A pendulum consists of a weight suspended from a rod or rope that swings back and forth. Pendulums are common in timekeeping devices such as metronomes, pendulum clocks, seismometers, and swing incense burners, and can be used to illustrate complex physics problems.
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Method 1 of 3: Understanding the Basic Pendulum
Step 1. Know that the pendulum is a weight hanging freely on the end of the string
Before you start using a pendulum, you need to know what a pendulum is and how it works. Fortunately, a pendulum is nothing more than a hanging weight that can swing back and forth. The rope is tied at a fixed point so that only the weight and rope move.
- Hold the end of a pendant necklace or yo-yo toy between your fingers and move the "weight" at the bottom. You have made your first pendulum!
- A common example of a pendulum is a large swinging weight on a pendulum clock.
Step 2. To use a pendulum, pull the weight back and then release it
Make sure you keep the rope tight and remove the weight without pushing it. The weight will swing back and forth, returning to almost the same height as when you dropped it.
- The pendulum will swing forever if nothing slows it down or changes its direction.
- In fact, external forces such as friction and air resistance will slow down the pendulum.
Step 3. Make a simple pendulum with a string, battery, and tape measure to better understand it
If you're learning through hands-on activities or want to teach kids how a pendulum works then you can quickly build a pendulum to experiment with:
- Tie one end of the rope to the center of a wooden tape measure or pole.
- Tie the other end to a battery or other small load.
- Balance the tape measure on the backs of two equal chairs so that the battery hangs freely between them and can swing without hitting anything.
- Lift the battery, keeping the rope taut, and release it so that it swings back and forth.
Step 4. Identify the scientific vocabulary for the pendulum
As with most scientific activities, understanding and using a pendulum is only possible if you know the words that describe it.
- Amplitude: The highest point the pendulum reaches.
- Bob: Another name for the load on the tip of the pendulum.
- Equilibrium: the midpoint of the pendulum; where the load is if it doesn't move.
- Frequency: The number of times the pendulum swings back and forth in a given time.
- Period: The amount of time it takes for a moving pendulum to return to the same place.
Method 2 of 3: Using a Pendulum to Teach Basic Physics
Step 1. Note that pendulum experiments are a great way to teach the scientific method
The scientific method has been the backbone of scientific research since ancient Greece, and the pendulum is an object that is easy to manufacture and produces results quickly. When conducting any of the following experiments, take the time to formulate a hypothesis, discuss which variable you are testing, and compare the results.
- Always experiment up to 5-6 times to ensure consistency of your results.
- Remember to only try one experiment at a time-- otherwise you won't know what changes the swing of the pendulum.
Step 2. Change the weight at the end of the rope to teach gravity
One of the simplest ways to learn about the effect of gravity is through a pendulum, and you might be surprised at the results. To see the effect of gravity:
- Pull the pendulum 10 cm and release it.
- Use a stopwatch to time the pendulum period. Repeat 5-10 times.
- Add a heavier bob to the pendulum and repeat the experiment.
- The period and frequency will be exactly the same! This is because gravity affects all loads equally. For example, a coin and a brick will fall at the same speed.
Step 3. Change where you drop the load to study the amplitude
As you pull the string higher, you have increased the amplitude or height of the pendulum. However, does that change how quickly the pendulum returns to your hand? Repeat the above experiment, but this time pull the pendulum 20 centimeters away and don't change the load.
- If you did everything correctly, the period of the pendulum will not change.
- Changing the amplitude doesn't change the frequency, a fact that will come in handy in trigonometry, sound science, and many other fields.
Step 4. Change the length of the rope
Repeat the experiment above, but instead of changing how much weight you add or how high you drop it, use a shorter or longer rope.
This time you will definitely notice the changes. In fact, changing the length of the string is the only thing that will change the period and frequency of the pendulum
Step 5. Learn more about pendulum physics to learn about inertia, energy transfer, and acceleration
For more senior students or aspiring physicists, pendulums are a great way to learn the relationship between acceleration, friction, and trigonometry. Search for “pendulum equations,” or design your own experiments to find them. Some questions to consider:
- How fast does bob move at its lowest point? How do you find the speed of the bob at each point?
- How much kinetic energy does the bob have at any point in the pendulum? As an aid, use the equation: Kinetic Energy = 0.5 x Bob's Mass x Velocity. Velocity2
- How can you predict the period of a pendulum based on the length of the string?
Method 3 of 3: Using a Pendulum to Take Measurements
Step 1. Adjust the length of the rope to measure the time
While pulling the rope back further and changing the load cannot change the period, lengthening or shortening the rope can change the period. This is how to make an old clock - if you change the length of the pendulum perfectly then you can make a period or full swing, which takes two seconds. Count the number of periods and you know how much time has passed.
- The pendulum clock is attached to the gear so that every time the pendulum swings, the second hand on the clock will move.
- In a pendulum clock, a weight that swings in one direction produces a "tick" and swings back to produce a "knock."
Step 2. Use your pendulum to measure nearby vibrations, including earthquakes
A seismograph, a machine that measures the intensity and direction of earthquakes, is a complex pendulum that moves only when the Earth's crust moves. While calibrating a pendulum just to measure plate tectonics is extremely complicated, you can turn almost any pendulum into a basic seismograph with just pen and paper.
- Glue a pen or pencil to the weight at the end of the pendulum.
- Place a piece of paper under the pendulum so that the pen touches the paper and makes markings.
- Gently rock the pendulum, but don't shake the string. The harder you shake the pendulum, the bigger the marks on your piece of paper. This is associated with a larger “earthquake”.
- The original seismograph had a rotating piece of paper so you could see the strength of an earthquake over time.
- The pendulum has been used to measure earthquakes since 132 AD in China.
Step 3. Use a special pendulum called Foucault's Pendulum to prove that the earth rotates
Although today people know that the earth rotates on its axis, Foucault's Pendulum is the earliest evidence of this concept. To replicate it you will need a large pendulum, with a minimum length of 4.9 meters and a weight of over 11.3 kg, to minimize external variables such as wind or friction.
- Move the pendulum in such a way that it can swing for a long time.
- As time goes on, you will notice that the pendulum swings in a different direction from when you started the swing.
- This happens because the pendulum moves in a straight line while the earth below rotates.
- In the northern hemisphere the pendulum will move clockwise and in the southern hemisphere the pendulum will move counter-clockwise.
- Although it's complicated, you can use Foucault's Pendulum to calculate latitude using trigonometric equations.
Tips
- You may need two people to perform this experiment accurately - one person using a pendulum and the other person keeping track of time.
- If you want to make a more accurate pendulum, use another rope to hold the weight at the desired height. Burn the end of the rope to “drop” the weight. This will prevent you from accidentally pushing the weight forward or sideways when you release it.
- Some people believe that the pendulum also has special divining powers.
