1. ## skid marks

Original: How can we use the skid marks of a car to estimate the car's speed before the brakes were applied."

I think instead of simple past the use of "present perfect" would be better. So what do you think?

Mine try: How can we use the skid marks of a car to estimate the car's speed before the brakes have been applied."

Source: Algebra & Trigonometry by Keedy/Bittinger.

Thank you.

2. ## Re: present perfect passive v. simple past passive.

The original is logical: we are now estimating the speed of the car at the time in the past when the brakes were applied.

The present perfect version is not logical. It says that, at a time before the brakes were applied, we will use skid marks that cannot yet exist to estimate the car's speed.

3. ## Re: present perfect passive v. simple past passive.

Originally Posted by hhtt21
Mine try
My try

4. ## Re: present perfect passive v. simple past passive.

Originally Posted by GoesStation
The original is logical: we are now estimating the speed of the car at the time in the past when the brakes were applied.

The present perfect version is not logical. It says that, at a time before the brakes were applied, we will use skid marks that cannot yet exist to estimate the car's speed.
Two two versions seems to me in meaning what you have explained here. It seems to me that the time adverb, before, implies this. So shouldn't it be "after" instead of "before" in two versions?

So I would like to ask what is the difference between

1.we are now estimating the speed of the car at the time in the past before the brakes were applied.
2.we are now estimating the speed of the car at the time in the past after the brakes were applied.

5. ## Re: present perfect passive v. simple past passive.

Isn't that more eye-catching?

6. ## Re: present perfect passive v. simple past passive.

There's no reason to change the original sentence.

7. ## Re: present perfect passive v. simple past passive.

But in both cases ie the present perfect or past simple, we are searching for the speed, the maximum speed just before the brakes. The brakes are applied. At that time, both brakes are applied and the car has its maximum speed and this is the time the skid marks beginning to form. What is wrong with this explanation?

Thank you.

8. ## Re: present perfect passive v. simple past passive.

Originally Posted by hhtt21
Original: How can we use the skid marks of a car to estimate the car's speed before the brakes were applied."

<snip>

How can we use the skid marks of a car to estimate the car's speed before the brakes have been applied."
Originally Posted by hhtt21
But in both cases ie the present perfect or past simple, we are searching for the speed, the maximum speed just before the brakes. The brakes are applied. At that time, both brakes are applied and the car has its maximum speed and this is the time the skid marks beginning to form. What is wrong with this explanation?
Your version is illogical. The car will produce skid marks after the brakes are applied. Since the skid marks are not there yet, you can't use them to estimate anything before the brakes have been applied.

9. ## Re: present perfect passive v. simple past passive.

Originally Posted by GoesStation
Your version is illogical. The car will produce skid marks after the brakes are applied. Since the skid marks are not there yet, you can't use them to estimate anything before the brakes have been applied.
If "before the brakes have been applied" is used the marks do not appear but if "before the brakes were applied" is used the marks appear. This is the point I cannot distinguish the difference and I do not know how I will distinguish. I cannot distinguish how this two tenses introduce very different situations.

So, would you please try to explain this "distinguishing point" in a different way?

Thank you.

10. ## Re: present perfect passive v. simple past passive.

I understand your confusion. I hope one of our actual teachers can jump in here to explain this. I'm sorry, but I don't know how.

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