This activity is related to a Teachable Moment from Nov. 15, 2018. See "NASA’s ‘Cyber Monday’ Mars Landing to Deliver Science Firsts."

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### Overview

Students observe the motions of spinning eggs to determine which are raw and hard-boiled. They will apply what they have learned to understanding how scientists determine whether the center of a planet is liquid or solid.

### Management

• Students should work in groups of two to three.
• Working with raw eggs has the potential to be very messy. Have paper towels available as well as antiseptic cleaner or wipes for cleanup.
• Model gentle egg spinning, as too much misdirected force could send an egg off of a desk or table and onto the floor. If desks are small, consider providing each team with an aluminum pie tin in which to conduct their experiment.
• If students working in groups is not feasible, this activity can be done as a demo for the class.

### Background

NASA’s InSight spacecraft will land on Mars in November 2018. Aboard InSight is a radio-science experiment called RISE, or Rotation and Interior Structure Experiment. RISE will help determine whether Mars’ core is liquid and which elements, besides iron, may be present. RISE will accomplish this by gathering information on Mars’ wobble as it orbits the sun. Just as Earth completes a wobble every 18 Earth years as it is pushed and pulled by the Moon, Mars completes a wobble every one Mars year (two Earth years). Learning more about Mars’ wobble will give us more information about the interior of Mars and the forces causing the wobble.

RISE works like a mirror, returning a signal sent to the lander from Earth. Changes in the signal, known as a Doppler shift, are measured and this information reveals the location (within centimeters) of the InSight lander and Mars in space. Scientists can use this information, collected over the course of InSight's mission, to understand just how much Mars wobbles in its orbit. Measuring the wobble tells us about the distribution of mass and materials inside the planet, outside forces acting on the planet, such as the gravitational attraction of moons, and whether there is any “sloshing” or moving of the center of mass, which would indicate a liquid core. This movement of the center of mass can be demonstrated by spinning a raw egg as indicated in the following activity.

The following activity also calls for a discussion of Newton’s first law, which states that an object in motion will stay in motion unless acted upon by an unbalanced force, and an object at rest will stay at rest unless acted upon by an unbalanced force. Inertia is the property of a body that makes it oppose any force that would cause a change in its motion. If a body is solid with no liquid parts, such as a hard-boiled egg, it will be relatively easy to place into motion or bring to rest because all the parts of the body act as one. Merely bringing the shell to rest will bring the whole egg to rest. However, if a body has interior liquid, such as a raw egg, it will be more difficult to place into motion or bring to rest because the liquid inside the egg experiences a delay in the application of the force applied to the shell. Stopping a spinning raw egg takes more force than stopping a spinning hard-boiled egg of the same mass because the liquid center of a raw egg will continue spinning after a force is applied to the shell.

### Procedures

1. Show students two identical-looking eggs and tell them that one is raw and one is hard-boiled. Ask them to guess which one is which. Ask them to explain their reasoning.
2. Ask how we might determine which is which aside from breaking the shells. Some students may wish to handle the eggs and try various ways of making the determination.
3. Explain that since we don’t know for sure which egg is raw, we must handle both eggs with care, as if they are raw. Distribute a pair of eggs, one raw and one hard-boiled, to teams of students for investigation. Have paper towels and antiseptic cleaner at hand in case of accidents.
4. Egg spinning technique. Credit: NASA/JPL-Caltech | + Expand image

5. Allow students to experiment with any method of differentiating the eggs that will not result in broken egg shells.
6. After students have experimented for a few minutes, ask them to make a mark on the egg they think is hard boiled. Ask them to explain their reasoning. Ask students to critique the reasoning of other groups and decide which, if any, have plausible ways to tell the difference.
7. Tell each team to spin each egg as shown in the video clip.
8. Ask them to determine if there is any difference in the way the two eggs spin. Observations might include that one egg spins faster than the other, that one wobbles or that one spins longer than the other. If students cannot see any difference, have them place a dot near but not on one of the ends of the eggs and spin the eggs again. Watching the dot as the egg spins might make spin differences more obvious.
9. Egg spinning interrupted. Credit: NASA/JPL-Caltech | + Expand image

10. Ask students if the spin differences offer any clues as to whether the interior of the eggs are liquid or solid. Ask them to hypothesize about what information spin differences might give. Invite teams to revise or confirm their decision about which egg is hard-boiled.
11. Tell teams to spin each egg again, but this time, once the egg is spinning, have them momentarily place a finger on top of the egg to stop the spin, then release. It’s important to only touch the egg to bring it to a stop and then release, not leaving their finger on the egg too long.
12. Ask students what differences they observe. Ask them to think about and say what this might mean about the interior of the egg. Invite teams to revise or confirm their decision about which egg is hard-boiled.
13. Engage students in a discussion about the center of mass of an object. A spinning object, such as an egg, will spin about its center of mass. If an object is solid, the center of mass is in one immovable location. If an object is part liquid, the center of mass can shift when the liquid shifts. Ask students what a shifting center of mass could mean for the motion of a spinning object (it would wobble as it spins about a point that is moving). Invite teams to revise or confirm their decision about which egg is hard-boiled, spinning the eggs again as needed.
14. Engage students in a discussion about Newton’s first law of motion: An object at rest stays at rest unless acted upon by an unbalanced force and an object in motion stays in motion unless acted upon by an unbalanced force. Explain that this resistance to a change in motion is called “inertia.” Discuss the applications of inertia to the current experiment. The egg will stay at rest until acted upon by an outside force (the spin provided by your hand). The egg will stay in motion until acted upon by an outside force (friction). Ask students to use their understanding of inertia to explain why one egg starts spinning after it is momentarily stopped by the touch of a finger and the other egg remains stopped. Students should be able to make the connection between a liquid inside continuing to move as a result of inertia, even though the shell is momentarily stopped. Invite teams to revise or confirm their decision about which egg is hard-boiled.
15. Have each team bring their eggs, one team at a time, to the front of the class and crack them over a large bowl or trash can to reveal if their hypothesis was correct.
16. Explain that we do not know if the core of Mars is solid or liquid, but the science instrument RISE on NASA’s InSight lander is about to find out. Ask students which method would be easier to use at Mars, the wobble or inertia, and ask them to explain why. Answer: wobble because we cannot stop and start the spinning of Mars.
17. Show an image of the InSight lander and point out RISE. Explain that it is a radio-science experiment that will precisely measure the location of InSight on the surface of Mars over the duration of the lander’s life, giving scientists information about the Red Planet’s wobble. These wobble observations will provide detailed information on the size of Mars' iron-rich core. They will help determine whether the core is liquid and which other elements, besides iron, may be present.

### Discussion

Ask students to discuss other practical uses of center of mass and inertia.

### Assessment

• Evaluate students' ability to tell the difference between a raw and a hard-boiled egg without cracking the shell.
• Evaluate students' ability to explain their reasoning using scientific terms.
• Evaluate students' ability to connect the egg experiment to planetary exploration.

### Extensions

Have students research what is known about the core of other planets in our solar system.