Does a truck's load affect its braking distance?

[mehgon]

Two trucks with the same futures,one loaded and one empty. They are traveling on same speed and when they brake on the same spot;which one stops earlier? Does a truck's load affect it's braking distance?
Thanks..

 

[DrClaude]

Is this a homework problem?

What do you think the answer is?

 

[newjerseyrunner]

The breaking distance has to do with how quickly it's velocity changes right? That's acceleration. The force that the breaks can apply does not change regardless of the size of the truck. The mass of the truck changes. force = mass * acceleration. So what happens to acceleration if force stays the same but mass increases?

 

[A.T.]

The force that the breaks can apply does not change regardless of the size of the truck.

The relevant force is the one that the road can apply.

 

[newjerseyrunner]

Oh geez, that's right. Okay, the amount of force applied to the truck is the force of kinetic friction. The friction coefficient times the normal force.

 

[A.T.]

Okay, the amount of force applied to the truck is the force of kinetic friction.

If it slides, which the OP doesn't specify.

The friction coefficient times the normal force.

So assuming sliding with the same friction coefficient, and ignoring all other retarding forces, what is the answer?

 

[rcgldr]

Is this a real world situation, or an idealized situation? In the real world, tire load sensitivity effectively lowers the coefficient of friction as the load increases. Wiki article:

http://en.wikipedia.org/wiki/Tire_load_sensitivity

 

[Lsos]

As mentioned, in the real world the stopping distance IS affected by the load. This is a classic example of how a textbook and real life are not in agreement. Whether the stopping distance actually increases or decreases with more load depends on many factors. For example, the coefficient of friction might decrease which lengthens the stopping distance, but the truck might have been designed to perform better with a load, so the suspension/ tire position might result in better overall grip and a shorter stopping distance.

Other things keep this from being a good homework question. For example, a wider tire might give a better coefficient of friction...but is this true on all surfaces? Is it true in the snow/ rain?

 

[sophiecentaur]

The relevant force is the one that the road can apply.

That's only true if the brakes are good enough to cause wheel slippage. Plenty of 'old' vehicles had the sort of brakes that would never cause a skid on a decent road surface. Under those conditions, the braking force could be considered independent of load (strength of the driver's right leg, in fact). But even if the limit is when the wheels slip, you can't treat the tyre / road friction as linear (not a constant coefficient of friction) so that makes good theory based predictions pretty difficult. With suitable choice of tyre and surface, I reckon you could get any answer you wanted to the OP.

 

[A.T.]

The relevant force is the one that the road can apply.

That's only true if the brakes are good enough to cause wheel slippage.

It's always true. Only external forces can slow down the truck.

 

[sophiecentaur]

It's always true. Only external forces can slow down the truck.

Are you saying that the torque that is present in the brakes plays no part in the braking process? You can't be!
That torque is caused by the brake pad friction which is caused by pedal force ( in a passive system). If there is no skidding, then it is the brake torque that governs the rate of negative acceleration. It can be the dominating factor and is independent of load.

 

[jbriggs444]

i.e. if the brakes are out, it doesn't matter how much force the road could otherwise provide.

 

[A.T.]

If there is no skidding

Which isn't specified.

 

[sophiecentaur]

Which isn't specified.

I dealt with both eventualities. You could even assume ABS. I think you have to admit that the majority of times that vehicles stop does not involve skidding.
You can hardly ignore the action of the brakes within the vehicle in any discussion like this one.

 

[A.T.]

I dealt with both eventualities.

My statement in post #4 also applies in both cases.

 

[sophiecentaur]

My statement in post #4 also applies in both cases.

That statement is actually wrong unless the wheels are slipping. In most circumstances the force is less than the limiting friction force between road and tyre. Can you possibly argue otherwise?

 

[A.T.]

That statement is actually wrong unless the wheels are slipping.

What is wrong about saying that only external forces can accelerate an object? That is true regardless of slipping or not.

 

[sophiecentaur]

What is wrong about saying that only external forces can accelerate an object? That is true regardless of slipping or not.

Nothing wrong with that. What is wrong is to associate that with the braking distance / mass relationship. The braking force is nearly always less than the limiting force on the tyres (which is proportional to the mass, where the simple basic law of friction applies). Mostly, the braking force is governed by the forces on the brake discs - which is independent of the mass. So the braking distance will be dependent on mass (surprise surprise).

 

[A.T.]

majority of times

In most circumstances

nearly always

I don't think questions like this want you to consider the infinite number of possible sub-optimal circumstances. The most reasonable way to compare the two braking situations is to assume optimal braking strategy in both cases, with the maximal ground friction possible in each case. Otherwise the answer becomes arbitrary.

 

[sophiecentaur]

I don't think questions like this want you to consider the infinite number of possible sub-optimal circumstances. The most reasonable way to compare the two braking situations is to assume optimal braking strategy in both cases, with the maximal ground friction possible in each case. Otherwise the answer becomes arbitrary.

I realize you don't want to let this go but most trucks, when brought to a halt on most occasions, do not skid. (That's not one of any 'infinite' set of circumstances; it's the majority.) To my mind, it is not an unreasonable situation to consider. You mention "optimal braking strategy". Doesn't that imply non skidding? That implies that the wheels are not locked and that the braking force is a function of the brakes.
We both have to admit, of course, that we are ignoring the real complexities of the way tyres and roads interact. A heavy load could have a big effect on how the tyres cope with any surface water, for instance. That sort of stuff is way beyond a simple discussion. With no extra knowledge, we have to consider the linear situation where the coefficient of friction is independent of load (at least, I think so).

 

[A.T.]

You mention "optimal braking strategy". Doesn't that imply non skidding?

It implies maximally possible friction force, which is usually a function of normal force and static friction coefficient.

That implies that the wheels are not locked and that the braking force is a function of the brakes.

The maximally possible external braking force, is limited by normal force and static friction coefficient. So there is no need to consider some internal forces to determine the minimally possible braking distance here.

 

[sophiecentaur]

It implies maximally possible friction force, which is usually a function of normal force and static friction coefficient.The maximally possible external braking force, is limited by normal force and static friction coefficient. So there is no need to consider some internal forces to determine the minimally possible braking distance here.

I see our mutual problem now. At the point of slipping and assuming that the friction coefficient is constant for all loads (we can't improve on that without knowing more) the friction force F will be proportional to the mass / weight but the braking acceleration will be proportional to F/ mass, which is independent of mass. In real life, the force from the brakes is always adjusted to avoid skidding and the force applied to the brakes will be limited to the force on the pedal, which is very relevant when answering the question "Is the braking distance affected by mass?"
It is possible that our relative ages are affecting our views on the subject. I have frequently been pressing as hard as possible on the brakes of a car with no servo assist and been all too aware of the fact that there were four passengers in it and an obstacle not very far in front. (Boys will be boys). With just myself on board, there would have been no problem stopping. That's were `I was coming from.

 

[jbriggs444]

I have frequently been pressing as hard as possible on the brakes of a car with no servo assist and been all too aware of the fact that there were four passengers in it and an obstacle not very far in front.

In my experience, without ABS, the tires of a passenger car will lock up with far less than maximum force applied to the pedal. With ABS, they will start pulsing with far less than maximum force. I've never had the brakes fade so badly that it was not possible to lock up the tires.

Truck brakes, however, may be a different story.

 

[A.T.]

In real life...

If you want to answer this based on real life, not on an idealized model, then It's almost trivially true that the load will have some effect.

 

[sophiecentaur]

If you want to answer this based on real life, not on an idealized model, then It's almost trivially true that the load will have some effect.

Yes. That has to be true. It's in the UK Highway Code, based on experience and evidence. But a good explanation for it has not been proposed in this thread (except mine ).
JBriggs' experience of non ABS cars must be based on modern cars or possibly poor road surfaces. There's no way I could get the wheels on my (1960)Morris 1000 to skid except on wet or icy roads. But modern cars, with disc brakes are a different matter. I still wonder about heavily loaded trucks, though. So called Air Brakes are certainly pretty fierce.

 

[gttjohn]

Yes the truck without the load will stop sooner I've tried it on bikes with two people and then one I've driven trucks with or without load and the unladen truck will take less distance to stop fact

 

[gttjohn]

A truck with a load will take longer to stop it just happened to me today

 

[Derektquestions]

Just wanted to add, Location or weight distribution of the "load" on the truck also becomes a factor when braking.

 

[Lok]

Of course a heavier truck will brake in a longer distance. Form an energetic point of view the heavier truck will dunk more energy into it's frictional components, and that means more heat. So on a constant brake pedal pressure your actual braking force will vary in time, rubber (road tires, not the F1 kind ) has a small braking coefficient increase for a few degrees, a peak and then a heavy drop in friction coefficient. Break disks too, although the coefficient peaks at about 300 C' it drops nicely thereafter, common break temperatures at the disk pad interface.

 

[sophiecentaur]

Of course a heavier truck will brake in a longer distance.

Yes. A truck is very unlike a block of wood on a lab bench and many of the comments on this thread have been assuming they are very similar.