# Spinning ball and its trajectory

pixel01
I play tabletennis and it is obvious that with top spin, the ball go down faster than the one without any spin. I have try to explain by bernoulli, but it seems the result is opposite: top spin ball will drift more!. Anyone can help?

Homework Helper
Gold Member
It is the same as a curveball.
Friction is important. That is why pitchers try to scruff up the ball.
The air flow is slowed down on the top side.
The faster flow on the bottom side reduces that pressure by Bernie's theorem.

Homework Helper
Gold Member
It is the same as a curveball.
Friction is important. That is why pitchers try to scruff up the ball.
The air flow is slowed down on the top side.
The faster flow on the bottom side reduces that pressure by Bernie's theorem.
Also turbulence at the top increases the pressure there.

Mentor
It is probably easier to use the rotational model/Newton's law. The spinning motion combined with the frictin directs the air upward behind the ball. Via action-reaction (conservation law), if air goes one way (up), the ball must go the other (down).

pixel01
It is the same as a curveball.
Friction is important. That is why pitchers try to scruff up the ball.
The air flow is slowed down on the top side.
The faster flow on the bottom side reduces that pressure by Bernie's theorem.
Also turbulence at the top increases the pressure there.

I agree with you that friction is important. But the force caused by friction isn't up or down is it?. Friction just slows down the spin. And turbulence, it is not so clear because sometimes the ball does not fly very fast and with not much of a spin, but the phenomenon is still observed. I don't know at which speed, the turbulence begins to occur in case of a ping pong ball

Mentor
I agree with you that friction is important. But the force caused by friction isn't up or down is it?.
Since the ball is rotating in the direction of motion, the friction force on the top surface is much larger than on the bottom surface. But even without that effect, the friction causes the air to rotate around the ball.