A compelling optical illusion called the illusion of velocity and size

Question

A compelling optical illusion called the illusion of velocity and size makes objects appear to be moving more slowly the larger the objects are. Therefore, a motorist’s estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a large truck approaching.

The conclusion above would be more properly drawn if it were made clear that the

(A) truck’s speed is assumed to be lower than the car’s

(B) truck’s speed is assumed to be the same as the car’s

(C) truck’s speed is assumed to be higher than the car’s

(D) motorist’s estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching

(E) motorist’s estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching

egmat · Accepted Answer

Hi Shelrod,

You are almost correct in whatever you said. There is a minor gap in your understand that withholds you getting to the right assumption. Let's understand it.

Conclusion: Time for small car < time for large truck

Basis of conclusion: Larger the vehicle size, the slower it appears.

Right?

Now, if you understand the basis of the conclusion, it means that if a car and a truck both are moving at, say, 100 miles per hour (mph). Then, the following speeds may appear to a motorist:

Car: 100 mph
Truck: 95 mph

Therefore, when motorist estimates time for both car and truck, he would estimate a larger time for the truck because truck's speed is lower than car's.

Now, what if originally, the car was moving at 20mph and truck at 100mph. What would a motorist see?

He would see:

Car: 20mph
Truck: 95mph

Now, in this case, he would not estimate a longer time for the truck.

Do you get this?

For the motorist to estimate a lower time for the car, the car must be moving at the same or almost same speed as the truck. This is a required assumption for the conclusion to hold true.

Does it help?

Thanks,
Chiranjeev

AjiteshArun · Answer

Hi  waytowharton ,

This question could have been clearer, but the key word for us here is   therefore  .
 
In the absence of any other information, if a truck is moving more slowly than a car (option A), a motorist can be expected to come to the right conclusion (that  the time available for crossing a highway with a small car approaching   <   the time available with a large truck approaching ). But that doesn't involve the illusion of velocity and size at all. The question stem is "the conclusion above would be more properly drawn if it were made clear that the", so we need to stick to the  illusion   →   estimate  path. In other words, we need something that helps us go from "illusion" to "a motorist's estimate...". If we choose option A,  we don't need the information about the illusion at all . We can go directly from A to the conclusion.

Option B, on the other hand, helps establish the link between the illusion and the conclusion. With it, we can read the argument like this:

1.  A compelling optical illusion called the illusion of velocity and size makes objects appear to be moving more slowly the larger the objects are.  Therefore ,  for a small car and a large truck moving at the same speed , a motorist’s estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a large truck approaching.

MamtaKrishnia · Answer

Ans : B.
In all the other cases the conclusion drawn may not be true.
Lets say option A was true.
In this case since the truck is slower than the car it will in any case take more time to reach the motorists irrespective of other factors like illusion of velocity or size.

priyankurml · Answer

Premise: due to illusion - object size makes objects appear to be moving more
slowly.
Conclusion: time to cross car is less than time to cross truck. Hence Car takes less time while truch takes more time to cover the distance.

A. if truck's speed is less, it is not illusion - Can not be the answer
B. If speeds are same, illusion acts - Answer
C. again Speed should be constant
D and E: Accuracy of Motorist's estimation is not be considered as we are making relation between illusion and time while speed is contant.

whamberto · Answer

I have chosen B for this question:

The optical illusion is only true if the 2 objects are moving at the same speed.

A. This is not an assumption made by the argument. If the truck's speed is slower, then what is the point of the optical illusion? Clearly, if the truck is moving slower, the motorist will have more time.

B. This is the correct answer. We have a good comparison now because we are comparing 2 objects moving at the same speed, so the larger truck will appear to be moving slower than the small car.

C. Same as A but reversed

D. How can we assume this? Nothing in the passage mentions the motorist's ability to estimate time.

E. Same as E.

talismaaniac · Answer

A compelling optical illusion called the illusion of velocity and size makes objects appear to be moving more
slowly the larger the objects are. Therefore, a motorist’s estimate of the time available for crossing a highway
with a small car approaching is bound to be lower than it would be with a large truck approaching.
The conclusion above would be more properly drawn if it were made clear that the
(A) truck’s speed is assumed to be lower than the car’s
(B) truck’s speed is assumed to be the same as the car’s
(C) truck’s speed is assumed to be higher than the car’s
(D) motorist’s estimate of time available is assumed to be more accurate with cars approaching than with trucks
approaching
(E) motorist’s estimate of time available is assumed to be more accurate with trucks approaching than with cars
approaching

(My only query - Read this after doing the Q why cannot the answer be D?. There are two things. First, either his estimate about cars is accurate (which should be if we have to prove that he has this optical illusion concerning large objects. Second, his estimate about cars is more accurate than trucks - which might mean that both his estimates are incorrect, hence he has optical illusion concerning large and small objects. Then this answer might not make sense??? --- please guide...

ankit0411 · Answer

I could not really get your statement, however, I don't think the accuracy of time is really important in this question.

The premise already says that   larger objects tend to appear to be moving slowly. b] . You have to take this at Face value .

So if the truck and the car are moving at the same speed , only then will the motorist estimate that the truck is a large object and hence, he estimates it to be moving slowly.
That said, if they are at the same speed , the vehicles' weight then come into picture.

Larger objects -> slowly 
Smaller objects -> faster .

Does that help you ?

margaretgmat · Answer

Blue Book CR67

Strengthen
Conclusion: Estimated that small car takes LESS time than large truck. Why?
Premise: (On the basis of illusion of v and s) Appeared that larger objects to have less speed, thus incurred more time.

Not A: If assumed that truck's speed less than car, then estimation is not necessary since the case presents that it must always be true that car will take LESS time.
It's B: If assumed that truck's speed is same as car (holding all other factors, in this case is velocity, constant), this is in accordance with the illusion theory that the estimation is based on the appearance that car takes LESS time than large truck.

Zarrolou · Answer

Yes this is an official question.

The  OE  states:

A If the truck is moving more slowly than the car, then the driver’s perception is accurate, and not affected by the optical illusion.
B   Correct  . This statement properly identifies an assumption implicit in the reasoning.
C If the truck is moving faster than the car, the motorist might judge that the two vehicles were moving at the same speed—or even that the truck was moving faster.
D It does not matter for which type of vehicle the driver’s estimate of lead time is more accurate; this assumption is not helpful in explaining the conclusion that the illusion makes larger objects appear to be moving more slowly than small objects.
E This contradicts information in the passage indicating that the larger the vehicle, the larger the distortion in the perceived speed.

shelrod007 · Answer

My analysis : illusion of velocity and size -> makes objects appear to move more slowly larger the objects are .

C : Motorcyclists estimate to cross the highway with a small car approaching is smaller then large truck approaching ...

Reason for conclusion to hold : Small car smaller size appears to move faster ( more velocity) , therefore smaller time required to cross it .

Large truck moves slowly ( less velocity ) more time to cross it .

Beyond this i am not able to get the assumption out of the argument ? Totally lost just choose B since the other option were all varying .

BrainLab · Answer

Straight (B) without even reading other answer choices. The argument was talking about the illusion - so, if stated example should be an illusion the speed of both a car and a truck must be  EQUAL . All other constellation it won't be an illusion, if there is a difference in speed.

nandetapuri · Answer

A compelling optical illusion called the illusion of velocity and size makes objects appear to be moving more slowly the larger the objects are. Therefore, a motorist’s estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a large truck approaching.

Assumption : Both of them are at same speed that is why the one if appearing to move slowly as compared to another because of the illusion. If it would have been opposite then, the illusion of velocity would have been different. Something has to be constant. 
 
The conclusion above would be more properly drawn if it were made clear that the

(A) truck’s speed is assumed to be lower than the car’s
 (B) truck’s speed is assumed to be the same as the car’s 
(C) truck’s speed is assumed to be higher than the car’s
(D) motorist’s estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching
(E) motorist’s estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching

TargetMBA007 · Answer

GMATNinjaTwo  nightblade354 - Hi Guys, I believe this should be tagged as an "Assumption" question. Please correct me if I am wrong.

waytowharton · Answer

KarishmaB   DmitryFarber   AjiteshArun   ExpertsGlobal5

Is there anything wrong with Option A? Wouldn't it make the conclusion, motorist’s estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a large truck approaching, MORE CLEAR. Firstly as per option A : Truck's speed is lower; moreover it would appear to be slower because of optical illusion and increase the difference.

I am unable to understand what is wrong with option A.

KarishmaB · Answer

I need to be able to draw the conclusion from the argument. If I were to assume (A), the argument is not required. I do not NEED (A) to be true. The actual assumption is (B) with which the argument too makes sense.

zoezhuyan · Answer

hi Chiranjeev,  egmat ,  egmat

and  GMATNinjaTwo ,  VeritasKarishma ,  MartyTargetTestPrep ,  AndrewN , VeritasPrepBrian , GMATRockstar

I am a bit confused you case 2, 
Car: 20mph
Truck: 95mph

obviously, truck is larger than car, according the prompt, truck will move slowly than car, that's why I am confused.

one more question, I am not sure this is assumption or strengthen question, 
the stem is "would be more properly drawn if it were made clear that ..." in other words, the conclusion is more likely concluded. I read the thread, some think it is an assumption question. 
 
would you clarify further ?

thanks in advance.

have a nice day

Sneha2021 · Answer

KarishmaB 
I am unable to eliminate D & E. How accuracy of estimation impact the argument?

(D) motorist’s estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching
(E) motorist’s estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching

Is it reasonable to say that D weakens the conclusion because we can't trust the estimated time of truck now. So conclusion can't be drawn.

KarishmaB · Answer

Premise:
An illusion makes objects appear to be moving more slowly the larger the objects are.

Conclusion: a motorist’s estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a large truck approaching.

The only fact given is that larger an object, the slower it appears to be moving. So a truck moving at 50 mph will appear to be slower than a car moving at 50 mph, which in turn will appear to be moving slower than a bike moving at 50 mph.

From this we conclude that estimate of time available to cross with a smaller car approaching will be lower than that with a truck approaching. 
So if I have to cross the road and a car is approaching, I might think that I have 5 secs available to cross. But if a truck is approaching, I might think I have 8 secs to cross (because the truck will seem to be moving slower)
When does this conclusion make sense? When they both are moving at the same speed. Then the motorist's estimate of time available will be lower in case of the car (it will seem to him that the car is faster than the truck)

What if the truck is actually moving much faster than the car? Then his estimate of time available may be same as that for the car or may even be smaller than that for the car. 
What if the truck is moving much slower than the car? Then his estimate of time available in case of car will be much lower than that in case of the truck. But then, what role does the illusion have to play in this? None. Even without the info on illusion we can conclude what we concluded.

The logic of the argument works only when the speed of the two is assumed to be the same. Answer (B)

(D) motorist’s estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching

(E) motorist’s estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching

Motorists will have different estimates based on their skill and experience. 
With 200 metres between him and a car moving at 20 meters/sec, he may estimate that he has 10 secs to cross or 20 secs to cross or 5 secs to cross (as per his skill and experience). How accurate his estimate is, we cannot say. But what we can say is that if a truck is 200 metres away and moving at 20 metres/sec, the same motorist is bound to estimate that he has more time. In both cases correct estimate would be 10 secs. 
That is, if he estimates 10 secs with a car, he may estimate 12 secs with the truck. (car estimate is more accurate)
If he estimated 5 secs with the car, he may estimate 8 secs with the truck etc. (truck estimate is more accurate)
If he estimated 20 secs with the car, he may estimate 30 secs with the truck etc. (car estimate is more accurate)

Hence, both (D) and (E) are incorrect.

zoezhuyan 
 Sneha2021

zoezhuyan · Answer

hi  KarishmaB 
and  egmat ,  egmat

GMATNinjaTwo ,  VeritasKarishma ,  MartyTargetTestPrep ,  AndrewN , VeritasPrepBrian , GMATRockstar

I reviewed your points over and over, and I am still have a question.
how can you know the fact that larger an object, the slower it appears to be moving (than its actual speed), why not the fact that larger an object, the slower it appears to be moving (than smaller's speed)

A compelling optical illusion called the illusion of velocity and size

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A compelling optical illusion called the illusion of velocity and size

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