Negative Nose Loads

Mar 14, 2005
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Just returning to Icaru5's hypothetical scenario where the tow car is loaded to 20kg above its GVW limit, but the trailer has a negative nose load which lifts the ball hitch and which will reduce the cars weight when coupled so it weighs less than its GVW limit.

This is actually more interesting than it might at first seem.

If the outfit were weighed, the car would appear to be within its GVW and it might get away with it. But this is where an anomaly might begin to appear.

The EU regulations as drafted by Brussels, refers to vehicle Masses not weights, and this is where the misuse of measurement units that is so common here leads to confusion and in this case legal difficulties.

Scientifically minded readers will know there is a difference between Mass and Weight. They are related but they are not the same thing. All matter has a mass, but it will only demonstrate a weight (which is a force) when it is subjected to an acceleration. In earthly terms we are used to the acceleration caused by gravity to wards the centre of the earth, and we use that as our reference. Scientifically if we have mass of 1kg it will be attracted to the centre of the earth with a force of 1 Kilogram force. (or more accurately 9.81 NM)

Good old Sir Isaac Newton worked it all out and gave us the basic formula of

Force = Mass x Acceleration.

Humans have decided that the acceleration due to earth gravity has a value of 1 and we compare all other gravitational systems to our own. So for example the gravity of the moon is approx 0.6 that of Earths, so your 1kg bag of sugar on earth will only weigh 600gm on the moon, but its Mass is still 1kg

The astute roadside tester will know this and should measure the axle loads of the car and caravan as an outfit, and just as the solo car, and armed with that information they can calculate the mass of the car and the caravan and the transfer of loads, and the direction of forces. By doing so they would discover the negaive nose load and the car with its excessve Mass over its MAM.

It also worthy to note that on the VIN plate whilst the what we conventionally call GVW is officially known as Maximum Authorises Mass, but the axles values are Loads not masses. so it is permissible to measure the axle loads by weighing them.
 
Nov 16, 2015
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So if you have a lorry filled with racing pigeons, and it weighs saw 20 tonnes, and they all decided to fly inside the lorry at the same time, how much does it weigh then ??
 
Mar 14, 2005
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EH52ARH said:
So if you have a lorry filled with racing pigeons, and it weighs saw 20 tonnes, and they all decided to fly inside the lorry at the same time, how much does it weigh then ??

:evil: It depends ..........

Stop reading now if you are not a geek..... :cheer:

...If its "filled" there won't be any room to fly. so its a mute question :blush: - and where will the driver sit? :S

.......on, if the lorry has free air movement in and out ( which to fly they must be alive and thus able to breath) in which case any difference in the air pressure caused by wing beats will almost certainly be able to escape the lorry and thus not transfer all its thrust to the bed of the lorry so it will weigh less.... But its mass including load will remain the same!!!!

What about the change in inertial load when the pigeons push off to fly and the impact when they land can you quantify that ? How much will it change the pressure in the trucks trailer tyres? :blink:
 
Nov 16, 2015
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The Prof asked, where does the driver sit, on the drivers seat. Dooh.

Ok Prof how do you get Four elephants into a Citron, 2 CV, ;) Oh two in the front two in the back, :p

PS. Tyre pressures do not change unless the pigeons take of and land enough to raise the tyre temperature. :woohoo:
 
Jun 26, 2017
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GD485 said:
Hi Prof,
Surely a bag sugar can only have a weight of 1Kg if it's on Earth!
As you stated force exerted by earth's mass on the sugar that results in a force of 1Kg can only be on this planet.
If it is elsewhere (or on Earth) it is mass will be 0.981 Newtons. On the moon it weight would equivalent to 600g but still have a mass of 0.981N.
Also the bag of sugar needs to be stationary if it has a velocity (not speed) then the mass will gain energy and therefore mass.
Also where are you observing the bag of sugar, are you on the same intertial plane.
In making these observations the properties of the sugar is altered by the observation itself? Heisenberg wasn't too sure or was he!!!
Assuming that gravity is a force but a bloke called Albert postulated that gravity is a distortion of time/space caused by the presence of a massive body. Which came first?

You seem to be getting Mass and Force mixed up a little GD ...

A bag of sugar that has a mass of 1KG has a mass of 1KG, whether it is on Mars, the moon, Venus or elsewhere.

What would differ if the bag of sugar was subject to a different gravitational pull is the force that the 1KG bag of sugar would exert on the ground.

Newton’s second law correctly states that F=M x A, (Force = Mass X Acceleration)

Since ...

The bag of sugar has a mass of 1KG

And, using widely accepted values, for the purposes of calculation,

Acceleration due to gravity on Earth is 9.81M/S/S
And
Acceleration due to gravity on the Moon is 1.62M/S/S

On earth, the bag of sugar exerts a downwards force of 1 x 9.81 = 9.81 Newtons.
On the moon, it exerts a force of 1 x 1.62 = 1.62 Newtons.

However, no matter where the bag of sugar is, it has a mass of 1KG.

To take it a step further, when somebody says their weight is 70KG, that is actually wrong. Their MASS is 70KG.

If they took their bathroom scales to the moon, and stood on them, the scales would read 11.56KG (70 / (9.81/1.62)) and their scales would be wrong. The reason being that everyone’s bathroom scales and other “weighing” equipment is calibrated for use on earth.

Despite being on the Moon, the person’s MASS is still 70KG, but despite displaying kilograms, the scales are not able to measure mass, only force, but because they will (most probably) only ever be used on earth, using M=F/A, they use 9.81 for A as a fairly safe bet to tell you your mass.

Unfortunately, if you want to use your bathroom scales on the Moon, you’ll need to have them (significantly) recalibrated !
 
Jun 26, 2017
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GD485 said:
Icaru
The unit kilogram is an alternative way of expressing the force acting on a mass that only occurs on earth. The force of 9.81 N is the force applied to specific mass that earthlings call 1kg
As a planet of smaller mass has a different gravitational force then the unit kg cannot exist
This was what Newton was trying to create, a uniform unit of mass independent of gravity or was gravy:)

Sorry, but your opening sentence is absolutely not true GD.

The Kilogram is the SI unit of MASS. The SI unit of force is the NEWTON.

Your second sentence makes sense, but to help you with the rest, please read the edited version of my previous post which uses bathroom scales as an example.
 
Mar 14, 2005
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GD485 said:
Hi Prof,
Surely a bag sugar can only have a weight of 1Kg if it's on Earth!
As you stated force exerted by earth's mass on the sugar that results in a force of 1Kg can only be on this planet.

No. the bag of sugar can have a weight of 1Kg anywhere it is subjected to an acceleration of 1G along the axis of acceleration. It just so happens that on earth we do experience an acceleration of 1G due to the mass of the Earth, any other body with the same mass as the Earth would produce an attraction or gravity of 1G

GD485 said:
If it is elsewhere (or on Earth) it is mass will be 0.981 Newtons. On the moon it weight would equivalent to 600g but still have a mass of 0.981N.

No. Mass is measured in kg, a Newton is a force. And what I actually wrote was

ProfJohnL said:
Humans have decided that the acceleration due to earth gravity has a value of 1 and we compare all other gravitational systems to our own. So for example the gravity of the moon is approx 0.6 that of Earths, so your 1kg bag of sugar on earth will only weigh 600gm on the moon, but its Mass is still 1kg
.

GD485 said:
Also the bag of sugar needs to be stationary if it has a velocity (not speed) then the mass will gain energy and therefore mass.

No it doesn't gain mass if it moves. In Newtonian physics the mas remains constant, however its velocity will affect its kinetic energy.

The bag of sugar on the surface of the Earth is following a circular path around the axis of the earth, so it maintains its position relative to the centre of the earth which attracts it, but the surface of the earth opposes the attraction with an equal and opposite force, and the resultant force is measured as its weight. But as it is rotating its velocity is constantly changing, thus it is experiencing a constant acceleration towards the centre of the earth.

GD485 said:
Also where are you observing the bag of sugar, are you on the same intertial plane.
In making these observations the properties of the sugar is altered by the observation itself? Heisenberg wasn't too sure or was he!!!
Assuming that gravity is a force but a bloke called Albert postulated that gravity is a distortion of time/space caused by the presence of a massive body. Which came first?

Neither General or Special Relativity changes the physical effects we believe we can practically see or measure.
 
Jun 26, 2017
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ProfJohnL said:
GD485 said:
Hi Prof,
Surely a bag sugar can only have a weight of 1Kg if it's on Earth!
As you stated force exerted by earth's mass on the sugar that results in a force of 1Kg can only be on this planet.

No. the bag of sugar can have a weight of 1Kg anywhere it is subjected to an acceleration of 1G along the axis of acceleration. It just so happens that on earth we do experience an acceleration of 1G due to the mass of the Earth, any other body with the same mass as the Earth would produce an attraction or gravity of 1G

GD485 said:
If it is elsewhere (or on Earth) it is mass will be 0.981 Newtons. On the moon it weight would equivalent to 600g but still have a mass of 0.981N.

No. Mass is measured in kg, a Newton is a force. And what I actually wrote was

ProfJohnL said:
Humans have decided that the acceleration due to earth gravity has a value of 1 and we compare all other gravitational systems to our own. So for example the gravity of the moon is approx 0.6 that of Earths, so your 1kg bag of sugar on earth will only weigh 600gm on the moon, but its Mass is still 1kg
.

GD485 said:
Also the bag of sugar needs to be stationary if it has a velocity (not speed) then the mass will gain energy and therefore mass.

No it doesn't gain mass if it moves. In Newtonian physics the mas remains constant, however its velocity will affect its kinetic energy.

The bag of sugar on the surface of the Earth is following a circular path around the axis of the earth, so it maintains its position relative to the centre of the earth which attracts it, but the surface of the earth opposes the attraction with an equal and opposite force, and the resultant force is measured as its weight. But as it is rotating its velocity is constantly changing, thus it is experiencing a constant acceleration towards the centre of the earth.

GD485 said:
Also where are you observing the bag of sugar, are you on the same intertial plane.
In making these observations the properties of the sugar is altered by the observation itself? Heisenberg wasn't too sure or was he!!!
Assuming that gravity is a force but a bloke called Albert postulated that gravity is a distortion of time/space caused by the presence of a massive body. Which came first?

Neither General or Special Relativity changes the physical effects we believe we can practically see or measure.

Sorry, but your opening statement is incorrect Prof.

Nothing weighs 1KG. Weight is a force, the SI unit of which is Newtons, NOT Kilograms.

A body that has a MASS of 1KG has a mass of 1KG irrespective of the gravitational force to which it is subjected.

On the moon, that same body would be subject to a gravitational force of only 0.16G, but it would still have a mass of 1KG.
 
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Icaru5 said:
Sorry, but your opening statement is incorrect Prof.

Nothing weighs 1KG. Weight is a force, the SI unit of which is Newtons, NOT Kilograms.

A body that has a MASS of 1KG has a mass of 1KG irrespective of the gravitational force to which it is subjected.

On the moon, that same body would be subject to a gravitational force of only 0.16G, but it would still have a mass of 1KG.

I agree the SI unit of force is the Newton, but Weight is a perfectly legitimate way of expressing the reaction force when a mass is acted on by gravity which is the specific point I was making.

Where I have suggested that a mass of an object would change under different gravitational circumstances. ?

The gravitational attraction on the surface of the moon is approximately 0.6G not as you state 0.16G.
 
Nov 11, 2009
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Before I hasten up into the loft and retrieve my books by S Timoshenko, or Beer and Johnston, does the recent redefinition of the "kilogram" affect this erudite debate? Studying in the era of mixed units which included "slugs" and "poundals", "Newtons" etc I always had difficulty figuring out the various relationships. Even my wheel bolt torque wrench serves to further confuse me with its mixed units. Perhaps I should take the dogs a walk. :evil:
 
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ProfJohnL said:
Icaru5 said:
Sorry, but your opening statement is incorrect Prof.

Nothing weighs 1KG. Weight is a force, the SI unit of which is Newtons, NOT Kilograms.

A body that has a MASS of 1KG has a mass of 1KG irrespective of the gravitational force to which it is subjected.

On the moon, that same body would be subject to a gravitational force of only 0.16G, but it would still have a mass of 1KG.

I agree the SI unit of force is the Newton, but Weight is a perfectly legitimate way of expressing the reaction force when a mass is acted on by gravity which is the specific point I was making.

Where I have suggested that a mass of an object would change under different gravitational circumstances. ?

The gravitational attraction on the surface of the moon is approximately 0.6G not as you state 0.16G.

I’m not being drawn into an argument here Prof, as it has happened too many times, on reflection, regrettably, but your opening statement made reference to a condition under which the 1KG would remain.

Regarding your figure for the gravitational force on the moon, I’m afraid it is wrong, and is indeed 0.16G as I correctly said.

If it was 0.6, we would be in serious trouble for many reasons ! - Not least unimaginable tidal forces.
 
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In view of the fact that we are all on the same planet where the gravitational forces on the whole vary negligibly, it is fairly irrelevant whether we talk about mass or the gravitational force acting on that same mass. So long as we remain consistent, the relationship between any two values, whether mass or force, will remain constant.
 
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My mistake.

I acknowledge that my reference to the moons gravitational force was wrong. it is 0.166 that of Earths or 1/6th.

Edited addition
All my other references to weight in KG are correct as I referenced them to a gravitational value of 1.

It is a long and accepted tradition that in non scientific circles the force exerted by gravity on a Mass is commonly referred to as weight.
 
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ProfJohnL said:
My mistake.

I acknowledge that my reference to the moons gravitational force was wrong. it is 0.166 that of Earths or 1/6th.

Edited addition
All my other references to weight in KG are correct as I referenced them to a gravitational value of 1.

It is a long and accepted tradition that in non scientific circles the force exerted by gravity on a Mass is commonly referred to as weight.

I entirely agree Prof, and on a Caravan forum, I wouldn’t normally dwell on the difference between mass and weight, but in this case, the OP (Don’t quite remember who it was ! :p ) made a point of distinguishing between the two and it continued on from there !

As is often the case Prof, we have once again found ourselves needlessly debating the same irrelevant point, long after the majority of those more sane than ourselves have disappeared due to lack of interest and rightly so ! :p
 
Nov 11, 2009
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Craigyoung said:
You's should write a book you's two ! ;)

Craig
There is no shortage of books on this subject. I have masses of them in the loft. Or should it be weights or forces of them sat 20 ft above my head? :)
 

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