I was using a concrete diamond cutting cup attached to an angle grinder at work the other day. The disk is heavy-ish (maybe a pound or two). When I turn the power on, it feels as though the thing gains 5 pounds. What is happening?
In: Physics
“Heavy” is kind of an imprecise term. But if you mean “Is a spinning disk harder to move than a stationary one?” the answer is yes. The spinning adds angular momentum, which you can just think of as weird momentum; a spinning disk will continue to spin, but that spinning will try to push the axle around. So if you are holding the axle in a certain direction (such as angle grinding), the momentum will be pushing your hold in addition to the overall weight of the tool. As the disk is heavier and faster, the momentum is increased.
It doesn’t make it any heavier, but it does make it much harder to rotate. You wouldn’t notice a difference in moving it straight up and down, but any kind of rotating will be more difficult.
If an object is moving in a straight line in one direction, that has no effect on that object moving in a straight line in a different direction. That’s not true with rotating. An object can only rotate in one direction at a time. So when you try to rotate your angle grinder, the fast rotating disc is resisting it.
Another way to think about it is if you think about something traveling on a straight road, like a car. That car has speed, and momentum, and in order to move it off it’s path, you need to apply a force to it.
When you spin something up, especially something which has a good amount of mass, you can think about it like a bunch of cars traveling on a circular road. They all have momentum and when you try to move them off the path they’re already going, you have to apply more force than you would if they were stationary.
To add on to what everybody else is saying about the gyroscopic effect, here is a great video by Vsauce which explains a lot of this in a very intuitive way. Pretty neat!
[https://www.youtube.com/watch?v=XHGKIzCcVa0](https://www.youtube.com/watch?v=XHGKIzCcVa0)
Rotational inertia and the gyroscopic effect. If the disk is spinning clockwise as you hold the drill and you try to angle the bit downward, all of the particles that make up the disk will have rotational inertia – they will continue spinning in the same plane unless acted on by an outside force.
So when the drill is powered off, to move it around you only have to overcome it’s inertial mass. When its spinning, you have to overcome its inertial mass and also its rotational inertia.
It’s not heavier, but it will resist rotation on any other plane. Any spinning weight causes a gyroscopic effect which in simple terms means it will want to maintain that attitude and resist rotation to another plane. This is very useful for many applications such as for example gyroscopic stabilisation and attitude control for precision machinery, targeting systems for weapons or high precision tools like telescopes or measuring/surveying equipment, or even spacecraft in space which have no surface to push against.
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