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Suppose I rotate a fidget spinner and let go of it in space. It will have some angular momentum. My question is will it move forward because of that angular momentum?

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I mounted a fidget spinner on a board using some toothpicks and after spinning it, I placed it in a bucket filled with water. Instead of moving forward because of the angular momentum,the board on which I mounted the fidget spinner was just rotating in one place instead of going in any direction, why did that happen?

Pranav K
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  • Angular momentum just shows body rotation axis and direction of angular speed. No force is acting along it, hence body will not experience any translational movement along $\mathbf L$ vector. – Agnius Vasiliauskas Jul 03 '23 at 08:49

2 Answers2

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My question is will it move forward because of that angular momentum?

No. Rotational and translational motion are independent of each other.

Imagine your spinner 'hanging' in space, not under the influence of a central gravitational field (or any other force or force field) and motionless. Now we introduce a torque $\tau$ about some axis.

This causes, as per N2L for rotation, angular acceleration $\alpha$ acc.:

$$\tau=I\alpha$$

where $I$ is the spinner's moment of inertia (about said axis).

This causes the spinner to start rotating about said axis, so it angular velocity $\omega$ increases, acc.:

$$\omega=\alpha t$$

Instead of moving forward because of the angular momentum,the board on which I mounted the fidget spinner was just rotating in one place

It is not supposed to start translating. Rotational movement doesn't magically induce translational movement in an object.

Gert
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Translation only occurs when there’s a non zero net force on the centre of mass of the object. When we have pure rotation, it is caused by a torque or a couple which have zero net force on the centre of mass.

So having a rotation does not imply there is translational motion.

Superfast Jellyfish
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