From: veritasium
Lifting heavy objects can be a significant physical challenge, especially when the object’s mass is considerable or its shape makes it awkward to handle. The perceived difficulty of lifting an object can also be influenced by its dynamic properties, such as rotation, due to phenomena like gyroscopic precession [00:01:20].
The 40-Pound Flywheel Experiment
At the University of Sydney, a mechanical engineering shop constructed a specific apparatus to demonstrate the challenges and surprising effects related to lifting heavy objects [00:00:01]. This apparatus consists of a 40-pound (19 kg) flywheel attached to the end of a meter-long shaft [00:00:08].
Holding a Stationary Heavy Object
Attempting to hold the 40-pound flywheel horizontally with just one hand at the end of the meter-long shaft is virtually impossible [00:00:14], requiring immense effort to counteract the torque created by its weight [00:00:22].
Similarly, lifting the non-spinning flywheel over one’s head with a single hand at the end of the shaft is extremely difficult, feeling “really hard” and requiring “a big effort” [00:01:44], [00:04:13]. One attempt barely succeeded, highlighting the significant strength required [00:01:49], [00:02:07].
The Effect of Gyroscopic Precession
When the 40-pound flywheel is spun up to a few thousand RPM, its behavior changes dramatically [00:00:37].
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Horizontal Stability: If held horizontally by one end while spinning, the shaft remains horizontal and moves in a circle, almost appearing weightless [00:01:03], [00:01:09]. This phenomenon is known as gyroscopic precession, where the weight of the wheel creates a torque that pushes it around in a circle instead of pulling it directly down [00:01:13], [00:01:17].
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Perceived Lightness During Lifting: Lifting the spinning flywheel over one’s head with one hand feels “incredibly, incredibly light” [00:03:05]. It feels as if the wheel “was just… wanting to go up by itself” [00:04:17], and the lifter perceives that they are “not putting in the amount of force that you’d think to lift that kind of weight” [00:04:23]. Despite this perception, the object is not truly weightless [00:04:34].
Measuring the Force of Lifting
To quantitatively assess the force exerted, an experiment was performed using a scale.
- Baseline Weight: The individual’s weight alone was about 72 kilograms [00:04:49].
- Weight with Stationary Flywheel: When the individual picked up the non-spinning 19-kilogram (42 pounds) flywheel, the scale reading increased to about 91 kilograms, which accounts for the combined weight [00:04:52], [00:04:59].
The experiment was set to determine if the scale reading would be more, less than, or equal to 91 kilograms when the spinning flywheel was lifted over the head [00:05:06]. This provides insight into the actual forces involved, contrasting with the subjective feeling of lightness.