Rest Mass Vs Movement

[hazelm]

Does moving from rest mass to velocity have an effect on weight?  What I mean is:  Once you force an object to start moving (ex:  pushing a stalled car)  it becomes easier to continue pushing it (force),  will it not also weigh less?  Is that how it air ships overcomes gravity?

[Moronium]

Weight really doesn't have anything to do with it.

 

This may sound funny, but doubling your speed quadruples the amount of your kinetic energy.  You therefore need 4 times as much energy.  If it took 2 gallons of gas to accelerate some heavy equipment from 0 mph to 30 mph, then it would take 8 gallons of gas to then to accelerate from 0 mph to 60 mph.  So that's 2 gallons from 0 to 30, and another 6 gallons to go from 30 mph to 60 mph, so it's not like the faster you go the less it takes to accelerate. Generally speaking it's just the opposite.  It makes more sense when you consider that the faster you are going, the more distance you cover in the same amount of time.  The need for more energy is a natural consequence.  It takes more work to move something one mile than it does 1/2 mile, for example.

 

A car is on wheels, so once it starts rolling it takes less energy to keep it rolling.  In addition to reducing friction, a wheel (or, better yet, a set of wheels attached to an axle) is a simple machine, so it serves to amplify the force being applied.

 

Mass is different from weight.  Mass, properly speaking, is just resistance to acceleration.  A wagon on wheels doesn't resist accelation to the same degree as a wagon with it's entire bottom surface sitting on the sidewalk.  But that's not really because the mass, per se, has changed.  It's just that the frictional forces you have to overcome are far greater.

 

Of course, due to the law of inertia, a body in motion tends to stay in motion, so you have that going for you too once you get a stalled car rolling.  Even if you quit pushing altogether is will still keep rolling for a while.

 

So, there are a number of factors involved, but neither weight or mass changes in everyday circumstances.  At very high speeds, mass can vary with speed--so called relativistic mass.

Edited June 7, 2018 by Moronium

[Moronium]

The weight of an object on earth is a function of gravity, which is a constant force.  Same with mass--it's a constant, inherent quality of matter.  These kind of things don't change with motion (unless extreme, like going into outer space or travelling near the speed of light).  So the answer to these types of questions will be found in the dynamics of the situation, which change with the circumstances.

Edited June 7, 2018 by Moronium

[hazelm]

Weight really doesn't have anything to do with it.

 

This may sound funny, but doubling your speed quadruples the amount of your kinetic energy.  You therefore need 4 times as much energy.  If it took 2 gallons of gas to accelerate some heavy equipment from 0 mph to 30 mph, then it would take 8 gallons of gas to then to accelerate from 0 mph to 60 mph.  So that's 2 gallons from 0 to 30, and another 6 gallons to go from 30 mph to 60 mph, so it's not like the faster you go the less it takes to accelerate. Generally speaking it's just the opposite.  It makes more sense when you consider that the faster you are going, the more distance you cover in the same amount of time.  The need for more energy is a natural consequence.  It takes more work to move something one mile than it does 1/2 mile, for example.

 

A car is on wheels, so once it starts rolling it takes less energy to keep it rolling.  In addition to reducing friction, a wheel (or, better yet, a set of wheels attached to an axle) is a simple machine, so it serves to amplify the force being applied.

 

Mass is different from weight.  Mass, properly speaking, is just resistance to acceleration.  A wagon on wheels doesn't resist accelation to the same degree as a wagon with it's entire bottom surface sitting on the sidewalk.  But that's not really because the mass, per se, has changed.  It's just that the frictional forces you have to overcome are far greater.

 

Of course, due to the law of inertia, a body in motion tends to stay in motion, so you have that going for you too once you get a stalled car rolling.  Even if you quit pushing altogether is will still keep rolling for a while.

 

So, there are a number of factors involved, but neither weight or mass changes in everyday circumstances.  At very high speeds, mass can vary with speed--so called relativistic mass.

Thank you, Moronium.  I am not ignoring your answer.  I am studying it, trying to figure it all out.  Lots of information there that want thought.  Mass has nothing to do with weight.  All right.  I knew - or thought I knew - that it takes less energy to keep something moving once you get it moving.  But it won't really be any lighter.  It will just seem lighter to the person doing the pushing (source of force).  That seems to be what you say in your second paragraph.  But the first paragraph confuses me. 

 

I'll think on it. 

[hazelm]

Ah!  I got it.  There is also energy in those moving wheels.  So, I need to re-word my question.  Forget stalled cars. Forget speeding up.  Let's take a heavy crate that needs to be moved from one side of the warehouse to the opposite side.  The only energy available is in the muscles of two humans (or even your wheel-less car on a sidewalk).  Now, once the two humans get that crate sliding,  will it take less, more, or the same amount of their energy to keep it moving?  Does it become easier to keep the crate sliding than it took to start it sliding?

 

While thinking on your other reply, I stumbled on a neat video.  A bit of a different line of thought while also shedding light on what you said.  So, I'll start a new thread.  It covers something I read some time ago.  Yes, you can go faster than the speed of light. 

[Moronium]

  The only energy available is in the muscles of two humans (or even your wheel-less car on a sidewalk).  Now, once the two humans get that crate sliding,  will it take less, more, or the same amount of their energy to keep it moving?  Does it become easier to keep the crate sliding than it took to start it sliding?

 

In my opinion :

 

1.  Yes it would take more energy to get it moving.  More force would be required because, again, you wouldn't have the law of inertia assisting you until you first got it moving.

 

2. "Does it become easier to keep the crate sliding than it took to start it sliding?"  Again, yes, for the same reason (so long as you're just keeping it going at the same speed).  It would, however, then take just as much energy (force) as it did to get in moving in the first place to completely stop it from moving at that point.

 

We often tend to think that something which is accelerating is "going faster," but that's not always the case.  Acceleration is just a change in either speed or direction (or both).  Slowing down, although often called "deceleration," is really just another form of acceleration.  There is no acceleration without an external force being applied.  It takes force to make something go faster, but it also takes force to make something go slower.

Edited June 7, 2018 by Moronium

[Moronium]

Mass has nothing to do with weight.  All right.  

 

 

Well, no, not exactly.  I just said that mass is not the "same thing" as weight, but they are closely related.  In free fall, nothing has any "weight" but it still has mass.  In free fall, if you let go of your water glass it will not "fall to the floor."  It will just float along beside you because it too is in free fall and hence has no "weight."

 

Your mass stays the same, wherever you go, but your "weight" will vary according the the gravitational field you're in.   You would weigh less on the moon than you do on earth, and you would weigh much more on the planet jupiter. That's because Jupiter is so much more massive than earth and therefore it "attracts" you (holds you in place, so to speak) with much more force.  It would therefore require more force to "pick you up" off of Jupiter's surface than earth's.

Edited June 7, 2018 by Moronium

[hazelm]

In my opinion :

 

1.  Yes it would take more energy to get it moving.  More force would be required because, again, you wouldn't have the law of inertia assisting you until you first got it moving.

 

2. "Does it become easier to keep the crate sliding than it took to start it sliding?"  Again, yes, for the same reason (so long as you're just keeping it going at the same speed).  It would, however, then take just as much energy (force) as it did to get in moving in the first place to completely stop it from moving at that point.

 

We often tend to think that something which is accelerating is "going faster," but that's not always the case.  Acceleration is just a change in either speed or direction (or both).  Slowing down, although often called "deceleration," is really just another form of acceleration.  There is no acceleration without an external force being applied.  It takes force to make something go faster, but it also takes force to make something go slower.

Thank you.  I do not remember right off the top of my head what I had read that made me ask the question but that is pretty much the picture my brain was forming.  It was something Lawrence Krauss was explaining in "A Universe From Nothing".  I'll think of it at 2;00 AM.

[hazelm]

Well, no, not exactly.  I just said that mass is not the "same thing" as weight, but they are closely related.  In free fall, nothing has any "weight" but it still has mass.  In free fall, if you let go of your water glass it will not "fall to the floor."  It will just float along beside you because it too is in free fall and hence has no "weight."

 

Your mass stays the same, wherever you go, but your "weight" will vary according the the gravitational field you're in.   You would weigh less on the moon than you do on earth, and you would weigh much more on the planet jupiter. That's because Jupiter is so much more massive than earth and therefore it "attracts" you (holds you in place, so to speak) with much more force.  It would therefore require more force to "pick you up" off of Jupiter's surface than earth's.

Thanks.  Then increase in speed - enough increase in speed - overcomes gravity and the space traveler weighs less. 

[Moronium]

Thanks.  Then increase in speed - enough increase in speed - overcomes gravity and the space traveler weighs less. 

 

Well, again, not exactly.  It's not an increase in speed that affects weight, but speed can increase mass.  Just like they were saying in the video you recently posted in another thread.

 

Again, mass is "a resistance to acceleration" (not the same thing as weight).  If, as is posited by SR, it is "impossible" to exceed the speed of light, then, at that speed, no amount of energy could ever force your speed to increase..  The resistance to further acceleration (mass) would be infinite.

 

On the other hand, if your "speed" took you to the moon, and landed, your weight would be less.  If it took you to Jupiter, your weight would be more.  If it took you into a black hole, your mass would, in a sense, be infinite (but actually you would just be toast).  Again, "weight" varies with the gravitational field you're in, not speed.

Edited June 7, 2018 by Moronium

[hazelm]

Well, again, not exactly.  It's not an increase in speed that affects weight, but speed can increase mass.  Just like they were saying in the video you recently posted in another thread.

 

Again, mass is "a resistance to acceleration" (not the same thing as weight).  If, as is posited by SR, it is "impossible" to exceed the speed of light, then, at that speed, no amount of energy could ever force your speed to increase..  The resistance to further acceleration (mass) would be infinite.

 

On the other hand, if your "speed" took you to the moon, and landed, your weight would be less.  If it took you to Jupiter, your weight would be more.  If it took you into a black hole, your mass would, in a sense, be infinite (but actually you would just be toast).  Again, "weight" varies with the gravitational field you're in, not speed.

Oh, yes.  I knew that.  I was just concentrating on the fact that enough of a boost can take one beyond Earth's gravity.