A pendulum consists of a mass suspended on a wire or cable that rocks back and forth. Pendulums are found in ancient clocks, metronomes, seismometers, and certain incense burners, and can be used to explain complex physics problems.
Steps
Method 1 of 3: Understanding the Basics of the Pendulum
Step 1. A pendulum is a mass held on the free end of a wire
Before you start using a pendulum, you need to know what it is and how it works. Fortunately, a pendulum is nothing more than a hanging mass that can swing back and forth. The wire is attached to a fixed point so that only the mass and the wire are moving.
- Hold the top of a pendant necklace or yo-yo between your fingers and move the "mass" at the bottom. You have created your first pendulum!
- A common example of a pendulum is found in old wall clocks.
Step 2. To use a pendulum, grab and pull the mass back and let go
Make sure you hold the thread it hangs straight on and let it go without pushing it. The mass will rock back and forth, returning to about the same height you dropped it from.
- A pendulum swings forever if nothing is done to slow it down or change its direction.
- In reality, external forces such as friction and air resistance end up slowing down a pendulum.
Step 3. Build a simple pendulum with a wire, a battery and a sprig for a better understanding
If you want to learn practical activities or want to teach children how pendulums work, you can quickly build a pendulum to experiment with:
- Tie one end of a thread about halfway down a branch or stick.
- Tie the other end to a battery or other small weight.
- Balance the branch between two identical chairs so that the battery hangs freely between them and can swing without hitting anything.
- Take the battery, keeping the wire taut, and let it go so that it rocks back and forth.
Step 4. Learn the scientific terms for pendulums
As with many scientific activities, it is possible to understand and use pendulums only by knowing the words that describe them.
- Amplitude: the highest point reached by the pendulum.
- Weight: another name for the mass that is at the bottom of the pendulum.
- Equilibrium: the central point of a pendulum; where the weight is when not moving.
- Frequency: the number of times the pendulum swings back and forth in a certain period of time.
- Period: the amount of time it takes for the pendulum to return to the same point.
Method 2 of 3: Using Pendulums to Teach the Basics of Physics
Step 1. Pendulum experiments are a great way to teach the scientific method
The scientific method has been the basis of scientific research since the time of the ancient Greeks, and the pendulums are easy to build and show instant results. When conducting any of the following experiments, take some time to formulate a hypothesis, talk about the variables you are testing, and compare the results.
- Always perform the experiments 5-6 times to make sure the results are consistent.
- Remember to go through one experiment at a time - otherwise you won't know what changed the pendulum's motion.
Step 2. Change the weight at the bottom of the wire to teach gravity
One of the simplest ways to learn the effects of gravity is with a pendulum, and the results may surprise you. To see the effects of gravity:
- Pull the pendulum 10 centimeters and then let it go.
- Use a stopwatch to measure the period of the pendulum. Repeat 5-10 times.
- Add more weight to the pendulum and repeat the experiment.
- The period and frequency will be exactly the same! This is because gravity affects all weights equally. A dime and a brick, for example, fall at the same rate.
Step 3. Changing where you let the weight go helps explain amplitude
When you stretch the wire higher, you have increased the amplitude, or the highest point, of the pendulum. But does this affect how quickly you get back into your hand? Repeat the experiment above, but pull the pendulum back 20 centimeters instead of changing the weight.
- If you have done everything correctly, the pendulum period will not change.
- The amplitude will change but not the frequency, a notion that comes in handy in trigonometry, in the study of sound and in many other fields.
Step 4. Change the length of the thread
Repeat the experiment above, but instead of changing the weight or height you drop it from, use a shorter or longer wire.
This time you will certainly notice a change. In fact, changing the length of the wire is the only way to change the period and frequency of a pendulum
Step 5. Dig deeper into pendulum physics to learn about inertia, energy transfer, and acceleration
For more advanced students or aspiring physicists, pendulums are a great way to learn the relationship between acceleration, friction, and trigonometry. Look for the "equations of motion of the pendulum," or create your own experiments to discover them. Some questions to consider:
- How fast does the weight move at its lowest point? How do you detect the speed of the weight at any point?
- How much kinetic energy does the weight in the pendulum have at any given moment? To help you, use the equation: Kinetic energy '=.5 x Mass of weight x Speed2
- How can you predict the period of a pendulum based on the length of the wire?
Method 3 of 3: Using Pendulums to Take Measurements
Step 1. Adjust the length of the thread to measure time
When you pull the yarn farther and change the weight, you don't affect the period, but shortening or lengthening the yarn does. This is how old watches were made - if you change the length of the pendulum perfectly you can create a period, or a full swing, of two seconds. Count the number of periods and you know how much time has passed.
- Pendulum clocks are attached to gears so that with each movement of the pendulum the second hand of the clock moves.
- In an antique watch, the weight swinging to one side creates the "tick" and the return creates the "knock."
Step 2. Use the pendulum to measure nearby vibrations, including earthquakes
Seismographs, the machines that measure the intensity and direction of earthquakes, are complex pendulums that only move when the earth's crust moves. Although calibrating a pendulum to only measure the movement of the tectonic plates is definitely complicated, you can turn almost any pendulum into a simple seismograph using a pen and paper.
- Attach a pen or pencil to the weight at the bottom of the pendulum.
- Place a piece of paper under the pendulum so that the pen or pencil touches it leaving a mark.
- Gently shake the pendulum, not the wire. The more you shake the pendulum, the wider the marks will be left on the paper. This corresponds to a major "earthquake."
- Real seismographs have a rotating piece of paper to show the power of the earthquake over time.
- Pendulums were used to measure earthquakes as early as 132 BC in China.
Step 3. Use a special Foucault pendulum to demonstrate the rotation of the Earth
Although it was known how the Earth rotates on its own axis, Foucault's pendulum was one of the first tangible proofs of this concept. To reproduce it, you will need a large pendulum, at least 5 meters long and weighing over 9 kilos, to minimize external variables such as wind and friction.
- Set the pendulum in motion, moving it enough for it to swing for a long time.
- As time goes by, you will notice how the pendulum swings in a different direction from the initial one.
- This happens because the pendulum moves in a straight line while the Earth below it is spinning.
- In the northern hemisphere the pendulum will move clockwise, in the southern hemisphere it will move counter-clockwise.
- Although complicated, you can use Foucault's pendulum to calculate your latitude using a trigonometric equation.
Advice
- You may need two other people to perform these experiments accurately - one using the pendulum and the other measuring time.
- If you want to make a more accurate pendulum, use another wire to keep the weight at the desired height. Burn the end of the line to "drop" the weight. This prevents you from accidentally pushing the weight forward or to the side when you let it go.
- Some believe that the pendulum also has special powers of divination.
