set be used to measure g?

Using one bob, time 20 oscillations at several lengths and calculate the period each time. Plot T squared against length. The slope equals 4 pi squared divided by g, so g is 4 pi squared divided by the slope.

Why do lighter bobs stop swinging sooner?

Air resistance acts on each bob, and a lighter bob has less energy to lose. The same drag therefore damps its swings more quickly, which is why dense bobs are preferred for timing experiments.

Which metals are the bobs made from?

The listing states six metals but does not name them or give their diameters. Sets like this usually use the same size in different metals so that only the mass changes, which keeps the experiment fair.

Last Updated: September 2026