Centripetal Force Science Project: Amazing Ways to Explore Circular Motion
String & Weight Orbit Lab
Whirl a weight on a string and discover how centripetal force changes with speed, mass, and the length of the string. This hands-on STEM activity lets students feel the force that keeps objects moving in a circle, making physics interactive, engaging, and easy to understand.
Overview
The String & Weight Orbit Lab is a classic centripetal force science project that helps students explore the physics of circular motion through direct observation and physical sensation. In this experiment, a rubber stopper (or similar object) is attached to one end of a string, while a set of hanging washers is attached to the other end. The string passes through a smooth tube held vertically in the hand.
As the orbiting mass spins in a horizontal circle, the tension in the string provides the centripetal force needed to keep it moving in its circular path. Students can actually feel this tension through the tube and observe how changing the speed, mass, or radius affects the force required to maintain equilibrium.
This activity is ideal for middle school and high school students learning about forces, circular motion, and Newton’s Laws of Motion. It can be performed as a simple demonstration or extended into a quantitative investigation by measuring rotation speed and comparing it with theoretical predictions.
Time : 25–35 minutes
Age Group : Middle school students
Complexity : Intermediate
Activity Materials Required
- One smooth plastic or metal tube (15–20 cm long, such as a pen casing or short PVC pipe)
- One 1.2-meter length of smooth nylon string
- One rubber stopper (or a small taped bundle) to serve as the orbiting mass
- Three to five metal washers or small weights
- Stopwatch or phone timer
- Ruler or measuring tape
- Small piece of colored tape (for marking the equilibrium position)
- Safety glasses
Step-by-Step Instructions
Step 1: Assemble the Apparatus
Thread the nylon string through the tube. Tie the rubber stopper securely to one end of the string and attach the metal washers to the opposite end.
Step 2: Set the Orbit Radius
Hold the tube vertically with one hand. Allow approximately 40 cm of string to extend above the tube for the orbiting mass.
Place a small piece of colored tape on the string just below the bottom of the tube. This tape serves as a reference marker to help maintain a constant orbit radius during the experiment.
Step 3: Create Circular Motion
Wear safety glasses before beginning. Swing the rubber stopper gently in a horizontal circle above your head while holding the tube vertically.
Gradually increase the spinning speed until the hanging washers remain at the same height and the tape marker stays aligned with the bottom of the tube.
This is the equilibrium condition, where the tension in the string exactly balances the weight of the hanging washers.
Step 4: Measure the Rotation Rate
Use a stopwatch to count the number of complete rotations in 10 seconds. Record the total number of rotations and calculate the rotation rate (rotations per second). Repeat the measurement two or three times and calculate an average value for greater accuracy.
Step 5: Observe the Effect of Speed
Spin the stopper slightly faster than the equilibrium speed.
Observe that the hanging washers move upward toward the tube, indicating that the increased centripetal force is pulling more string through the tube.
Now slow the stopper below the equilibrium speed.
Notice that the hanging washers move downward because there is not enough centripetal force to keep the system balanced.
Step 6: Investigate Different Variables
Change only one variable at a time and repeat the experiment.
Try the following investigations:
- Increase or decrease the orbiting mass.
- Add or remove hanging washers.
- Increase or decrease the length of the string above the tube (orbit radius).
For each change, determine the new spinning speed needed to keep the hanging washers stationary.
Record your observations in a table.
Step 7: Compare Your Results
Discuss the following questions:
- Which setup required the fastest spinning speed?
- How did increasing the orbiting mass affect the required speed?
- What happened when the orbit radius became larger?
- How did adding more hanging washers change the equilibrium condition?
The Science Behind the Activity
When an object moves in a circle, it is constantly changing direction. Even if its speed remains constant, its velocity changes because velocity depends on both speed and direction.
This continuous change in direction requires an inward force called centripetal force.
The magnitude of centripetal force is given by:
Fc = mv²/r
where:
- F = Centripetal force
- m = Mass of the orbiting object
- v = Speed of the object
- r = Radius of the circular path
In this experiment, the tension in the string provides the centripetal force.
The hanging washers create this tension by pulling downward through the tube.
When the stopper spins at exactly the correct speed, the upward tension in the string equals the downward weight of the hanging washers. This is known as the equilibrium condition.
If the stopper spins too quickly, greater centripetal force is required, causing the hanging washers to move upward.
If it spins too slowly, the washers move downward because the existing tension is no longer sufficient to maintain equilibrium.
This activity clearly demonstrates how speed, mass, and radius work together to determine the centripetal force required for circular motion.
Pro Tip
Place a small piece of colored tape on the string just below the tube before beginning the experiment.
During spinning:
- If the tape moves upward, you are spinning too fast.
- If the tape moves downward, you are spinning too slowly.
- If the tape remains aligned with the bottom of the tube, the system is in equilibrium.
This simple marker helps students maintain a constant orbit radius and makes the concept of centripetal force much easier to observe.
Expected Results
Students will observe that:
- Increasing the spinning speed requires greater centripetal force.
- Increasing the hanging mass increases the tension in the string.
- A larger orbit radius changes the spinning speed required for equilibrium.
- A heavier orbiting mass also affects the speed needed to maintain circular motion.
These observations match the mathematical relationship between centripetal force, mass, speed, and radius, helping students connect theoretical physics with real-world motion.
Conclusion
The String & Weight Orbit Lab transforms a physics equation into a hands-on learning experience.
Instead of simply calculating centripetal force, students can feel the tension in the string, observe how equilibrium changes, and understand how mass, speed, and radius interact during circular motion.
The concepts explored in this activity explain many real-world applications, including satellites orbiting Earth, cars turning around curves, amusement park rides, and athletes swinging a hammer or discus.
By combining observation, measurement, and experimentation, this centripetal force science project builds a strong foundation for understanding circular motion and prepares students for more advanced studies in physics and engineering.
Frequently Asked Questions
1. What provides the centripetal force in this experiment?
The tension in the string provides the centripetal force. The hanging washers create this tension by pulling downward through the tube, while the string pulls inward on the orbiting mass.
2. Why do the hanging washers move upward when I spin faster?
Spinning faster requires greater centripetal force. The increased tension pulls more string through the tube, causing the hanging washers to rise toward the tube.
3. What happens if I spin too slowly?
The string tension becomes too small to balance the hanging washers, so they move downward and the system is no longer in equilibrium.
4. What would happen if the string broke?
The orbiting mass would continue moving in a straight line tangent to the circular path at the instant the string broke. This demonstrates Newton’s First Law of Motion.
5. How is this experiment similar to satellites orbiting Earth?
In this experiment, string tension provides the centripetal force. For satellites, Earth’s gravity provides the centripetal force that keeps them in orbit.
6. Is centripetal force the same as centrifugal force?
No. Centripetal force is the real inward force that keeps an object moving in a circle.
Centrifugal force is an apparent outward force experienced only from the viewpoint of a rotating reference frame. From the perspective of an outside observer, only the inward centripetal force acts on the orbiting object.