Materials Included In Kit

Additional Materials Required

Prelab Preparation

Cut twenty 60-cm lengths of string. Each student group should receive two 60-cm pieces of string. Students may also cut their own string if this is appropriate for your classroom setting.

Safety Precautions

Caution students to be aware of their surroundings as they spin the Greek Waiter’s Tray. Students should practice the swinging technique with slow, steady motions before attempting more dramatic swings (i.e., above the head or in a complete circle). Clean up water spills immediately. Students should wear safety glasses when performing this activity.

Disposal

The materials should be saved for future use.

Lab Hints

• Enough materials are provided in this kit for 30 students working in groups of three or for 10 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. All materials are reusable.
• To prevent water spills, use clay at the bottom of the cup instead of water for beginning or practicing students. Students may “graduate” to water after they have proven their skill with the Greek Waiter’s Tray. If you do not feel comfortable with your students performing the experiment with water, have them use clay, and then demonstrate the Greek Waiter’s Tray to the class using water.
• Demonstrate the proper swinging technique before students begin the activity.
• Adding food dye to the water will make the water more visible, but it may also be more difficult to clean up if the water spills.
• Make sure students place the cup in the center of the tray to maintain proper balance.

Teacher Tips

• Once students master the Greek Waiter’s Tray technique, have a relay race in which students must maintain a rotating Greek Waiter’s Tray as they pass it from partner to partner while walking from one place to another. This activity can be fun as well as frustrating and will drive home the principles of force, friction and centripetal motion. Make sure students understand the principles of centripetal force before trying this fun extension. (It may be best to perform this part of the activity outdoors.)

Sample Data

Observations
As the Greek Waiter’s Tray is swung like a pendulum, the water level in the cup remains parallel to the plate. It appears that “down” for the cup is always perpendicular to the plate, instead of perpendicular to the ground.

As the tray and cup swing, a large force is felt on the hand. The force is attempting to pull the string out of the hand. The faster the tray is swung, the larger the force feels.

When the cup is swung in a complete circle, the water level continues to remain parallel to the plate and no water spills out of the cup. The cup and water do not tip or spill. A large force was felt as the tray was spun in a big circle.

Answers to Questions

1. Name and describe the forces acting on the cup of water as the Greek Waiter’s Tray is spinning.

   Three forces act on the cup as it spins. The force due to gravity is always acting on the cup. Frictional forces act between the cup and the plate. A centripetal force exists between the hand and the cup and tray. The centripetal force points toward the hand that is spinning the tray.

2. Why do you feel a force on your hand as the tray is spinning in a circle?

   The force that is felt on the hand is actually the reaction force of the swinging plate (Newton’s third law of action–reaction). The centripetal force causes the tray to move in a circle (instead of in a straight line). This force changes the motion of the tray and cup, and with every force, there is an equal and opposite force. The centripetal force points toward the hand spinning the cup and tray, whereas the reaction force, the apparent or fictitious centrifugal force, points away from the hand. (“Centrifugal force” makes it appear as if the tray and cup are initiating a force, when in reality they are reacting to a change in motion.)

3. Describe how the water responds to the motion of the Greek Waiter’s Tray.

   The water level remains parallel to the plate, even when the cup and tray are tipped on their side or completely upside-down. As long as the tray is moving in a circle, the cup feels a “downward” pull that is pointed towards the plate. This “downward” force is equal and opposite to the centripetal force that causes the tray to swing.

Introduction

Centripetal force causes objects to spin in a circle. Build a “Greek Waiter’s Tray” to observe and study the effects of circular motion on an object.

Concepts

• Rotational motion
• Acceleration due to gravity
• Newton’s laws of motion
• Centripetal force
• Gravity simulation

Background

An object in motion will stay in motion unless acted on by an outside force. This is Newton’s first law of motion. The outside force may act in two ways: it can change the speed of the object without affecting its direction (linear acceleration), or it can change the direction of an object with or without affecting its speed (centripetal acceleration). When a force pushes or pulls an object into a circular motion, this force is known as a centripetal force (see Figure 1). Centripetal means “toward the center.” As an object moves in a circle, the change in its direction of motion is always toward the center of the circle, while the direction of its motion (its velocity) is always perpendicular to the radius of the circle.

The centripetal force is related to both the speed of rotation and the radius of rotation as given by Equation 1.

F_c = centripetal force
m = mass
v = velocity
r = radius

Another common term associated with circular motion is centrifugal force. This term is used to describe the tendency of an object to be thrown outward when spun in a circle. However, this centrifugal force is not a force at all, but rather an apparent pseudo-force that is a consequence of Newton’s first law of motion (inertia). The inertia of an object will cause an object to continue to travel in a straight line until acted on by an outside force.

The “Greek Waiter’s Tray” takes advantage of centripetal force, inertia, and friction to keep a cup of water from spilling as the tray is spun around in circles. The forces acting on the cup of water in a Greek Waiter’s Tray are illustrated in Figure 2. As long as the Greek Waiter’s Tray is not bumped or jerked, the cup will remain stable on the Greek Waiter’s Tray as it is swung.

Materials

Safety Precautions

Follow the teacher’s guidance when preparing to and when swinging the Greek Waiter’s Tray. Use caution when rotating the Greek Waiter’s Tray. Practice the swinging technique with slow, steady motions before attempting more dramatic swings (i.e., above the head or in a complete circle). Clean up water spills immediately. Wear safety glasses when performing this activity.

Procedure

Assembly

1. Obtain two 60-cm pieces of string and the plastic platform with holes.
2. Insert one end of one piece of string through one of the holes in the platform, and then through the hole diagonally across (see Figure 3).
   {13532_Procedure_Figure_3}
3. Repeat step 2 using the other piece of string, inserting the string into the two remaining holes in the plate. Make sure the ends of the string come out on the same side of the plate.
4. Gather the four ends of string and use the string to pick up the platform (see Figure 4).
   {13532_Procedure_Figure_4}
5. Hold the platform parallel to the ground and then tie the four ends of string together near their ends using a simple overhand knot as shown in Figure 4. The result will be that when the string is held by the knot, the plate should hang parallel to the ground.

Experiment

1. Obtain an empty plastic cup and place a thumb-size piece of clay at the bottom or, if directed by your instructor, fill the cup half-full with water.
2. Place the cup in the center of the plate and pick the plate up by the knot in the string (see Figure 2 in the Background section).
3. Carefully swing the tray back and forth like a pendulum a couple times. Be careful not to bump the tray or hit anybody or anything with the tray. How does the water level in the cup behave as the cup twirls? Record observations in the worksheet. (If water is not being used, observe the teacher demonstration and record observations in the worksheet.)
4. After several minutes of practice, attempt larger swings or circles. Do you feel a force on your hand? Record your observations in the worksheet.
5. Once you feel comfortable enough, attempt to swing the tray in a complete circle. How do the cup and water behave as the tray was swung in a complete circle? Record observations in the worksheet.
6. Allow each person in the group to experiment with the “Greek Waiter’s Tray.”
7. Consult your instructor for appropriate storage procedures.

Student Worksheet PDF

13532_Student1.pdf