Disc Golf Course Review How do you calculate disc weight at a certain speed
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#21
03-09-2020, 06:50 PM
 Blobfish Eagle Member Join Date: Jul 2016 Location: Pittsburgh, PA Years Playing: 3.9 Courses Played: 18 Throwing Style: RHBH Posts: 791 Niced 264 Times in 151 Posts

Quote:
 Originally Posted by ALT-J I kinda knew this was coming because I could not state the question properly because I dont know physics too well. But everyone knows a disc will feel very heavy for a split second in a good throw and objects inside the car become heavier in a crash etc. Thats the phenomena I tried to describe. Dont know what the actual name for it is.
Inertia.

Mass is the way we quantify inertia (for all intents and purposes, inertia and mass are synonymous).

A qualitative description of inertia is an object's resistance to a change in its state of motion. Newton's First Law of Motion (aka the Law of Inertia) is the one that says, "an object at rest stays at rest unless acted upon by an external force" or "an object at constant velocity stays at constant velocity unless acted upon by an external force".

Here, the disc starts off at rest. A good reachback isn't necessarily one where the disc gets thrust backwards; in a lot of cases, the disc stays in place relative to the ground as you move around it to set up your reachback. But still, the point is, the disc is at rest and it wants to stay at rest due to Newton's First Law.

Newton's Second Law of Motion is the one that folks remember as F=ma, but that's because a simple product is easier to remember than a fraction. However, rearranging the equation gives us a=F/m where a is acceleration, F is force that must be applied to create the acceleration, and m is mass of the object being accelerated.
Now we generally think of acceleration as "speeding up" but in fact, acceleration is the change in velocity with respect to time (where velocity is speed with a direction). As such, a positive acceleration changes velocity in the positive direction and negative acceleration changes velocity in the negative direction.
For now, let's call forward the positive direction.

So... we have a disc that is at rest. We want to get it up to 80 mph. We have the length of our reachback and pull and release to get it from 0 mph (initial velocity) to 80 mph (final velocity)
In order to get that large change in arm speed (final velocity-initial velocity) in such short real estate, the time interval has to be really small.
So remember, I said acceleration is the change in velocity with respect to time (change in velocity over change in time (dv/dt))
The smaller you make the time interval, the bigger the fraction becomes for a fixed change in velocity.
Anyway, dV/dt is the very definition of velocity.

Back to Newton's Second Law which said that a=F/m.
If we know the acceleration (change in velocity over time), then that value must equal the force needed to create that acceleration (your pull) divided by the mass of the object being accelerated (your 175 g disc)
For the same size disc, if you want to crank up the acceleration, it means you need to apply more force. This is common sense, I know, but I'm trying to put the concepts all together.
If you're really interested, the second law is actually the Impulse Momentum Theorem: Fdt=mdV because F=ma and we substituted dV/dt for a and then multiplied both sides by dt. The impulse momentum theorem says that apply a force for a given amount of time, it will change the objects momentum (mass x velocity).

Anyway, Newton had a third law of motion: We call this the law of action and reaction, but really, what it says is that forces occur in pairs and every force has an equal but opposite (in direction) reaction force. Why this is crucial is the greater the applied action force, the greater the reaction force (WHICH IS WHAT YOU FEEL)
Wait, what?
Ok, bear with me. When you are in a car and you hit the gas, you do so to make your car accelerate forwards. But it FEELS like you are thrown backwards. Really, what you are experiencing is Newton's First Law all over again. Your body wants to stay at whatever velocity (or at rest), but the car moves and so it presses into your back and you feel like you are thrown backwards. Similarly, when you stomp the brakes (negative acceleration), you feel like you are thrown forwards, because again, the law of inertia says that you want to keep going at constant velocity but the car wants to slow down, so you feel like you're thrown forwards.

The point of that example is that you FEEL a force that is in the opposite direction of the acceleration.

So when you pull on your disc with your maximum acceleration that you can muster, the disc feels the heaviest it can because of the disc's inertia.

 Niced: (4)
#22
03-09-2020, 07:07 PM
 Innova63 Eagle Member Join Date: Aug 2011 Location: South-eastern, Disconsin Years Playing: 35 Courses Played: 260 Throwing Style: RHBH Posts: 652 Niced 173 Times in 101 Posts

Quote:
 Originally Posted by ThrowBot Or for us dumb Muricans: about 0.39 pounds(force) at 32.2 ft/s^2.

 Niced: (2)
#23
03-09-2020, 07:23 PM
 BogeyNoMore * Ace No More * Join Date: Jul 2009 Location: Walled Lake, MI Years Playing: 16 Courses Played: 330 Throwing Style: RHBH Posts: 10,725 Niced 3,509 Times in 1,606 Posts

Is it OK if I just throw my disc?

Kidding aside, while I really do think it helps to understand the basic physics of disc flight, the physics pretty quickly gets more complex than your basic two semesters of Physics 101 & 102 addresses.... which is why most people won't be able to follow the complete physics of disc flight.

However, you can get a great deal of practical knowledge by accumulating a wealth of observational info. That helps build a sort of internal database, which ultimately leads to a feel for your discs, and the game.

I believe most people's best throws are about "feel" rather than about their comprehension of disc physics.

 Niced: (1)

Last edited by BogeyNoMore; 03-09-2020 at 07:25 PM.
#24
03-09-2020, 07:25 PM
 ru4por * Ace Member * Join Date: May 2012 Location: Dearborn Michigan Years Playing: 37 Courses Played: 248 Throwing Style: RHBH Posts: 4,377 Niced 2,855 Times in 1,013 Posts

Quote:
 Originally Posted by etdefender19 So it's more overstable, then?
Wait...does is fly like a TeeBird?
#25
03-09-2020, 07:52 PM
 Shamis Eagle Member Join Date: Mar 2012 Years Playing: 9.9 Courses Played: 71 Throwing Style: RHBH Posts: 622 Niced 174 Times in 87 Posts

Quote:
 Originally Posted by ThrowBot Uhhh...I'm getting closer to 1.72 Newtons. At least on earth's surface, with the acceleration of gravity of 9.81 m/s^2. Or for us dumb Muricans: about 0.39 pounds(force) at 32.2 ft/s^2.
#26
03-09-2020, 09:29 PM
 ThrowBot Double Eagle Member Join Date: Jun 2015 Location: Billings, MT Years Playing: 13.9 Courses Played: 145 Throwing Style: RHFH Posts: 1,606 Niced 1,109 Times in 474 Posts

Quote:
Wolfram Alpha is better, my friend.

F=ma

F=175g * 9.81 m/s^2

Go to Wolfram, type in "175g * 9.81 m/s^2" and hit enter. It will spit out the answer in a variety of units.

 Niced: (1)
#27
03-10-2020, 12:23 AM
 ejvogie * Ace Member * Join Date: Sep 2008 Location: Toledo, OH Years Playing: 11.8 Courses Played: 133 Throwing Style: RHBH Posts: 4,678 Niced 805 Times in 369 Posts

Quote:
 Originally Posted by ru4por Wait...does is fly like a TeeBird?
Yes, but longer

 Niced: (1)
#28
03-10-2020, 12:42 AM
 R-Ogre Eagle Member Join Date: Jun 2019 Location: Juneau, AK Posts: 550 Niced 727 Times in 270 Posts

Quick google search has a golf swing at .25 sec and a baseball swing at .15 sec. For funsies let’s split the difference and call it .2 sec, and say an am can throw 80 kph (22 m/s). With a weight of 175 gm we’ve got all we need to ballpark it. My extremely rusty college physics and some google gives 19.25 Newtons or about 4 lbs.

I’ll admit it’s extremely possible that I missed a decimal place or 2 somewhere in there.
#29
03-10-2020, 01:07 AM
 R-Ogre Eagle Member Join Date: Jun 2019 Location: Juneau, AK Posts: 550 Niced 727 Times in 270 Posts

And of course as pointed out you’d have to know a lot more to account for the force you put in to getting the disc spinning.

#30
03-10-2020, 09:05 AM
 ALT-J Par Member Join Date: Nov 2015 Posts: 110 Niced 37 Times in 24 Posts

Quote:
 Originally Posted by Blobfish Inertia. Way too long answer follows:

Thank you for your detailed answer. Really refreshed some memories from way back, especially the classic Newton laws and the delta sign that represents change in velocity and time. Good times over 15 years ago. So how do you calculate the force and mass you need to resist if you know the length of the swing, the weight of the disc and roughly the time span where the acceleration happens? How would that force translate into inertia and mass in kilograms that a thrower will feel momentarily? Dont know if these are even valid questions but lets try.

The concept of acceleration is kind of hard. In disc golf its probably easiest to see the effect in putts. The best and most consistent and repeatable putts always feel like that the time span for the actual throw is very small and compact. Yet the disc flies 60-70 feet with a very small movement. Its like going from 5 mph to probably 40 mph in a fraction of a second that makes the disc fly. Thats very confusing and counter intuitive for me for some reason.