When science and gravity meet

This exercise is a part of Educator Guide: Galileo Experiment Re-created in Space / View Guide

Directions: After students have had a chance to review the article “Galileo experiment re-created in space,” lead a classroom discussion based on the questions that follow. Before you begin the discussion, show students “Apollo 15 proves Galileo correct” on YouTube.

Discussion questions:

1. What is the equivalence principle?

The equivalence principle states that an object’s inertial mass (Minertia) and its gravitational mass (Mgrav) are equal, Minertia = Mgrav. How much a force (F) makes an object accelerate (a) depends on the object’s inertial mass, F = Minertia a. How much gravitational force pulls on an object depends on the object’s gravitational mass (Mgrav) and the gravitational acceleration (g), Fgrav = Mgrav g. Considering only gravitational force on an object, F = Fgrav, gives Minertia a = Mgrav g. If Minertia = Mgrav, an object’s gravitational acceleration does not depend on its mass, a = g.

2. What are aerodynamic drag and terminal velocity?

Aerodynamic drag is the force air molecules exert on an object as it moves through the air. Drag generally increases with the square of an object’s velocity: If the object goes faster, it experiences much more drag. Terminal velocity is the speed at which the aerodynamic drag on a falling object becomes so large that the force balances the opposing gravitational force, so the object stops accelerating and falls at a constant velocity. An object’s terminal velocity depends on aerodynamic drag and the object’s mass. For example, terminal velocity is low for parachutes and higher for cell phones.

Extension prompts:

3. What is quantum gravity?

Quantum gravity is a class of unproven theories that combine both general relativity (which describes gravitational fields) and quantum physics (which describes very small particles). These theories attempt to describe the behavior of things that are both very small and subjected to strong gravitational fields. Very small things that are subjected to weak gravitational fields, generally behave by Newtonian gravitational principles.

Discussion questions:

1. The article mentioned that the two cylinders are made of alloys of platinum and titanium. What are the densities of pure platinum and pure titanium? Why is it possible that scientists used these materials for this experiment?

Because these substances are alloys of two different elements, they have different physical and chemical properties. The difference in their composition alone could lead to an equivalence principle violation. Platinum has a density of 21.45 g/cm3, whereas titanium has a density of 4.51 g/cm3. A large difference between their densities may be useful for an experiment in which composition is the independent variable.  

Extension prompts:

2. What are some ways that microgravity (the condition in which people or objects can achieve weightlessness) could be used for chemistry and materials science applications?

In microgravity, molten metal would form spheres (due to surface tension) and could cool that way to make ball bearings. Semiconductor crystals and optical crystals in microgravity could form lattices with fewer defects. Phase change processes ranging from boiling to condensation would work differently, as would chemical processes such as combustion.

3. What are the effects of long-term weightlessness on the human body? How is artificial gravity created for the astronauts on the space station?

Prolonged weightlessness in a microgravity environment can cause health problems including bone loss, muscle atrophy and decreases in red and white blood cells. Getting lots of exercise while on a space station helps to minimize those problems. And artificial gravity can be created by spinning the station.

Discussion questions:

1. Name the independent and dependent variables in the MICROSCOPE experiment described in “Galileo experiment re-created in space.”

The independent variables are the compositions of the two cylinders — one cylinder is made of a platinum alloy and the other is made of a titanium alloy. The dependent variables are the cylinders’ rates of acceleration.

2. What sorts of confounding variables can limit the precision of the MICROSCOPE experiment and other previous experiments testing the equivalence principle?

On Earth, groundwater flow can alter the mass, and hence, the gravitational pull of surrounding terrain. On Earth and in space, temperature changes can make things expand and contract, limiting the precision of measurements. If the experiment is not performed in a perfect vacuum, air resistance may alter the acceleration of objects based on their structure and geometry.

3. On Earth, you can measure the mass of an object using a two-pan balance, a triple-beam balance, a spring scale or a digital scale. If you used those same tools on the moon or Mars, would you get the same results?

Gravity on the moon is roughly one-sixth that of Earth, and gravity on Mars is roughly one-third that of Earth. The mass of objects measured using a two-pan balance or triple-beam balance would be the same on the moon and Mars as it is on Earth because gravity pulls on objects on both sides of the balance. A spring scale or digital scale assumes the Earth’s usual gravitational force is pulling an object down, and measures the distance the object is pulled down. There would be less downward pull on the moon or Mars, so the mass readout would be erroneously low.

Extension prompts:

4. How else might measuring gravitational acceleration with high precision be useful?

Sensors such as gravity gradiometers are used to map small changes in the Earth’s gravitational acceleration. Those acceleration changes can indicate alterations in the density of matter nearby, and could be used to locate dense mineral deposits, measure changes in groundwater, spot underground cavities and distinguish between loaded and unloaded vessels, for example.

5. Imagine that you could use some process to alter the inertial mass (or possibly the gravitational mass) of an object. How would those alterations affect the object’s acceleration?

If the inertial mass were reduced, the same amount of applied force could cause a larger acceleration.