(Please visit one of our advertisers)

No donations or subscriptions are required

   OR   
   
Subscription choices:
Board Karma = 40  (3488 positive of 3870 votes is 40 %pts higher than a neutral 50%)
All Things (Safety Oriented) Motorcycle   
Username:
Password:
Save Password
Forgot your Password?

You can the entire collection of Safety Tip articles in a 33 Megabyte PDF Portfolio

 All Forums
 Motorcycle Safety
 Physics and the theoretical
 Weight vs Mass
Member Previous Topic Discussion Topic Next Topic  

James R. Davis
Male Administrator
17282 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 07/03/2012 :  3:58 PM Follow poster on Twitter  Join poster on Facebook as Friend                        Like
During a braking presentation I gave last week the question of weight vs mass came up. I was talking about traction and stated that it was a function of how much weight was pressing the tire rubber against the roadway.

During that discussion I made sure that the audience knew that weight and mass are different. That mass is a constant and is the same for any given object anywhere in the universe, but that weight was location dependent. That is, weight is the result of the interaction of a mass with gravity and that gravity is different from place to place. The closer two masses are to each other (that is, their centers), the greater will be their gravitational pull on each other.

The poles of the earth are about 13 miles closer to the center of the earth than is the equator. Therefore, an object weighs more at the north pole, for example, than if it was located on the equator.

Furthermore, because the earth is spinning, an object located on the equator experiences a modest centrifugal force from that rotation that also reduces weight as compared to that same object sitting at a pole.

For perspective, I announced that Houston is located at approximately 29 degrees North and if my bike and me combined had a mass of 1,000 lbs., it would weigh approximately 1,001 lbs. parked in my driveway. On the other hand, If my bike and I were located in Bolton, England (at 53 degrees north and just to the north of Manchester), our combined weight would be approximately 1,003 lbs. That 2 pound difference (a kilo) was due entirely to location.

Here is a graphic that demonstrates this concept:



One could effectively argue that it takes more braking energy to stop my bike in England than it does in Houston, but other than that I can see no practical value in knowing this concept. But maybe some of you can. What do you think?

DataDan
Advanced Member
540 Posts
[Mentor]


Central Coast, CA
USA

Yamaha

FJR1300

Posted - 07/03/2012 :  8:47 PM
quote:
Originally posted by James R. Davis

What do you think?
I think you posted this just to get the nerds in the audience to identify themselves.

A more practical application of the weight/mass dichotomy is in understanding braking on a hill. Mass is always the same, and on the flat, braking force needed for a given deceleration rate depends only on mass. It would be the same in England, in Houston, or on the moon. But to achieve that same deceleration on a hill, more braking force is required when going downhill and less when going uphill. That's because gravity--a neutral party when braking on the flat--enters the picture. That is, a small fraction of the motorcycle's weight (sine of inclination * m * g) thwarts your braking effort going downhill and assists it going uphill.

Hope this helps.
Go to Top of Page

James R. Davis
Male Administrator
17282 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 07/03/2012 :  9:11 PM Follow poster on Twitter  Join poster on Facebook as Friend  
I think I misstated what I was trying to say above. Thank you.

As I was talking about traction, which is a function of weight, not mass, I meant to say that because there is a trivially higher amount of traction available in England than in Houston, the rider COULD need to use more braking energy to effect the same deceleration rate there than in Houston. But perhaps even that is not accurate.

As it is clear to me that adding weight does NOT, necessarily, increase stopping distance or distance (unless it changes the location of the CG) - since more braking energy can be used to accomplish the same rate.

So, perhaps it is more correct to say that the bike in England COULD attain a slightly higher deceleration rate before it began to slide?

Confused. (Doesn't it irritate you as a fellow nerd, when reading about braking dynamics, to see the phrase 'wheel dynamic mass'? There is nothing dynamic about mass - they are talking about weight.)

By the way, it took me forever to work out the computations that allowed me to determine weight based on Latitude.

Go to Top of Page

James R. Davis
Male Administrator
17282 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 07/03/2012 :  11:15 PM Follow poster on Twitter  Join poster on Facebook as Friend  
I created an Excel spreadsheet (here) for your use to see how the weight was computed.
Go to Top of Page

James R. Davis
Male Administrator
17282 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 07/04/2012 :  7:36 AM Follow poster on Twitter  Join poster on Facebook as Friend  
Ah! Eureka!

What this says, in layman's terms, is that it may be easier to move to Houston than diet, if weight is your problem.
Go to Top of Page

twowheelsbg
Male Junior Member
50 Posts


Burgas, Burgas
Bulgaria

Suzuki

Posted - 08/15/2013 :  3:36 AM
It is a fun topic indeed, but also tangent to others, more practical.
Let me sketch.

Imagine a man with mass 100kg enters stationary elevator.
I mean the mass of the man is 100kg, not the scenario he carries 100kg suitcase
Imagine also our elevator has weight scale on the floor,
calibrated in Newtons. Accepting the gravity as g=9.81m/s/s aprox.
the scale would indicate mass*g=981 N.
If the elevator starts going up accelerating with 10% of gravity,
the scales would indicate mass*(g+10%g)= 981+98.1=1079.1 N.
If the elevator starts going down accelerating with same 10% of gravity,
the scales would indicate mass*(g-10%g)=981-98.1=882.9 N.

Here is the question, how much is the weight of the man,
is it 981, 1079 or 883 N? Some guys would say weight could be each of those, as the scales showed depending on the situation. Others would argue this is rubbish, weight should be 981 because other figures are derived in accelerating frame. So, not to argue unnecessary, we have to choose terminology and to ask others when speak of 'weight' to be precise - do they allow it to variate with the observation frame or not. I personally prefer to use term weight as mass times gravity acceleration. So, my weight does not change. In case observation frame accelerates, I tend to use not 'weight', but 'load' or similar word. This way elevator example seems to me as : weight 981N, load ( on scales ) 1079 or 883 or whatever depending on elevator ( observation frame ) acceleration.

Going back to bikes, there is similar term mismatch due to different terminology. Take a look at the popular concept of braking/acceleration. Does weight go/transfer back and forth? Depends on your terminology. For me it looks more clear and simple for understanding that weight does not transfer. Mass center ( ignoring suspension and tires flex ) does not move, so weight ( due to gravity because this is what I accept - weight could be produced only by gravity ) distribution does not change. What changes is the load at the tires, because braking/accelerating dynamics affects the load produced by weight alone, so the actual push to the ground changes.
That's why I prefer the term load transfer, not weight transfer.

Frankly speaking, I understand what others mean using weight transfer under braking as minor terminology confusion, but it could be not so minor for others especially when there are not one but couple concepts without preliminary agreed terminology convention.

Actually, not to hijack the fun element from the topic,
do you acknowledge that even in a stationary bike the tires do not push straight down ( the normal ground reaction is slightly abnormal )? Rephrasing, braking performance seems affected by the direction of travel
Go to Top of Page

JMalmsteen
Female New Member
13 Posts


Long Island/Lancaster, NY/PA
USA

Harley-Davidson

XL1200

Posted - 02/14/2014 :  5:12 AM
You gave a great example that I will use in class. I teach physics and the kids are fascinated with stuff like this. We normally talk about how to lose weight in physics and I'll tell them to go upstairs (or brush their hair, since they will lose electrons). We talk about taking staplers to space. My favorite example is why superman has super powers based on gravitational differences between Earth and Krypton. If the kids go to the moon, they can also turn into Superman.

In physics, deceleration is a four letter word. An acceleration is a change in velocity over time. You can have positive or negative accelerations, but slowing down is still accelerating.

As far as the elevator example, we talk about apparent weight and usually use rollercoaster or free fall examples in class. And, we do a lab using elevators and scales. You actually do see the differences on the scale even in a three story building.

Edited by - JMalmsteen on 02/14/2014 5:19 AM
Go to Top of Page
  Previous Topic Discussion Topic Next Topic  
Jump To:
All Things (Safety Oriented) Motorcycle © Master Strategy Group Go To Top Of Page
  This page was generated in 0.45 seconds. Powered By: Snitz Forums 2000 Version 3.4.05