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Skill 12 of 14
In Progress

# Gravity

Bring a loaded gun, a tennis ball, a barbell and a bathing ball and

• throw the tennis ball straight sideways,
• shoot the bullet straight sideways,
• drop the barbell and also
• drop the bathing ball.

Which one hits the ground first?

The answer might surprise you: They all land at the same time!1

If you aren’t convinced yet, here’s a NASA recording from 1971 where an astronaut drops a hammer and a feather on the atmosphere-free moon[6,7] 2 (with no wind or air resistance to interfere):

Mind-blown?

How can this be? Shouldn’t the obviously heavier object fall faster and reach the ground first?

No, gravity doesn’t pull equally much in all objects. It pulls equally much in all “bits” or “particles” of all objects. When skydivers fall, they don’t suddenly start falling faster just because they hold on to each other. In the same way, all “particles” fall equally fast, and they don’t suddenly start falling faster just because they are put together to form an object. All objects fall equally fast, because all the “particles” they consist of fall equally fast.3

And how fast is this ‘equally fast’ then? If you measure it, it turns out that gravity speeds things up with 9.8 metres-per-second every second. Let’s name it the gravitational acceleration and symbolise it $g$:

$$g=9.8\,\mathrm{m/s^2}$$

This value differs slightly at different locations on Earth. $g$ is $9.82\,\mathrm{m/s^2}$ in Denmark nearer the north pole, but $9.78\,\mathrm{m/s^2}$ in Mexico nearer the Equator.[8] 4 And the value of course also differs depending on which planet, moon or star you are standing on/nearby; the gravitational acceleration on the Moon is around 6 times smaller, $g_\text{moon}=1.6\,\mathrm{m/s^2}$, because gravity up there is around 6 times weaker.[9,10]

References:

1. Who Was the First Person to Discover Gravity?’ (web page), Laura Gee, Sciencing, 2018, www.sciencing.com/first-person-discover-gravity-23003.html (accessed Aug. 7th, 2019)
2. Galileo Galilei’ (encyclopedia), Peter Machamer, The Stanford Encyclopedia of Philosophy, 2017, plato.stanford.edu/entries/galileo (accessed Aug. 7th, 2019)
3. Galileo Galilei on Motion and Mechanics: Comprising De Motu (ca. 1590) and Le Meccaniche (ca. 1600)’ (book), I.E. Drabkin, The University of Wisconsin Press, 1960, www.amazon.com/Galileo-Galilei-Motion-Mechanics-Comprising/dp/B007BOFKJ4
4. Galileo Galilei and Motion: A Reconstruction of 50 Years of Experiments and Discoveries’ (book), Vergara Caffarelli & Roberto Vergara, Springer, 2014, ISBN 978-3642427305
5. The Measurement of Little g: A Fertile Ground for Precision Measurement Science’ (article), James E Faller, Journal of Research of the National Institute of Standards and Technology, vol. 110, issue 6, 2005, www.ncbi.nlm.nih.gov/pmc/articles/PMC4846227, DOI 10.6028/jres.110.082, pages 559–581
6. David Scott does the feather hammer experiment on the moon’ (film), Youtube, Science News, 2019, www.youtube.com/watch?v=Oo8TaPVsn9Y (accessed May 9th, 2020)
7. The Apollo 15 Hammer-Feather Drop’ (web page), Dr. David R. Williams, NASA Goddard Space Flight Center, 2019, nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html (accessed May 9th, 2020)
8. New ultrahigh-resolution picture of Earth’s gravity field’ (article), Christian Hirt, Sten Claessens and others, Geophysical Research Letters, vol. 40, issue 16, August 28, 2013, doi.wiley.com/10.1002/grl.50838, DOI 10.1002/grl.50838, pages 4279–4283
9. Acceleration Due to Gravity Formula’ (web page), SoftSchools.com, http://www.softschools.com/formulas/physics/acceleration_due_to_gravity_formula/54 (accessed Jan. 14th, 2020)
10. Value Of g On Moon’ (web page), Byju's, 2019, www.byjus.com/physics/value-of-g-on-moon (accessed Jan. 14th, 2020)
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