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I just saw an experiment where a heavy object (bowling ball) and a light object (feather) are dropped in a vacuum and they both fall at the same speed (almost like a slow motion video) and reach the bottom at the same time. But when the same two objects are dropped in a non vacuum environment, the ball hits the ground immediately while the feather has not even covered half the distance. I know this is somehow related to air resistance and terminal velocity and he air resistance reduces the acceleration of the ball to zero. but what I do not understand is that how come air resistance helps in increasing the speed of the ball ? Isn't it a kind of resistance ? Is it that in the vacuum experiment, the ball reaches the ground very quick and only the video is in slow motion ?

Thought of adding the video link as well: https://www.youtube.com/watch?v=E43-CfukEgs

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  • $\begingroup$ Either slow motion, or filmed on the moon. $\endgroup$ – Previous Nov 30 '16 at 3:08
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    $\begingroup$ The experiment in vacuum was probably filmed at slower speed. Drag is always opposite to the direction of motion so it can only slow free falling things down. $\endgroup$ – Floris Nov 30 '16 at 3:18
  • $\begingroup$ Please comment on the reasons for the downvote ! $\endgroup$ – Crusaderpyro Nov 30 '16 at 3:33
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    $\begingroup$ Probably because they had difficulty believing this was a serious question. But indeed, the video does not explicitly say this was slow motion. Would be better if they did. $\endgroup$ – Pieter Nov 30 '16 at 8:41
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Well, the experiment was obviously filmed at a slower speed or shown at a slower speed. Both feather and ball should accelerate at around $9.8~\mathrm{m/s^2}$ and their velocities will be the same at all times. When there is air, the feather falls at much slower rate compared to the ball. Air resistance will decrease the acceleration of both but the effect of it will be much more on the feather.

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  • $\begingroup$ "Fall at 9.8 m/s^2" is terrible wording. $\endgroup$ – Jasper Nov 22 '18 at 15:15
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    $\begingroup$ @Jasper Corrected it. $\endgroup$ – Apoorv Potnis Nov 23 '18 at 9:48
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    $\begingroup$ It is quite terrible that the educational video does not mention this. It is not at all obvious to lay people. The general population tends to associate vacuum with space and "weightlessness". This expensive video only adds to the confusion. $\endgroup$ – Pieter Nov 23 '18 at 9:57
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    $\begingroup$ @Pieter Yeah. I actually wanted to see the feather falling down as fast as the ball at normal speed but was disappointed by the video. $\endgroup$ – Apoorv Potnis Nov 23 '18 at 10:12
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Actually, to begin with, you were correct, @Crusaderpyro, in your question, that as the speed of a falling object increases, your air resistance increases. The reason this is is that as the object falls faster, it meets up with more and more air molecules per given amount of time. This will manifest as an increase in air resistance. However, we also have to include the factor of mass in the equation. Since the ball has more mass and less surface area, even though it also experiences more air resistance, it will be affected by gravity more and reach the ground before the feather, which has very little mass and more surface area and is therefore less affected by gravity. They both start accelerating at 9.8 m/s2, but the feather gets slowed down by the air resistance much more as you can see in the first part of the video, due its small mass and large surface area. Of course, they slowed down the video in the second part, or the action would have been over in a fraction of a second. ( You can see this in the end of the video as they start to fall quickly together before the film gets slowed). But the main point was to show that, after the air is removed and thereby the air resistance removed, the ball and feather will fall at the same velocity.

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  • $\begingroup$ It must be a huge feather if it has more surface area than a typical bowling ball. ;) You should make it clear that you are talking about relative surface area, i.e., the ratio of surface area to volume (or to mass). $\endgroup$ – PM 2Ring Nov 22 '18 at 16:00
  • $\begingroup$ Downvoting because the experiment was in vacuum. Old question, please read previous answers. $\endgroup$ – Pieter Nov 23 '18 at 10:11

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