# How does a ball rotate in a waterfall?

• infinitetime
In summary, the conversation discusses the phenomenon of a ball rotating under a mini-waterfall in a pool. The ball is pushed by the force of the water hitting its surface, which creates a torque force and causes the ball to rotate at a constant velocity and direction. Other factors, such as the ball's mass and the buoyancy of the water, also play a role in balancing the ball. The speaker also suggests that the Bernoulli principle may be at work in this phenomenon, as seen in the example of the 'Johnny Astro' toy.
infinitetime
I needed to check if I was correct in my thoughts about how a ball rotates under a waterfall.

Let me describe the situation: there is a mini-waterfall in my pool that has water that flows like a sheet (smooth water without any gaps, if that makes sense). Whenever a ball is placed near the waterfall, it glides over and starts rotating at a constant velocity and direction directly underneath and inbetween the water's flow.

Why does this happen?

So far, I have found out that it is because the presence of torque force. If you use a vector to represent the force of the water hitting the ball's surface, the vector can be extended to the ball's center of mass. From there, an x component and a y component can be created; the x component is the force applied to the surface of the ball, while the y component is the resultant torque force.

Also involved in this phenomenon is the force of the ball's mass or gravity which pulls the ball downwards, and buoyancy of the water's surface due to surface tension that pushes the ball upwards; these cancel each other out to balance the ball.

I don't know if I am missing anything - of course there are probably a lot of other factors influencing the ball to rotate (such as velocity of the water, etc), but I'm not sure exactly. Please help with what you can offer!

infinitetime said:
I needed to check if I was correct in my thoughts about how a ball rotates under a waterfall.
I think that you're dealing primarily with Bernoulli here. Don't take it as a fact, but I suspect that the water flowing around the ball acts like air over a wing; the pressure drops because it has farther to go. The ball is then kept centred by external air pressure pushing it back into the stream.
The reason that I think this is that I used to have a toy called 'Johnny Astro' that used a joystick-controlled fan to manoeuvre a balloon done up like the hot air variety. Varying the speed of the fan brought the balloon closer or sent it farther out, and it always stayed centred in the airflow. It was accurate enough that I could drop it six feet away, snag something with the built-in hook, and take off again. (By the way, I was about 12 at the time.)

Your understanding of the situation is correct. The rotation of the ball under the waterfall is due to the presence of torque force. Torque is the rotational equivalent of force, and it is created when a force is applied at a distance from the center of mass of an object. In this case, the water hitting the surface of the ball creates a torque force that causes the ball to rotate.

As you mentioned, there are other factors at play as well, such as the velocity of the water and the mass and buoyancy of the ball. These factors can affect the magnitude and direction of the torque force, and therefore, the rotation of the ball. Additionally, the shape and surface of the ball can also play a role in how it interacts with the water flow and rotates.

Overall, the rotation of the ball under the waterfall is a result of the complex interactions between various forces and factors. It is a great example of how physics can help us understand and explain natural phenomena.

## 1. How does the ball maintain its rotation in a waterfall?

The ball maintains its rotation in a waterfall due to the force of gravity and the downward flow of the water. As the ball moves through the water, the downward force of the water pushes against the bottom of the ball, causing it to rotate.

## 2. Does the shape of the ball affect its rotation in a waterfall?

Yes, the shape of the ball can affect its rotation in a waterfall. A round ball will rotate more smoothly and consistently compared to a non-spherical shape, which may experience more turbulence and erratic rotation.

## 3. Why does the ball rotate in the same direction as the waterfall?

The ball rotates in the same direction as the waterfall because of the direction of the water flow. The water flow creates a current that pushes against the bottom of the ball, causing it to rotate in the same direction as the water.

## 4. Can the speed of the waterfall impact the ball's rotation?

Yes, the speed of the waterfall can impact the ball's rotation. A faster and stronger waterfall will create a stronger current and therefore a faster rotation of the ball. A slower waterfall will have less of an impact on the ball's rotation.

## 5. How does the surface tension of the water affect the ball's rotation in a waterfall?

The surface tension of the water does not directly affect the ball's rotation in a waterfall. However, it can contribute to the overall flow and movement of the water, which in turn can impact the ball's rotation. For example, if the surface tension is high, it may create a smoother and more uniform water flow, resulting in a more consistent rotation of the ball.

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