Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta
    • Thread Starter
    Offline

    19
    ReputationRep:
    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?

    Name:  IMG_2188.jpg
Views: 21
Size:  284.3 KB
    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.
    Attached Images
     
    Offline

    12
    ReputationRep:
    (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.
    • Thread Starter
    Offline

    19
    ReputationRep:
    (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.
    Offline

    11
    ReputationRep:
    (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 \sum F=ma 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.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: October 15, 2017
Poll
Are you going to a festival?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.