# Question about objects and motion

• cvw
In summary: Whizzing the ball around the inside of the tube at 90 degrees to the tube long axis, the ball goes round and round the inside circumference until it runs out of steam. This continues at 90 degrees, staying on the orginal plane. Firing the ball into the tube, while giving it some rotation too I expected it to spiral all the way to the other end. Along plus Round equals Corkscrew?No.If you fire the ball in at an angle, not only does it travel around the circumference but it tries to return to the thrower at a similar angle. This means that after a few rot
cvw
Hi guys,

Please could you help with with a puzzling question for a non physics person. I am sure it has a simple answer.Today at the park, my kids and I discovered a strange physical phenomenon while playing with a tennis ball and a large stainless steel tube (the kind you crawl through).

Whizzing the ball around the inside of the tube at 90 degrees to the tube long axis, the ball goes round and round the inside circumference until it runs out of steam. This continues at 90 degrees, staying on the orginal plane. Good.

Firing the ball into the tube, while giving it some rotation too I expected it to spiral all the way to the other end. Along plus Round equals Corkscrew?

No.

If you fire the ball in at an angle, not only does it travel around the circumference but it tries to return to the thrower at a similar angle. This means that after a few rotations, the ball ends up traveling around the inside of the tube at 90 degrees to the long axis again.

It seems impossible to get the ball to corkscrew to the other end AND get a number of rotations around the tube - the ball always returns to the 90 angle of travel.

This behaviour must have a name. What can I look for to find out more?

CheersRob

and two small children

Last edited:
That was quite difficult to understand, but I think I got there in the end. Basically, whenever you try to get the ball to corkscrew through the tube to come out the other end, it ends up just spiralling inside the tube but moving nowhere?

Edit: If that is the case, my initial guess would be that it's to do with the spin of the ball. It's really hard to explain, but if you think about how the ball is rolling as it starts its ascent up the side of the tube, its spin will be such that it acts to stop the ball from moving along the tube. Try to follow the direction of the spin as it moves through the tube and you might catch my drift. Basically, by the time the ball is at the top of the tube it is spinning in the wrong direction to the direction of travel acting to brake the balls movement. I've never considered this before. It's really quite interesting...

Last edited:
cvw said:
This behaviour must have a name. What can I look for to find out more?
It is a consequence of http://en.wikipedia.org/wiki/Angular_momentum" conservation. If the ball rolls within a plane, it's spin axis is constant. But rolling in a spiral would require the spin axis to change, and the ball resist this change. Just like a spinning top refuses to fall over. If the ball was sliding, not rolling, it would spiral trough.

Last edited by a moderator:
A.T. said:
If the ball rolls within a plane, it's spin axis is constant. But rolling in a spiral would require the spin axis to change, and the ball resist this change.

Thanks for the replies. Your understanding of the setup was correct and the ball definitely rolls around, rather than skidding.

Although the ball didn't do what I expected, it make some kind of sense. The ball sent in at an angle, the round tube still caused the "round and round" motion but as an elongated elipse rather than a circle. As the elipse becomes less elongated each turn, the rotation axis swings back toward the 90 degree mark.

When we are next at the play park, we will try the same thing with a sliding object like a coin. This surely should corkscrew.

Thanks very much once again.

Rob and the kids

## What is the difference between speed and velocity?

Speed is a measure of how fast an object is moving, while velocity is a measure of both speed and direction. In other words, velocity indicates not only how fast an object is moving, but also in what direction it is moving.

## How is acceleration calculated?

Acceleration is the rate at which the velocity of an object changes. It is calculated by dividing the change in velocity by the time it took for that change to occur. The formula for acceleration is: a = (vf - vo) / t, where a is acceleration, vf is the final velocity, vo is the initial velocity, and t is the time.

## What is Newton's first law of motion?

Newton's first law, also known as the law of inertia, states that an object will remain at rest or in motion with a constant velocity unless acted upon by an external force. In other words, an object will continue to do what it is doing unless a force is applied to it.

## How do forces affect the motion of an object?

Forces can cause a change in an object's motion by either speeding it up, slowing it down, or changing its direction. The size and direction of the force applied to an object determines how it will move.

## What is the difference between distance and displacement?

Distance is the total length of the path travelled by an object, while displacement is the distance and direction of an object's final position from its starting position. In other words, distance is a scalar quantity, while displacement is a vector quantity.

• Mechanics
Replies
8
Views
2K
• Mechanics
Replies
5
Views
2K
• Mechanics
Replies
10
Views
1K
• Mechanics
Replies
2
Views
1K
• Mechanics
Replies
20
Views
2K
• Aerospace Engineering
Replies
6
Views
2K
• Special and General Relativity
Replies
6
Views
1K
• Sci-Fi Writing and World Building
Replies
13
Views
2K
• Mechanics
Replies
11
Views
2K
• Mechanics
Replies
11
Views
2K