# Rotational Motion Question

#### americanforest

Why is it that when you spin a piece of plastic bent slightly upwards on a flat surface, it spins normally for a little while, slows down, and then spins the other way for a bit before stopping? My professor did a demo of this in class and didn't really explain it.

#### Meir Achuz

Homework Helper
Gold Member
The plastic has a slightly asymmetric shape. The actual mechanism of the effect is very complicated. It only works if you spin it in one direcction, but not the other. You can observe the asymmetry by looking at the reflection of a light in the curved plastic.

#### americanforest

Can you explain more thoroughly how asymmetry would affect the rotational motion and cause it to spin in the reverse direction? I was thinking it had something to do with Static Electricity. Is that possible?

#### Meir Achuz

Homework Helper
Gold Member
Nothing to do with electricity. The asymmetry causes it to lean over enough so that it jams in its rotation and rebounds the other way. It only happens when you start spinning it in one direction.

#### vijay123

omg...this is nth to do with asymmtry....i think u must draw a free body diagram....then list down the forces...assume that the prof gave a force at the top of a cylnder for example...the cynlinder woud start to slide...u know that kinectic friction acts at the bottom of the ball...opposite in direction to the force once applied by the prof....the torque applied by the force of prof would start to reduce while the torque of fric. would increase until one point wen it starts to roll....this is wen STATIC FRICTION comes into the pictor...since the ball is at rest in every point of it's motion since it is rolling...static friction is used...this would go on until once point wen the force exerted by the prof. is zero caused by the friction force...since the cynlider is still moving...the friction torque is still being applied...causing the cynlinder to move in the other direction for some time bfore coming to a stop

phew...hope that explains it....i think i am not very clear with the explaination...but life wut meir achuz...the actual mechanism is a wee bit complicated

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving