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# M1: Car and trailer (N2L) watch

1. Why is the resistance force and the braking force shown as acting 'forwards', yet the braking force is acting 'backwards' as you would expect it to?

Attachment 695950695952

I assume that it's something to do with it being a deceleration, but I don't really understand it.

Any help is appreciated.
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2. (Original post by W. A. Mozart)
Why is the resistance force and the braking force shown as acting 'forwards', yet the braking force is acting 'backwards' as you would expect it to?
As far as I can make out, that diagram is wrong.

Firstly, it is pointing in the opposite direction to the original picture. This doesn't make it wrong, but it does make it confusing. This answers your question about why the forces are acting in the "wrong" directions - they aren't but because the picture is the other way around from the original one, it looks like they are.

But if they are going to reverse the picture like this, then the acceleration should be positive if they want it to point in the same direction as their acceleration arrow. They've ignored the negative sign in their calculation (which they should do, as it shouldn't be there!).

If you tried to solve this question in the normal way, and agree with their (correct) answer, then you're doing fine, and it is only their confusing diagram that is the issue.
3. (Original post by Pangol)
As far as I can make out, that diagram is wrong.

Firstly, it is pointing in the opposite direction to the original picture. This doesn't make it wrong, but it does make it confusing. This answers your question about why the forces are acting in the "wrong" directions - they aren't but because the picture is the other way around from the original one, it looks like they are.

But if they are going to reverse the picture like this, then the acceleration should be positive if they want it to point in the same direction as their acceleration arrow. They've ignored the negative sign in their calculation (which they should do, as it shouldn't be there!).

If you tried to solve this question in the normal way, and agree with their (correct) answer, then you're doing fine, and it is only their confusing diagram that is the issue.
Yeah it seems really odd...

But thanks, I managed to figure out what I was doing wrong in the end. When the car is decelerating, the tensions in the towbar act outwards from the centre rather than inwards towards the centre (when accelerating), which is why I got the wrong answer initially.
4. (Original post by Pangol)
But if they are going to reverse the picture like this, then the acceleration should be positive if they want it to point in the same direction as their acceleration arrow. They've ignored the negative sign in their calculation (which they should do, as it shouldn't be there!).
They made a mess of that picture. The rule to follow for 1D problems is to

a) choose a +ve direction
b) draw vectors pointing in the direction you wish, +ve or -ve
c) label all vectors with +ve magnitude
d) take account of direction in by making the sign of the vector +ve or -ve according to its direction.

They seem to have pointed the arrows as they wish, but also tried to take account of direction in the magnitude. This is going to lead to much confusion on the part of students (and the person who wrote the question, I think) as is then too easy to "double count" the -ve directions.

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