Why heavy cars take more time than lighter cars to stop?

[Mohamed Essam]

Why heavy cars took more time to stop than lighter cars to stop when braking ??
Is it related by Inertia , if yes then why it's related by Inertia because i don't understand the physical concept of inertia

 

[rcgldr]

Assuming the tires were the same size and the limiting factor, then there's an issue where the coefficient of friction decreases with an increase in load, called tire load sensitivity.

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

 

[Mohamed Essam]

Sorry but i don't understand , my professor of physics in my university tells us that it related by it's moment of inertia , so what it's the relation between Inertia and friction ?!

 

[berkeman]

moment of inertia

Well, Moment of Inertia is almost certainly the wrong term. Perhaps you could ask your instructor to show you the math behind their assertion...?

 

[Mohamed Essam]

Well, Moment of Inertia is almost certainly the wrong term. Perhaps you could ask your instructor to show you the math behind their assertion...?

Okay , but if it's wrong , could you tell me the reason behind this by a simple way or tell me what i have to search on the internet and understand it to know the reason ?!

 

[256bits]

Okay , but if it's wrong , could you tell me the reason behind this by a simple way or tell me what i have to search on the internet and understand it to know the reason ?!

Newton's laws of motion.
http://www-istp.gsfc.nasa.gov/stargaze/SNewton.htm

 

[rcgldr]

Inertia isn't the direct issue here. A heavy car has more inertia, but the heavier car weight results in more force between the tires and pavement. If the tires coefficient of friction was not affected by the load, or if the heavier car had different tires, it could stop in the same distance or even less distance as the lighter car. The issue is the ratio of stopping force versus weight, which is a function of the available friction from the tires.

 

[sophiecentaur]

It's a very complicated business with many variables involved.
The first thing to 'forget' is the notion of a 'coefficient of friction' which is constant for all loads on a tyre. Tyres do not behave like blocks of wood on an inclined plane in the lab.
The friction force between tyre and road is very dependent on the road surface, the tyre design and the mass of the car. For instance, a very light car may not be applying enough pressure on the contact area to squish water out of the footprint. That will cause the light car to travel much further when braking. Heavy vehicles may have no trouble on a light covering of snow where a light car can be all over the place.
For older cars, in particular, the limiting force (torque, actually) for braking was due to the brakes themselves. Cars with drum brakes all round and without servo (very common, fifty years ago). A heavy car would take much longer than a light car to stop from the same speed.
Your professor should, perhaps, be specifying the problem in more detail before expecting an answer. I suspect that, at your level, you couldn't be expected to be able to cope with all the extra complexities for a definitive answer. (Me, too - I can only point out the list of difficulties but not offer a cast iron solution)

 

[Mohamed Essam]

It looks like it's more complicated than what i think , my instructor mentioned this as example of inertia not a main reason , but thank you anyway.

 

[sophiecentaur]

It looks like it's more complicated than what i think , my instructor mentioned this as example of inertia not a main reason , but thank you anyway.

I think your obvious answer is that the tyres aren't slipping and it's down to the brakes and the brakes on each car are the same. Now, if he had talked about a fully loaded vs lightly loaded car . . . . . . .. Students can get confused by poorly worded questions, which isn't fair.

 

[Chadi B Ghaith]

Okay , but if it's wrong , could you tell me the reason behind this by a simple way or tell me what i have to search on the internet and understand it to know the reason ?!

As per my point of view Mr. Mohamed Essam is right. Because I believe, the breaks convert the kinetic energy into internal energy which is then transferred to the atmosphere by heat. But because a larger vehicle has more kinetic energy the temperature increase of the breaking system is higher which can cause it to overheat and results in a loss of braking power.

 

[sophiecentaur]

As per my point of view Mr. Mohamed Essam is right. Because I believe, the breaks convert the kinetic energy into internal energy which is then transferred to the atmosphere by heat. But because a larger vehicle has more kinetic energy the temperature increase of the breaking system is higher which can cause it to overheat and results in a loss of braking power.

But that presupposes the two braking systems are the same. In reality, big cars have bigger brakes (large discs and bigger calliper cylinders).

 

[Chadi B Ghaith]

But that presupposes the two braking systems are the same. In reality, big cars have bigger brakes (large discs and bigger calliper cylinders).

I only referred this link : https://en.wikipedia.org/wiki/Brake_fade00000

 

[Vanadium 50]

This thread is very unlikely to be helpful.

First, the OP is confused about something his instructor said, and we are trying to both a) guess what he actually said, and b) what he must have meant. Guessing is not going to help the OP. What we should be doing is telling him to ask his instructor. If we know what he said and it is still not clear, then we can help.

Second, the premise is not true. It is not true that heavy cars universally take longer than light cars to stop. We are again reduced to guessing to trry and figure out what he really meant.

I know you're trying to help, but guessing will be less helpful to the OP than having him go back to the source to find out what he said and what he meant.

 

[Mohamed Essam]

This thread is very unlikely to be helpful.

First, the OP is confused about something his instructor said, and we are trying to both a) guess what he actually said, and b) what he must have meant. Guessing is not going to help the OP. What we should be doing is telling him to ask his instructor. If we know what he said and it is still not clear, then we can help.

Second, the premise is not true. It is not true that heavy cars universally take longer than light cars to stop. We are again reduced to guessing to trry and figure out what he really meant.

I know you're trying to help, but guessing will be less helpful to the OP than having him go back to the source to find out what he said and what he meant.

My instructor was talking to us in inertia and he said that the example of heavy cars took more time to stop than lighter cars can be a good example of inertia, because heavy cars had greater inertia than light ones so their inertia wants to keep it moving and resists to be stopped , is that wrong ?!

PS: i didn't ask that question to know if my instructor is right or not , i asked because i want to know the reason behind this and aslo to know if it's related to inertia or not .

 

[Nugatory]

My instructor was talking to us in inertia and he said that the example of heavy cars took more time to stop than lighter cars can be a good example of inertia, because heavy cars had greater inertia than light ones so their inertia wants to keep it moving and resists to be stopped , is that wrong ?!

It's not exactly wrong, but it isn't a great example because it's potentially confusing (you were confused by it, and this thread shows some evidence of confusion as well).

Yes, if everything else is the same (brakes, tires, wheel size, ...) so the weight is the only difference then it will take longer for a heavier car to stop than a lighter one, and its greater mass and inertia is why. An easy way to see this is start with Newton's ##F=ma##; ##F##, the stopping force from the brakes will be the same, ##m## is bigger, so ##a##, the rate at which the car slows down must be smaller.

Perhaps a better example of inertia would be if I rolled a soccer ball ("football" outside the US) and a bowling ball towards you, both at the same speed. The bowling ball is much more massive and has much more inertia. Which one will be harder to stop?

 

[Austin Z W]

Moment of inertia is not correct.

However, all else being equal the heavier vehicle will take longer to stop than a light vehicle because of 2 reasons. 1. It does have more inertia. 2. It also has more momentum. Which is a measure of how hard it is to stop something. Momentum(p) is equal to mass(m) times velocity(v)

P=mv

Hope that helps

 

[Vanadium 50]

Nugatory is right - it's not a great example. It's true if everything else is equal, but that doesn't happen in real life. His ball example is better.

 

[Austin Z W]

Yes it is, but I was trying to stay with the car example.

 

[houlahound]

Following Nugatory's post above, we should clearly define the problems being discussed.

Eg two identical cars one with extra mass added.

- no braking, just equal speed and all thrust ceased.
- equal breaking force applied to wheels.

The other possible scenarios are too incomparable.

 

[sophiecentaur]

Moment of inertia is not correct.

However, all else being equal the heavier vehicle will take longer to stop than a light vehicle because of 2 reasons. 1. It does have more inertia. 2. It also has more momentum. Which is a measure of how hard it is to stop something. Momentum(p) is equal to mass(m) times velocity(v)

P=mv

Hope that helps

I didn't want to introduce this but it really is crucial to the thread.
The term 'Inertia' is not really suitable for simple mechanics discussions. It doesn't have a definition (or units) that's applicable here. We have Momentum and we have Mass. Those two quantities are very well defined and can produce satisfactory outcomes from this sort of problem. Inertia is mostly a hand waving term and it tends to be used by people who don't want to get involved with the actual Physics of a situation. PF, by definition, aims at bringing Physics into anything that's discussed.

In my dictionary, Inertia is what keeps you in your chair when there are jobs to be done out in the garden.

 

[Vanadium 50]

Yes it is, but I was trying to stay with the car example.

Staying with a bad example tends to generate misconceptions at least as fast as it clears them up. That's why we call them that.

 

[sophiecentaur]

"Moment of Inertia"
This matter should be cleared up, I think.
Moment of Inertia, in dynamics, is a term which specifically refers just to rotational motion. It is the equivalent concept to Mass in linear motion. The acceleration of a Mass, depends on the Force applied whilst the Angular Acceleration depends on the Applied Torque and the Moment of Inertia.

 

[256bits]

In my dictionary, Inertia is what keeps you in your chair when there are jobs to be done out in the garden.

But the term "inertia" is often used is physics, ( even if its meaning has changed somewhat over the years, even from before it had the name inertia )

In the end, inertia, as we know it, explains why Aristotle's viewpoint was questioned and superceded eventually by that of Galileo and Newton.

Two terms used in physics:
Inertial mass
Inertial reference frame.
One has to understand to what "inertia" is referring, to lay down a base for concepts in Newtonian physics.
( Which is why I directed the OP, who asked about inertia, to read Newton's laws. )

PS. My Insert Quotes click doesn't work.

 

[sophiecentaur]

My insert button doesn't work either. I must let someone know. . . . . . . inertia at work again lol - but what would its units be? (Yawns, perhaps?) I thought it was a fault with my browser or OS X.
You quote the two terms
"Inertial mass
Inertial reference frame."
But 'inertia' is used there as an adjective and not a noun. I think that is very relevant. Furthermore, what could be an alternative? Mass Frame or Mass Reference Frame? I know of no formula that uses a quantity Inertia so I feel it has no place in our Mechanics when used on its own. "Moment of Inertia" is a term that's used and for which there isn't an equivalent. "Moment of Mass" might be better, imo, but it hasn't ever been used in my experience. Also MI is a term in static mechanics when describing the strength of Beams relating to their dimensions - just because formula has the same dimensions, I think.
To me, Inertia is a term that is as inappropriate as the word Force in emf, when it is spoken explicitly ("electromotive Force") It isn't a Force (so many arguments and misunderstands / attempted justifications about this in the past). But the two terms are so rooted in our Physics that they are too well established to be edited out.
If someone could offer a difference between what they mean by Inertia and either Mass or Momentum, I would pay attention but so far I haven't come across anyone who can. I'm happy to go along with it, as long as inertia keeps its place.

 

[Lsos]

Yes, heavier cars are harder to stop because of inertia. In fact, how hard something is to stop is basically the definition of inertia. Inertia is a property of matter, so the more matter (weight), the more inertia, and the harder the car is to stop. By harder, I mean it takes more force.

The reason a car braking is a bad example of inertia, is that while a heavier takes more force to stop, its brakes also generally do, in fact, have more force. In the end you get a heavy car that should brake in the same distance as a light car. Sometimes this is true, sometimes it is not, but the reasons behind it not being true are much more nuanced than simply more inertia = more braking distance.

 

[sophiecentaur]

because of inertia

Can you give a definition of Inertia that's not Momentum or Mass? (With a Unit, thrown in)

 

[Khashishi]

A heavier car has more momentum than a lighter car (moving at the same speed). There are two regimes: (1) limited by brake pad friction, and (2) limited by road friction. The latter case is called skidding, and is dangerous, and we have anti-lock brakes to try to avoid that regime.
In (1), if the brakes of the heavier car and lighter car are the same, then a heavier car will take longer to slow down. You can easily see this from
F=ma.
If the brakes are different, then it depends on what kind of brakes are installed. Roughly speaking, the mass of a vehicle increases as the cube of the size, and the force of brakes increases as the square of the size. So bigger vehicles need much bigger/more brakes to stop in the same time.

 

[sophiecentaur]

A heavier car has more momentum than a lighter car

True, but it's Kinetic Energy that needs to be dissipated by the brakes (Torque times number of revolutions turned by the wheels during braking)

There's little point in trying to come up with a statement that covers all these possible variables. It's just not Science if you can't isolate a relationship between just two variables, keeping all the others constant.

 

[OldYat47]

This kind of question makes my head hurt. The accurate statement (which does not fit the teacher's "example") is that to stop a car you need to reduce its momentum to zero. It take more braking effort to stop a heavier car than a lighter car. Full stop.

There is no doubt that my Honda Odyssey can stop more quickly than the 1964 VW beetle I used to have, in spite of the significant weight difference.

 

[sophiecentaur]

This kind of question makes my head hurt. The accurate statement (which does not fit the teacher's "example") is that to stop a car you need to reduce its momentum to zero. It take more braking effort to stop a heavier car than a lighter car. Full stop.

There is no doubt that my Honda Odyssey can stop more quickly than the 1964 VW beetle I used to have, in spite of the significant weight difference.

You have proper brakes on the new one - probably the Beetle would have had no servo assist and drums all round. I had an old (at the time) Morris 1000 in 1967 with minute drums all round which would over heart and fade at the drop of a hat. In Physics terms - not a fair test.

 

[OldYat47]

My '64 beetle was brand new at the time (revealing my age). And at that time the beetle was among the better-stopping cars around (compared to American cars, for example). Which brings us around to the main point, that heavier does not equal longer stopping distances. Many other factors contribute. Probably no servo assist? That made me smile. The windshield washer had to be pressurized with a bicycle pump. We're talking basic and primitive.

 

[sophiecentaur]

I used to get a lift in a woman's lh drive beetle that let water into the heating duct. Condensation On the windscreen made it impossible to see out and she used to drive (in U.K.) with her body where it would have been in a rh drive car. In my (rh ) passenger seat, I was in the path of oncoming traffic. Scary.

 

[OldYat47]

Talk about scary! The only place you would be getting water from is the exhaust leaking into the air shrouds.

 

[sophiecentaur]

I don't think CO was a problem (no headaches). It was after rain, which came in through rust holes! and trickled into the hot air ducts. I nearly offered to drill some drain holes but I didn't want to get involved.
I have had most of the classic cars that you find on British roads (including a 2CV - my lovely new 2CV) but I never found a Beetle when I needed a car. I did have a VW camper with the same basic engine, though. The brakes on that were fine. - but they had to be, on a commercial transporter chassis.