Can a moving car's weight be accurately measured on a scale?

[Femme_physics]

I know it doesn't change mass, but does it change weight?

Example:

Let's say I put a weight sensor under the road as the car drives on it. Will it feel the same weight if the car stood still and if it drove really fast, or would it feel different weights?

 

[I like Serena]

I know it doesn't change mass, but does it change weight?

Example:

Let's say I put a weight sensor under the road as the car drives on it. Will it feel the same weight if the car stood still and if it drove really fast, or would it feel different weights?

It depends on your trajectory.
If you're moving horizontally on a road, the weight sensor would show the same weight regardless of speed.
But if you're going up an incline, or going over a hill, the weight sensor would show a different force.
Note that the term "weight" is used to identify only the force due to gravity, but a weighing sensor does measure the force and not the mass.

 

[Femme_physics]

If you're moving horizontally on a road, the weight sensor would show the same weight regardless of speed.

Good, and makes sense.

But if you're going up an incline, or going over a hill, the weight sensor would show a different force.

Really? What would be bigger?

 

[Quinzio]

I know it doesn't change mass, but does it change weight?

Example:

Let's say I put a weight sensor under the road as the car drives on it. Will it feel the same weight if the car stood still and if it drove really fast, or would it feel different weights?

Exactly the same weight.

ILS is right to say that it really depend on a number of factor, if the road if curved, if the road goes uphill, if, if, if...but let's take a straight horizontal flat road, a scale is always "under the car" then the weight is the same.
Put in another way, the car dumpers doesn't feel less weight when the car moves.

 

[I like Serena]

Good, and makes sense.

Really? What would be bigger?

Well, if you're going up an incline, the "weight" would be bigger. (Think as if you're in an elevator going up.)

If you go over the top of a hill, the weight would be less. Wheeee!

If you go through a valley, the weight would be greater.

 

[ShayanJ]

I know it doesn't change mass

In fact special relativity says mass is of two kind.Invariant mass does not change with speed but if I move with respect to you,you will measure my mass a different number.

But about your main question.Your answer lies in equivalence principle.
Imagine your in an elevator.if it starts to move downward with an acceleration equal to a,you will feel the acceleration of gravity,g-a.you will feel lighter.

Wait,my answer was for accelerated motion!

If your moving horizontally,no change happens.But if your going up or down a hill,a sensor will show a different number because it does not measure your weight,but the normal force its surface is applying to your body and that will be different in a slope.

 

[Femme_physics]

Imagine your in an elevator.if it starts to move downward with an acceleration equal to a,you will feel the acceleration of gravity,g-a.you will feel lighter.

Makes sense :)

Well, if you're going up an incline, the "weight" would be bigger. (Think as if you're in an elevator going up.)

If you go over the top of a hill, the weight would be less. Wheeee!

If you go through a valley, the weight would be greater.

The weight of what? Of the moving car, the standing car? Hmm.. it does make sense that going up the weight of the car is bigger than if it just stood still on the inclined hill. Is that what you mean? If so, I think I understand now :)

If your moving horizontally,no change happens.But if your going up or down a hill,a sensor will show a different number because it does not measure your weight,but the normal force its surface is applying to your body and that will be different in a slope.

Ah, "normal force" is what the sensor measures? Interesting :) Makes sense, too.

 

[I like Serena]

The weight of what? Of the moving car, the standing car? Hmm.. it does make sense that going up the weight of the car is bigger than if it just stood still on the inclined hill. Is that what you mean? If so, I think I understand now :)

A weighing sensor measures the force it generates.

If you're simply standing on top of a weighing sensor, the force on it is the force of gravity. The sensor itself must generate the normal force to balance the forces.
The sensor measures the normal force that in this case is equal to your weight.

And actually, I said it wrong before. Sorry!
If you're going up an incline at constant speed, the net resultant force is zero, and again you'll be measuring just your weight.

If you're accelerating, you'll have a resultant force. You'll not only measure your weight but also the resultant force.

If you're going through a valley, you'll be measuring your weight combined with the centripetal force, although actually you'll be measuring the normal force generated to balance those forces.

Ah, "normal force" is what the sensor measures? Interesting :) Makes sense, too.

Very apt!

 

[Lsos]

I thought the car WOULD weigh more as its speed increases due to E=mc^2

Is this not the case?

 

[ShayanJ]

Hey serena you're telling sth wrong.when a car is going up or down a valley,the sensor shows a different number for a person sitting in the car but not because of a new force.
imagine a right triangle(as is common for showing an incline).put x-axis parallel to the incline and y-axis prependicular to it.The weight of a mass sitting on the incline is a vector headed straight down.write that vector as the some of a vector parallel to x and another to y axis.
The sensor in a car moving on an incline,shows the vector component parallel to y-axis and because of that,it shows a smaller number.

and Lsos.the mass of an object increases with its velocity but not because of the formula you wrote.there is a lorentz transformation for mass too which I don't remember now.
I should say that einstein defined a quantity called rest mass or invariant mass which does not change with speed and that is the mass you measure when you are at rest with respect to that object.

 

[I like Serena]

I thought the car WOULD weigh more as its speed increases due to E=mc^2

Is this not the case?

There is indeed a relativistic effect, but we're in the subforum Classical Physics right now.

Hey serena you're telling sth wrong.when a car is going up or down a valley,the sensor shows a different number for a person sitting in the car but not because of a new force.
imagine a right triangle(as is common for showing an incline).put x-axis parallel to the incline and y-axis prependicular to it.The weight of a mass sitting on the incline is a vector headed straight down.write that vector as the some of a vector parallel to x and another to y axis.
The sensor in a car moving on an incline,shows the vector component parallel to y-axis and because of that,it shows a smaller number.

I would depend a bit on how the sensor works.
In a car it would be something like measuring the springs on the wheels.
We wouldn't be measuring perpendicular to the road surface, but effectively vertical, and we would be measuring a combination of the normal force and friction.

The weight would be the same whether we're on an incline or not, as long as there's no resulting force.

 

[ShayanJ]

the sensor is placed on the incline and so is parallel to its surface.so the force that it measures is prependicular to the surface which means the normal force.

 

[I like Serena]

the sensor is placed on the incline and so is parallel to its surface.so the force that it measures is prependicular to the surface which means the normal force.

Ah, I see I did not read the initial post properly!
Good point.

So yes, if only the normal force is measured, you're right. ;)

 

[ZealScience]

You just consider the nature of weight, it's just a force, right? Nothing changes this nature unless you change the position. However, there is also a equivalence weight caused by acceleration in that direction, ignoring the relativistic effect, unless you have an acceleration in vertical direction, weight won't change.

 

[ShayanJ]

thanks zeal.that was the point I wanted to mention but got to answer every time.
because we are in classical physics forum,I forget about relativity.
No speed does not change weight.The change is in the number the sensor shows us.
No matter your going down or up a hill,the sensor shows a smaller number.

 

[Shark 774]

I know it doesn't change mass, but does it change weight?

Example:

Let's say I put a weight sensor under the road as the car drives on it. Will it feel the same weight if the car stood still and if it drove really fast, or would it feel different weights?

Just a little thing, and something you may look into in the future: When you consider special relativity, speed DOES change mass! Very interesting stuff.

 

[skanda9051]

I know it doesn't change mass, but does it change weight?

Example:

Let's say I put a weight sensor under the road as the car drives on it. Will it feel the same weight if the car stood still and if it drove really fast, or would it feel different weights?

hi but i was watching into the universe discovery channel they mentioned that when we travel at the speed of light object weight changes... so what you guys think?...

 

[Lsos]

Yes, a few people already mentioned this, but as had been pointed out, we're in the "classical physics" subforum. I guess this means we don't consider relativistic effects.

 

[vanhees71]

It depends on how you define "classical physics". In my opinion everything is "classical physics", which is not "quantum physics", i.e., classical electromagnetism (as, e.g., treated in Jackson) is also classical physics although it's relativistic. The same holds true for the classical relativistic theory of gravitation, i.e., the General Theory of Relativity. So I feel free to give my 2 cts. to the question.

Of course, the weight, i.e., the gravitational interaction of matter, depends on its energy and momentum since gravity couples universally to any kind of energy and momentum.

Contrary to that the mass of a body does not change with motion (except if there are intrinsic excitations, like temperature changes of macroscopic bodies or the charge of a capacitor, etc.). Sometimes one reads statements about an old-fashioned notion of the quantity mass, where they define the total energy of a particle divided by c^2 as "relativistic mass". This is an idea from before 1908, when Minkowski analyzed the mathematical structure of special relativity, and from then on it should be clear to every physicist that mass is a scalar quantity while the energy is the time component of a four vector.

 

[Dale]

Let's say I put a weight sensor under the road as the car drives on it. Will it feel the same weight if the car stood still and if it drove really fast, or would it feel different weights?

Actually, even ignoring relativity and inclines or anything a weight sensor will measure twice the actual weight for a dynamically applied load. But this is due to elasticity and strain rather than due to any actual increase in the weight, so it is probably interesting but not relevant to your question.

 

[Drakkith]

hi but i was watching into the universe discovery channel they mentioned that when we travel at the speed of light object weight changes... so what you guys think?...

This is incorrect. There is a big misunderstanding with "relativistic mass" as it is called. It is easily resolved by putting yourself in the frame of the moving object. In its own frame, it is NOT moving, so how could it gain mass if it isn't moving?

Just a little thing, and something you may look into in the future: When you consider special relativity, speed DOES change mass! Very interesting stuff.

I don't believe this is correct. See above.

 

[Drakkith]

Yes, a few people already mentioned this, but as had been pointed out, we're in the "classical physics" subforum. I guess this means we don't consider relativistic effects.

Nonsense! The only reason why you wouldn't post here is if it was specifically about something you knew dealt primarily with relativity. If the thread is deemed to be primarily about a subject in another forum then the admins will move it. Just because we are in the "Classical" forum doesn't mean we ignore other fields!

 

[I like Serena]

Nonsense! The only reason why you wouldn't post here is if it was specifically about something you knew dealt primarily with relativity. If the thread is deemed to be primarily about a subject in another forum then the admins will move it. Just because we are in the "Classical" forum doesn't mean we ignore other fields!

Not nonsense!
This thread has derailed from the OP who explicitly posted in "Classical".
However, I do not think that the OP really minds.

 

[DaveC426913]

To sum (and it depends on thea the OP is interested in), there are three factors to consider:

1] If this is a practical experiment (below 1000mph), results could be quite easily corrupted if not carefully controlled. Simply driving a car over a weigh scale would have far too many variables to provide an accurate result.

2] If this is extended to higher speeds (10,000mph+), one might want consider how one is moving relative to the Earth. Fast enough, and the curvature of the Earth will affect the result.

3] If this is extended to relativistic speeds (1,000,000mph+), the mass will be affected by relativistic effects.

 

[skanda9051]

To sum (and it depends on thea the OP is interested in), there are three factors to consider:

1] If this is a practical experiment (below 1000mph), results could be quite easily corrupted if not carefully controlled. Simply driving a car over a weigh scale would have far too many variables to provide an accurate result.

2] If this is extended to higher speeds (10,000mph+), one might want consider how one is moving relative to the Earth. Fast enough, and the curvature of the Earth will affect the result.

3] If this is extended to relativistic speeds (1,000,000mph+), the mass will be affected by relativistic effects.

hi so we are assuming and only we have theoretical solution ... there is no practical solution for this question right....

 

[DaveC426913]

hi so we are assuming and only we have theoretical solution ... there is no practical solution for this question right....

It depends on the OP's needs. They asked about a car going over a scale, so that's why I listed that as #1. I suspect that the OP will draw a conclusion on some real-world test they have observed without factoring in all sorts of confounding variables. But if one were to want to measure the weight of a moving vehicle accurately enough to determine that it does not change when in motion, it should be doable.