Students will investigate the action-reaction principal (Newton's third law of motion) by creating a water-propelled engine. By observing the device in action and changing certain variables, students will explore the properties of engines and the dynamics behind directionality and thrust.
- Important safety note: Always wear heavy work gloves when handling cans and nails, as the holes created will be sharp and can cut. Instruct any students handling cans and nails to wear heavy work gloves.
- With older students, divide the class into small groups. Set up one or more water tubs around your classroom and fill the tubs with about 20 cm of water. Have no more than one or two teams test their engines at one time. Discuss the importance of keeping the water in the tubs.
- With younger students, do this lesson together as a class or, if you have additional adult assistants, divide the class into groups led by an adult who will manage the nails and poking holes in the cans.
- When engines are filled, they should not be raised any higher than the rim of the tub. This will keep water that's coming out of the holes from falling on the surrounding floor. Be sure to recycle the cans at the conclusion of the activity.
- Practice making holes in a can in advance of doing the lesson so you will be able to demonstrate and assist with the technique.
This activity simulates the operation of what's known as the aeolipile (pronounced "ee-oh-li-pile") engine invented by Hero of Alexandria more than 2,000 years ago.
Hero’s engine was a spinning copper sphere that was propelled by a thrust produced by a jet of steam. The engine was an early demonstration of the action-reaction principle (Newton's third law of motion) described by Sir Isaac Newton 1,700 years later: For every action, there is an equal and opposite reaction.
Hero’s invention was not self-contained because external heat was applied. Therefore, it was not a true rocket device. However, it's a useful example for exploring the dynamics of engines, such as the ones that power rockets NASA uses to launch spacecraft.
In this activity, students will create a device inspired by Hero's engine, giving them the opportunity to observe and experiment with some of these dynamics, such as directionality and thrust.
To make the engine, angled holes are punched into the side of a soda can. A string is tied to the pull tab to support the can and permit it to rotate.
To power the engine, the can is immersed in water and pulled out. Gravity draws the water through the angled holes, and streams shoot out in either a clockwise or counterclockwise direction. The streams produce an action force that is accompanied by a reaction force that causes the can to spin in the opposite direction.
There are many potential variables with the soda can engine. Hole size, hole angle, number of holes, and the placement of the hole above the base of the can all affect the thrust produced.
The most significant of these variables is the hole placement. The greatest thrust occurs when the holes are punched just above the bottom of the can. This is due to the effects of gravity. The strength of the water stream (thrust) is based on the pressure and water pressure in a container is the greatest at the bottom.
The pressure at the top of the water in the container is zero (ignoring air pressure in this example). Water will dribble out of a hole near the top of the water column. The water stream gets stronger the closer the hole is to the bottom of the container.
Thrust stops when water drains out to the level of the holes. Holes at the bottom of the container produce thrust for a longer time. However, the magnitude of the thrust diminishes as the water column lowers (pressure drops with column height).
The effects of the other variables are numerous. For example, more holes means more water streams out of the can, but the water drains more quickly. Large holes drain water more quickly than small holes. Holes angled in different directions counteract each other. Holes that are not angled produce water streams that leave the can perpendicular, and no rotation occurs. The objective is to have students discover the effects of the different variables themselves.
Soda Can Engine Demonstration
- Don heavy work gloves and explain to students that we will be punching holes in cans using sharp nails. These holes can be very sharp and can easily cut skin. Explain that anyone who handles the cans or nails must wear heavy work gloves.
- Demonstrate the procedure for punching holes in the cans. The idea is to punch the hole without crushing the can sides. Place the nail point near the bottom rim of the can. Apply pressure with the nail, rotating it, if necessary, to make the hole. (Note: You'll create angled holes at a later step.)
- Rotate the can a quarter turn, and punch a second hole.
- Repeat the steps above two more times to make four holes in total. (Cans may have different numbers of holes.)
- Tie a string to the can's pop top.
- Immerse the can in the tub of water.
- When the can is full of water. Pull it out by the string and have students describe what is happening.
- Ask students what would happen if you punched a hole closer to the top of the can.
- Ask students if they can think of a way to make the can rotate. Try their suggestions if you have enough cans.
- Explain that you'd like to try creating an angled hole in the can to see what will happen. Insert the nail into one of the holes and pull or push the nail down toward the can to make a hole that is angled to one side.
- Repeat the experiment of filling the can with water and pulling it out by the string. Have students describe what is happening.
- Ask students what they'd like to try next. Try various suggestions, repeat the experiment, and have students describe what is happening and why.
- If students do not suggest it themselves, suggest angling all the holes in the same direction. Repeat the experiment and have students describe what is happening and why.
Student Team Experiment
- Provide each student team with copies of the worksheet.
- Review the instructions on the page and discuss the objective to design an experiment to increase the number of rotations a soda can engine makes.
- Make a list of student ideas for variables to test (e.g., hole size, number of holes, etc.). Discuss the importance of changing only one thing at a time. The first engine they create will serve as the baseline experiment. The second and third engines will vary just one thing each. For example, Can 1 will have medium size holes, Can 2 will have smaller holes, and Can 3 will have larger holes.
- Discuss ideas for keeping track of the number of rotations the cans make. For example, place a large bright mark on one side of the can, record a timelapse video.
- Give teams time to pick their experiment, devise their hypothesis, and write the procedures they will follow on their worksheet. Remind students that a hypothesis is what they expect will happen given a set of circumstances (e.g., a particular placement or number of holes will result in more can rotations).
- Remind students of the importance of wearing heavy work gloves at all times when handling cans and nails. Distribute the materials to the teams and have them begin their investigation.
- What provides the force that causes the cans to rotate?
There are a combination of factors that contribute to the force that causes the cans to rotate. The most important is the force of gravity. It attracts the water in the can and causes it to stream out of the holes. The shape of the holes directs the water streams. The diameter of the holes determines how fast the water streams out, etc.
- Which of Newton’s laws of motion explains why the can rotates in the opposite direction from the direction of the water streams?
Newton’s third law of motion
- Based on the results of the individual team experiments, what could you do to maximize the number of rotations of the soda can engines?
Possible answers: Combine best hole size with the right number of holes, best placement, etc.
- How might the results differ if we used an aluminum can that differs in thickness or overall dimensions from a soda can?
- Ask teams to state their experiment hypotheses, explain their procedures, and present their results. Make a list of the different ways one can increase the number of rotations their engine makes.
- Have teams submit their completed data sheet with their written conclusion based on their results.
- Have students explain the connection between the engine's action-reaction and how a rocket lifts off the ground.
- Secure other sizes of aluminum cans, such as those for tea or energy drinks, and have students hypothesize about performance. Then repeat the experiment using these cans.