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

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1. Jan 12, 2017

### 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

2. Jan 12, 2017

### rcgldr

3. Jan 12, 2017

### 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 ?!

4. Jan 12, 2017

### Staff: Mentor

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

5. Jan 12, 2017

### Mohamed Essam

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 ?!

6. Jan 12, 2017

### 256bits

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

7. Jan 13, 2017

### 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.

8. Jan 13, 2017

### 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)

9. Jan 13, 2017

### 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.

10. Jan 13, 2017

### sophiecentaur

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.

11. Jan 13, 2017

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.

12. Jan 13, 2017

### sophiecentaur

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

13. Jan 13, 2017

Last edited by a moderator: May 8, 2017
14. Jan 13, 2017

Staff Emeritus

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.

15. Jan 13, 2017

### Mohamed Essam

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 .

16. Jan 13, 2017

### Staff: Mentor

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?

Last edited: Jan 13, 2017
17. Jan 13, 2017

### 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

18. Jan 13, 2017

Staff Emeritus
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.

19. Jan 13, 2017

### Austin Z W

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

20. Jan 14, 2017

### 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.