WEBVTT 00:00:00.000 --> 00:00:07.460 A curious astronaut arrives on the Moon with two very different objects: a light feather and a 00:00:07.460 --> 00:00:08.571 sturdy hammer. 00:00:08.571 --> 00:00:13.730 You may have watched a feather drift slowly through air on Earth. 00:00:13.730 --> 00:00:16.905 That could make the answer seem obvious. 00:00:16.905 --> 00:00:19.603 Surely the hammer will land first! 00:00:19.603 --> 00:00:22.619 But the astronaut does not answer yet. 00:00:22.619 --> 00:00:25.873 Both objects are held at the same height. 00:00:25.873 --> 00:00:33.095 A fair comparison starts by noticing which conditions are the same and which are different. 00:00:33.095 --> 00:00:37.063 Here on the lunar surface, there is almost no air. 00:00:37.063 --> 00:00:40.000 Make your prediction before we begin. 00:00:40.000 --> 00:00:43.933 This question belongs to more than a fictional story. 00:00:43.933 --> 00:00:50.390 During the Apollo fifteen mission, astronaut David Scott performed a hammer-and-feather 00:00:50.390 --> 00:00:52.319 demonstration on the Moon. 00:00:52.319 --> 00:00:57.440 He released both objects together from approximately the same height. 00:00:57.440 --> 00:01:01.521 They reached the lunar surface at nearly the same time. 00:01:01.521 --> 00:01:04.490 The record gives us evidence to examine. 00:01:04.490 --> 00:01:11.095 Our friendly character and toy stage are original designs, rather than a copy of the real 00:01:11.095 --> 00:01:12.801 astronaut's appearance. 00:01:12.801 --> 00:01:19.629 Press release to watch the scene, then pause and compare the two objects as carefully as you 00:01:19.629 --> 00:01:20.000 like. 00:01:20.000 --> 00:01:22.927 The lunar surface is close to a vacuum. 00:01:22.927 --> 00:01:28.705 It does not have the noticeable air resistance that slows a feather on Earth. 00:01:28.705 --> 00:01:35.760 Released from rest together at the same height, both objects accelerate downward equally under 00:01:35.760 --> 00:01:36.360 gravity. 00:01:36.360 --> 00:01:40.188 That does not mean there is no gravity on the Moon. 00:01:40.188 --> 00:01:46.867 Now add an illustrative air-resistance model to our scene while keeping the same gravity. 00:01:46.867 --> 00:01:49.118 The feather falls more slowly. 00:01:49.118 --> 00:01:55.647 This comparison isolates the effect of drag; it is not a measurement of another planet. 00:01:55.647 --> 00:02:00.000 Switch between the conditions and make another prediction. 00:02:00.000 --> 00:02:04.110 The astronaut invites you to check the comparison again. 00:02:04.110 --> 00:02:10.569 Return to lunar vacuum, start both objects at the same height and release them together. 00:02:10.569 --> 00:02:12.991 Pause or reset whenever you want. 00:02:12.991 --> 00:02:17.174 Observing means more than noticing an interesting result. 00:02:17.174 --> 00:02:23.046 It also means describing the conditions and asking whether a comparison is fair. 00:02:23.046 --> 00:02:29.284 Our scene uses simplified calculations to explain a principle, not to make laboratory 00:02:29.284 --> 00:02:30.239 measurements. 00:02:30.239 --> 00:02:36.844 At home, you can explore air resistance using one sheet of paper, first held flat and then 00:02:36.844 --> 00:02:40.000 crumpled into a ball, with an adult nearby. 00:02:40.000 --> 00:02:46.538 The astronaut packs away the feather and hammer and leaves you with another question. 00:02:46.538 --> 00:02:52.692 Make the lunar stage, astronaut and props to create a table-top science theatre. 00:02:52.692 --> 00:02:57.692 One family member becomes the observer and another the presenter. 00:02:57.692 --> 00:03:03.769 Ask for a prediction, compare conditions in the scene and explain what changed. 00:03:03.769 --> 00:03:10.154 Printed props help you tell the story; they do not change the gravity in your room. 00:03:10.154 --> 00:03:16.308 Keep your astronaut for a later adventure about Moon phases, planets or shadows. 00:03:16.308 --> 00:03:20.000 Where would you like to take your next question?