# 3 ways to use a pendulum

A pendulum consists of a weight suspended from a rod or rope that swings back and forth. Pendulums are found in time-keeping devices (for example, metronomes, grandfather clocks, seismometers, and oscillating incense burners) and can be used to illustrate complex physics problems.

## Steps

### Method 1 of 3: Understand Basic Pendulums

#### Step 1. Keep in mind that a pendulum is a weight that hangs freely at the end of a string

Before you start using a pendulum, you should know what it is and how it works. Fortunately, a pendulum is nothing more than a dangling weight that is allowed to swing back and forth. The rope is attached to a fixed point so that the only thing that moves is the weight and the rope.

• Hold the end of a necklace with a charm or yo-yo between your fingers and move the "weight" at the bottom. You will have made your first pendulum!
• A common example of a pendulum is the large weight that swings on a grandfather clock.

#### Step 2. Pull the weight back and release to use the pendulum

Be careful to keep the rope taut and release the weight without pushing it. The weight will swing from side to side and return to about the same height from which you released it.

• A pendulum will swing forever if nothing happens to slow it down or change direction.
• In reality, external forces (for example, friction and air resistance) will slow down a pendulum.

#### Step 3. Build a simple pendulum using string, a battery, and a folding rule to get a better understanding

If you are learning through hands-on activities or want to teach children how pendulums work, you can quickly build a pendulum to experiment with:

• Tie the end of a string to the center of a folding rule.
• Tie the opposite end to a battery or other small weight.
• Balance the folding rule on the backs of two identical chairs so that the battery hangs freely between them and can swing without hitting anything.
• Pick up the battery keeping the string taut and release it so that it swings from side to side.

#### Step 4. Recognize the scientific vocabulary for pendulums

Like most science activities, pendulums can only be understood and used if you know the words that describe them.

• Amplitude: the highest point that the pendulum reaches.
• Pendulum mass: another name for the weight at the end of the pendulum.
• Balance: the center point of a pendulum, where the weight rests when it is not moving.
• Frequency: the number of times the pendulum swings back and forth in a set amount of time.
• Period: the amount of time it takes for a moving pendulum to return to the same place.

### Method 2 of 3: Using Pendulums to Teach Basic Physics

#### Step 1. Keep in mind that pendulum experiments are a great way to teach the scientific method

The scientific method has been the backbone of scientific inquiry since Ancient Greece, and pendulums are easy to experiment with and see immediate results. When conducting any of the following experiments, take the time to formulate a hypothesis, discuss the variable you are testing, and compare the results.

• You should always do the experiment 5 or 6 times to make sure the results are consistent.
• Don't forget to only try one experiment at a time. Otherwise, you won't know what changed the pendulum swing.

#### Step 2. Shift the weight on the end of the rope to teach about gravity

One of the easiest ways to learn about the effects of gravity is by using a pendulum, and the results may surprise you. If you want to observe the effects of gravity, do the following:

• Pull the pendulum 10 cm back and release it.
• Count the period of the pendulum using a stopwatch. Repeat 5-10 times.
• Add a heavier pendulum mass to the pendulum and repeat the experiment.
• The period and the frequency will be exactly the same. This is because gravity affects all weights equally. For example, a penny and a brick will fall at the same speed.

#### Step 3. Change where you release the weight to learn about range

By pulling the string further up, you increase the width (or high point) of the pendulum. However, does this change the speed at which the pendulum returns to your hand? Repeat the previous experiment, but this time, instead of changing the weight, pull the pendulum 20 cm back.

• If you do everything correctly, the period of the pendulum will not change.
• Changing the amplitude does not change the frequency, a fact that will come in handy in trigonometry class, in sound science, and in many other areas.

#### Step 4. Change the length of the rope

Repeat the above experiment but don't change the amount of weight you add or the height from which you release it but instead use a much shorter or longer rope.

### This time, you will definitely observe a change. In fact, changing the length of the string is the only thing that will change the period and frequency of a pendulum

#### Step 5. Delve into the physics of pendulums to learn about inertia, energy transfer, and acceleration

For older students or aspiring physicists, pendulums are a great way to learn about the relationships between acceleration, friction, and trigonometry. Search for "pendulum equations" or create your own experiments to find them. Here are some questions to consider:

• How fast does the pendulum mass move at the lowest point? How do you find the velocity of the mass at any point?
• How much kinetic energy does the pendulum mass have at any point on the pendulum? If you want help, use the following equation: kinetic energy = 0.5 x weight of pendulum mass x speed2.
• How can you predict the period of a pendulum based on the length of the string?

### Method 3 of 3: Using Pendulums to Take Measurements

#### Step 1. Make adjustments to the length of the string to measure time

Pulling the rope further back and changing the weight does not change the period, but it does extend or shorten the rope. This is how old clocks are made: by changing the length of the pendulum perfectly, you can make a period (or a full swing) take two seconds. Count the number of periods and you will know how much time has passed.

• Pendulum clocks are attached to gears so that the second hand of the clock moves each time the pendulum swings.
• In a grandfather clock, the weight produces the "tick" when oscillating in one direction and the "tock" when oscillating in the other direction.

#### Step 2. Use the pendulum to measure nearby vibrations, including earthquakes

Seismographs are machines that measure the intensity and direction of earthquakes, and are complex pendulums that move only when the Earth's crust moves. Calibrating a pendulum to only measure plate tectonics is extremely tricky, but you can turn almost any pendulum into a basic seismograph using a pen and some paper.

• Glue a pen or pencil to the weight at the end of the pendulum.
• Place a sheet of paper under the pendulum so that the pen touches it and can leave marks.
• Gently rock the pendulum but not the rope. The greater the force with which you swing the pendulum, the larger the marks on the sheet of paper. This corresponds to a larger "earthquake".
• Real seismographs have a rotating sheet of paper so that you can see the strength of the earthquake over time.
• Pendulums were used to measure earthquakes even as early as 132 AD. C. in China.

#### Step 3. Use a special pendulum known as the Foucault pendulum to check that the Earth rotates

People knew that the Earth rotated on its own axis, but Foucault's pendulum was the first visible proof of this concept. If you want to reproduce it, you will need a large pendulum, at least 5 m (16 ft) long with a weight greater than 11 kg (25 pounds) to minimize external variables (for example, wind or friction).

• Set the pendulum in motion enough that it can swing for a long time.
• As time goes by, you will notice that the pendulum swings in a different direction from the one in which it started.
• This occurs because the pendulum moves in a straight line while the Earth below it rotates.
• In the northern hemisphere, the pendulum will rotate clockwise, and in the southern hemisphere, it will rotate counterclockwise.
• It is complicated, but you can use the Foucault pendulum to calculate your latitude using a trigonometric equation.