# Physics help?

#### Patrick Johnson

##### Fish of the Day
Silver Member
Gotta ask, why is this important? Played for 40yrs and never seen this asked.
I don't know about the original question, but it has generated some useful info. For instance, it's good to know that a death grip doesn't add anything to the force of your shot, so you're free to use whatever grip suits you best for other reasons.

I think most of what people think of as "pool science" info is valuable mostly in this passive way - it isn't stuff you obsess over or even think about while playing; it's "background" info that informs and supports how you practice, learn, visualize and, as a result of that, play.

pj
chgo

#### goettlicher

##### AzB Silver Member
Silver Member
So if I can move my cue stick at 10mph into contact. How fast in mph does the cue ball leave, assuming a fairly loose grip?

#### vinay

##### Registered
I think Bob and Dave and Mike are saying the soft skin of the hand effectively isolates the cue from your hand/arm during the millisecond of contact.

Yes exactly, and I figured out what the cue velocity should be from my equations and found that it closely matches the experimental data mentioned in Dr. Dave's video. It seems the forces at play at the moment of impact are large enough that your hand might as well be made of jello.

#### Patrick Johnson

##### Fish of the Day
Silver Member
...the stick does almost instantly go down to 50% of its initial speed and then over a period of about 20 milliseconds go back up to about 80% of its initial speed as the hand and wrist finally get more force into the stick after contact.

...contact takes only 1-2 milliseconds.
So it takes 10-20 times the contact time for the cue to regain most of its speed. Pretty convincing that we're essentially throwing the cue at the ball.

pj
chgo

#### dr_dave

##### Instructional Author
Gold Member
Silver Member
The fact that cue speed drops by about 40% actually is a very good indication that the weight of the arm doesn't contribute meaningfully to the impact. Given a cue that's 3x as heavy as the ball and a coefficient of restitution of 0.75, you'd expect the cue to retain 56% of its speed after impact. If the arm doubled the effective mass of the cue, you'd expect it to retain 75% of its speed.
Good point.

Regards,
Dave

#### Patrick Johnson

##### Fish of the Day
Silver Member
So if I can move my cue stick at 10mph into contact. How fast in mph does the cue ball leave, assuming a fairly loose grip?
13-15 mph with any grip or none.

pj
chgo

#### vinay

##### Registered
So if I can move my cue stick at 10mph into contact. How fast in mph does the cue ball leave, assuming a fairly loose grip?

Assuming a hard leather tip and an 18oz cue, about 13mph. Maybe about 14mph with a phenolic tip.

It might be possible to verify this with one of those apps that measures break speed along with some careful video analysis to figure out what speed the cue is moving at before impact, or some custom timing hardware I wish I had access to a real high speed camera, but those things are incredibly expensive.

I am actually curious about what's driving your interest here. Just curiosity or something else?

#### dr_dave

##### Instructional Author
Gold Member
Silver Member
I think Bob and Dave and Mike are saying the soft skin of the hand effectively isolates the cue from your hand/arm during the millisecond of contact.
And any force that the grip hand might be exerting on the cue during contact would be too small to have any meaningful effect anyway compared to the extremely large force between the cue tip and CB. And many (if not most people) are exerting very little force with the grip right before contact anyway since the cue has already reached the desired speed at contact with the ball. That is clear in the acceleration plots in the various resources on the stroke acceleration resource page.

For those interested, here are some other resources that help back up some of these claims:

effects of light vs. tight grip

cue tip contact time

TP B.22 – How peak tip contact force and contact patch size vary with shot speed, and drop tests

There is some good discussion in this thread,
Dave

#### duckie

##### GregH
Silver Member
What’s a firm stroke?

What’s a medium stroke?

What’s a soft stroke?

This is a major issue for a lot of your test scenario..... Subjectivity on stroke speed. Not everyone will have the same opinion on what those speeds actually are.

Plus, why use a mph scale on the graph? In pool, we don’t deal in miles or hours, but in inches and seconds. Why not use inches per second?

Plus......does anyone know what range of speeds they stroke at? How can you gage if you are stroking at 1 mph or rather 17.6 inches per second.

I play a lot of 14.1, which requires a lot of slow rolling the balls. My firm stroke in playing 14.1 might be considered medium stroke for others playing 8 ball.

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#### logical

##### apart of their 'semi public'
Silver Member
What’s a firm stroke?

What’s a medium stroke?

What’s a soft stroke?

This is a major issue for a lot of your test scenario..... Subjectivity on stroke speed. Not everyone will have the same opinion on what those speeds actually are.

Plus, why use a mph scale on the graph? In pool, we don’t deal in miles or hours, but in inches and seconds. Why not use inches per second?

Plus......does anyone know what range of speeds they stroke at? How can you gage if you are stroking at 1 mph or rather 17.6 inches per second.

I play a lot of 14.1, which requires a lot of slow rolling the balls. My firm stroke in playing 14.1 might be considered medium stroke for others playing 8 ball.
Feel free to post your research results.

Sent from my SM-G950U using Tapatalk

#### goettlicher

##### AzB Silver Member
Silver Member
Vinay:
I was presented with this question in Pool School this weekend.
I was not prepared to give them an answer. I needed to know the real truth of the matter.
Now I am better armed because of this discussion board.