Lecture 7: Weight - Perceived Gravity - Weightlessness Free Fall - Zero Gravity in Orbit (misnomer)

author: Walter H. G. Lewin, Center for Future Civic Media, Massachusetts Institute of Technology, MIT
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008,   recorded: September 1999,   views: 48432
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)

See Also:

Download Video - generic video source Download mit801f99_lewin_lec07_01.m4v (Video - generic video source 107.5 MB)

Download Video - generic video source Download mit801f99_lewin_lec07_01.rm (Video - generic video source 109.0 MB)

Download Video Download mit801f99_lewin_lec07_01.flv (Video 108.6 MB)

Download Video Download mit801f99_lewin_lec07_01_352x240_h264.mp4 (Video 149.8 MB)

Download Video Download mit801f99_lewin_lec07_01.wmv (Video 428.8 MB)

Download subtitles Download subtitles: TT/XML, RT, SRT

Help icon Streaming Video Help

Related Open Educational Resources

Related content

Report a problem or upload files

If you have found a problem with this lecture or would like to send us extra material, articles, exercises, etc., please use our ticket system to describe your request and upload the data.
Enter your e-mail into the 'Cc' field, and we will keep you updated with your request's status.
Lecture popularity: You need to login to cast your vote.



1. What is Weight?

Weight is the force exerted on you by a bathroom scale. Your weight increases when you are in an elevator which is accelerated upward; you weigh less than your normal weight when the elevator is accelerating downward, and your weight is zero when the elevator is in free fall. When you hang from a rope, your weight is indicated by the tension in the rope.

2. Tension in Massless String:

Consider a string of negligible mass suspending two objects with different mass on either side of a frictionless pin. The tension in this string is everywhere the same because the string is "massless" and the pin frictionless. The masses of the two objects are NOT equal but the weight of the two objects IS THE SAME!

3. Weight, when Swinging around on a String:

An object is swirled around on a string in a vertical plane. The tension in the string is evaluated when the object is at the top and when it is at the bottom of its circular trajectory. The tension at the bottom is always higher than the normal weight of the object, but the tension can be zero when the object is at the top in which case the object is weightless.

4. Objects in Free Fall are Weightless:

Exploring the weight of a tennis ball being tossed in the air, and of a bottle of water in Professor Lewin's hands when he jumps off a table. The bottle and Lewin are in free fall, thus both are weightless.

5. Weight Measurements of a Free Falling Object:

Bathroom scales have too slow a response time to indicate your zero weight when you weigh yourself while jumping off a table. Professor Dave Truemper has built a scale with a response time of 10 ms (it uses pressure gauges instead of springs). Professor Lewin places a 10 lb object on the scale, and we see that the scale indicates this weight. He then tapes the object firmly to the scale and drops the scale with the object from about 2 meters. The weight of the object is zero during free fall!

6. Professor Young's "Zero Gravity" Experiments:

NASA has sponsored "zero gravity" experiments to study motion sickness. These are zero weight environments (not zero gravity) created in air planes which free fall for 30 seconds. The air plane's (a KC135) trajectory is discussed in detail, and one of Professor Young's video clips is shown.

Link this page

Would you like to put a link to this lecture on your homepage?
Go ahead! Copy the HTML snippet !

Reviews and comments:

Comment1 guohua Zhang, December 25, 2011 at 6:55 a.m.:

Thanks to Professor Lewin for the excellent courses, i learned form they very much!

Comment2 Davor form VideoLectures, December 18, 2017 at 10:26 a.m.:

Hi all!

We have translated this entire course for you from English into 11 languages.

Check this video and give us some feedback in this short survey https://www.surveymonkey.co.uk/r/ZYPHH62

Write your own review or comment:

make sure you have javascript enabled or clear this field: