Can any engineers answer this nagging question?


#1

A couple of months ago my father and I were watching a show about American Truckers driving the high passes of India. At one point they came to a small dilapidated bridge. The driver’s spotter got out and guided the driver slowly across the bridge. When they got about half way across I turned to my dad and said “Why don’t they just gun it.” He called me an idiot and said I had forgotten my physics because increased speed equals increased mass. He’s smarter than me, so I just shut up.

The thing is, the question has been nagging at me. The increase in mass would be negligent and less time spent on the bridge means less time spent under load. But I’m also afraid of impact stress as the bridge surface was uneven. Lastly, the faster you travel, the better chance you have to reach the other side if the bridge does indeed collapse.

The more I contemplate the question the more complicated it becomes. So, I hope someone smarter than me (and perhaps my dad) can give me a final verdict. Faster or slower? Thanks.


#2

Collecting data here,what was your reason for saying “why don’t they just gun it”?, OK to get across quicker,I ask why would getting across quicker be better?.Your father mentions “increased mass” is he saying that the increased speed of the truck puts more stress on the bridge? I would agree that increased speed puts more stress on the bridge but not from a mass increased due to speed. You are going to have to lay both sides of the argument out better so as I do not have to make any of my own conclusions about what you (or your Dad) is saying and why you/him are saying what you are.


#3

Thanks for the reply. It was an issue of safety. The bridge looked unsafe and the show made a big deal about how scared the truckers were crossing. I thought a faster traverse might put less stress on the bridge and give the driver a better chance of making it across if the bridge began to fail. My father’s reply was saying that the increased mass from the speed would put more load on the bridge and I just didn’t think that would be a major factor. We left it there so it hasn’t really been an argument and I’m not trying to be right or win a bet. I’m really just curious. The more I think about it, the more factors seem to come into play and at this point I’m throughly confused. Thanks.


#4

Increase speed = increase mass. Einstein came up with that. But it doesn’t apply here. At such low speed, dropping a feather on that bridge would add more mass than increased speed.

Think about what happens when you fire a gun. A puff of gas pushes a bullet forward as well as the gun itself backward. Now, the truck here would be the bullet and the bridge is the mount. If they simply “gun it”, the truck goes forward and the bridge goes backward. If that bridge isn’t properly mounted on either side, the bridge moves off of its mount, and you can imagine what happens next.


#5

An increase in speed does not increase the mass of an object(perhaps as we approach the speed of light Einstein would have something to say about this but we are not talking about light speed here). An increase in speed will increase the stress placed on the bridge (because the same mass is moving faster more energy in the form of momentum exists). Parking on the bridge(the ultimate example in going slow) would not be a good idea as who knows just when a support member is going to decide to break but increasing your speed so much as the truck places an unsupportable amount of stress on the bridge also is no good. Slow and steady is the best approach.If you are crossing a shallow creek a good bit of spped would help out as the momentum increase will help but you don’t want to go so fast as control is lost.

This article speaks of a TINY (the word it uses) increase of mass at speeds way below the speed of light. I don’t at all think this TINY increase of mass is what your father had in mind. It is the increase in energy in the form of momentum that plays the largest part here.The change in mass indicated by Einsteins equation is called immeasureably small for everyday use.I was envisioning the “gunning it” part happening while the vehicle was only on the approach to the bridge,not on the bridge itself.Gunning it while on the bridge would make it so that the extra speed obtained by the truck was gained at the expense of the stability of the bridge,don’t gun it while you are on the bridge if gunning it is your plan

http://galileoandeinstein.physics.virginia.edu/lectures/mass_increase.html


#6

Saw the same show.

I was told, that the reason you go slow was give the bridge time to adjust to the load. As you drive over a marginal bridge or you’re really close or over it’s weight limit, it’s going to shift and move as you go over it. So by going slow the bridge can adjust and move, if you go fast it doesn’t have time to adjust and you may over stress a member and it will fail.

That what I was told and it may be wrong.

My opinions are subject to change with new facts.


#7

All I can say is you don’t want to increase or decrease speed while on the bridge because you will be adding inertia forces, possibly adding stain.


#8

The idea that an increase in speed would increase your chances to reach the other side, I believe is theoretically correct. A bullet will fall at a rate of 32 ft/sec/sec regardless of it’s horizontal velocity. The same gravitation constant is working on your truck. Spread over the length of the bridge, the faster you move across the bridge and decrease the amount of time on each foot of the bridge the less your force over each foot of the bridge and theoretically, the increase in your chances to get to the other side. Anyone who has skated on a pond across thin ice or snowmobiled over such knows this and increases speed when the ice begins to crack.
As mentioned, just like a snowmobile can traverse water w/o sinking, increasing this velocity with a large truck puts horizontal loads (re rwee200 and jos) on the bridge, which may decrease its integrity for the next traveler. So, if you’re the last person who will ever use the bridge, go for it. Other wise go slow.

Because mass increase is an exponential function and is only relevant at near light speed, don’t worry unless you’re driving a Tesla. And even then, the amount of force applied to the bridge would be infinitely small over the time sent on the bridge…still no problem.

Though there may be lots of holes in my reasoning, I’m still sticking to the “gun it in theory.” but not in practice. PS, accelerate (gun it) BEFORE you hit the bridge though, not during. Keeping speed constant, is more important to not increasing stress on the bridge and in reality it will not work w/o a run way per say to achieve maximum velocity. You may be better off decreasing the air pressure in your tires, in theory again,


#9

Mass of the object is a constant…it can’t increase or decrease.

Increase Speed DOES = Increase in Force…BUT Only in the direction you’re going in. There would be more Potential Force in the forward direction of the truck NOT in the downward direction.

The reason you go slow…I’ve seen that show and I saw that bridge…The bridge is also NOT that smooth…At a higher speed it may cause the truck to bounce…that bouncing will cause a LOT MORE STRESS put on the bridge. If it was nice smooth bridge…then accelerating across would be safer…but for that bridge…slow and easy with someone else driving…I sure wouldn’t do it.

According to the show something like 20 people die every day on that road…


#10

Increasing speed increases momentum, not increased mass, in a Newtonian system. Newtonian systems are moving too slowly to be relativistic. I buy the idea that increased speed will increase bouncing, and therefore increased momentary load. Going slowly also means that you might stop, get out of the truck, and make a run for it if it starts to collapse.


#11

Let’s make this easy. Think of the bridge as a person and the vehicle as a baseball. How much more “force” is applied to your body if you are hit with a baseball traveling at 20mph as opposed to a baseball traveling at 100mph. There is a lot of difference. Increased speed equals increased force. Slow down over old bridges.


#12

One that I like to run through my mind when I have daydream time is “will you get hit by more rain drops if you run or if you walk”? If you run you decrease the overall exposure time but running has the potential to put you in a position to get hit by a rain drop that wuld otherwise miss you if you were walking.


#13

“Mass of the object is a constant…”

No it isn’t, yes it is…

I think OP was into this…“There is sometimes confusion surrounding the subject of mass in relativity. This is because there are two separate uses of the term. Sometimes people say “mass” when they mean “relativistic mass”, mr but at other times they say “mass” when they mean “invariant mass”, m0. These two meanings are not the same. The invariant mass of a particle is independent of its velocity v, whereas relativistic mass increases with velocity and tends to infinity as the velocity approaches the speed of light c.”

So if mass is defined to be resistance to acceleration, then yes it does vary. When that mass regardless of how defined as determined by its velocity, it has no affect on it’s weight or the influence of gravity, then no it doesn’t. Otherwise, we or any car could travel at speed greater than light speed w/o air resistance with minimal HP, which at this time we can’t. So the relativistic mass does increase. Too many people equate mass directly with weight which is in part were the confusion lies.


#14

I saw that show. It appeared to me that the spotter was helping the driver keep the wheels over the strongest parts of the bridge. At one point, the bridge deck started to bend, and the driver had to back up and reposition. If the truck had been going faster, there wouldn’t have been time to see the problem and correct for it.

In the animation that was shown, a wheel broke through a weak point, and the truck fell. Once the truck started falling, it hit the bridge and added a sudden load that caused the whole structure to fail.


#15

I can stand that show. I worked on heavy trucks for a long time and never ran into a bunch of imbaciles(??spell??) like that lot. Dram for the sense of drama only…not a bit of reality in it. I’m waiting for “Quantum Truckers”…you know, the ones that can be hauling a different load in more than one place at the same time. They’re my real heros.


#16

Has that show ended it’s run? was there a winner or a loser at the end of the show? This concept was a good one but would have been better realised if different locations we used (perhaps not in the budget). These shows that detail the “work is everything concept” (I speak of the Alaskan fishing show, the logging show,the truck driving show) lose me when the boss/employee relationship becomes so abusive(some abuse may be for the camera but I have seen how it goes at the logging site). I guess it is OK to be the one dealing out the abuse but being on the other end, not so good. I will put up with a lot of abuse from the boss if the pay is good enough. For me to be setting choakers for any of those guys on the “AX men” show I would want $50.00 an hour with a couple of weeks pay put in escrow. In the 80’s I saw men setting choakers for $5.00 an hour, and happy they had the job. Lucky for me I was fixing cars for much more money, less danger,and did not suffer at all in dating because I did not have a “macho” type job.


#17

Thought experiment. There are snowmobile contests across open water. Naturally a snowmobile would sink in open water but at speed they can go hundreds of feet. Now translate that to a lake with thin ice, they may be able to cross the lake at speed, but if they stop they would fall through the ice. Now why is it safer to cross the bridge at slow speed. Watching the show there may be boards missing, or uneven boards. The force of hitting the board after the missing one would probably break it at a high speed, causing disaster. Also if you watch they are going slowly to avoid potential weaknesses. Got to go with stuff that works, and on those bridges they found slow and cautious works.


#18

They seem to skip like rocks over a pond. The friction of the water decreases the speed rapidly, so as they approach 40 mph they have to be on land, shallow water or swimming.
They can do it because of the tremendous amount of traction afforded by the belt. Those of you who ride them know how awesome accelerators they are. There are few land vehicles that do as well.


#19

"A bullet will fall at a rate of 32 ft/sec/sec regardless of it’s horizontal velocity.

The same gravitation constant is working on your truck.

Spread over the length of the bridge, the faster you move across the bridge and decrease the amount of time on each foot of the bridge the less your force over each foot of the bridge"

Are you sure about that last part?


#20

“Let’s make this easy. Think of the bridge as a person and the vehicle as a baseball. How much more “force” is applied to your body if you are hit with a baseball traveling at 20mph as opposed to a baseball traveling at 100mph. There is a lot of difference. Increased speed equals increased force. Slow down over old bridges.”

How much more “force” is applied to your body if you someone rolls a baseball over your body traveling at 20mph as opposed to a rolling a baseball over your body at 100mph?

You’re not throwing the truck at the bridge.