- Forums
###### Mechanics Resolving Forces

Watch

1 year ago

This is from the answer sheet

Shouldn't the direction of friction be in the direction of the deceleration?

Because even though its decelerating the direction of motion is to the right?

The workings show that positive direction taken to the right. But they show the acceleration as negative. And the frictional force also as negative so does not make sense here.

Was wondering if I am missing something?

Here is my diagram for this

Friction opposes motion (opposite of velocity), it doesnt matter whether the object is accelerating or decelerating. In this scenario it is the sole cause the deceleration so the frictional force is in the opposite direction to positive velocity..

For their diagram, Id guess the positive direction etc is to the left as friction is acting to the right. Similarly, a represents an arbitrary (positive) acceleration. For this example it turns out to be negative as it must equal the deceleration caused by the friction. When they use v-u+at, ithe sign of a is correctly worked out and it is decelerating as the question states. Note for a force diagram, once the positive direction is clearly stated, you'd mark on the force directions (and their magnitudes) relative to that.

Edit - as a related example, imagine you threw a ball up in the air and its motion was solely determined by gravity. Assuming positve is up, you'd mark an arbitrary positive acceleration a as upwards, g (9.8) acting downwards and youd get an equivalent diagram to what they have here. Then newton 2 gives

a = -9.8

so it is decelerating (upwards) or equivalently accelerating downwards.

For their diagram, Id guess the positive direction etc is to the left as friction is acting to the right. Similarly, a represents an arbitrary (positive) acceleration. For this example it turns out to be negative as it must equal the deceleration caused by the friction. When they use v-u+at, ithe sign of a is correctly worked out and it is decelerating as the question states. Note for a force diagram, once the positive direction is clearly stated, you'd mark on the force directions (and their magnitudes) relative to that.

Edit - as a related example, imagine you threw a ball up in the air and its motion was solely determined by gravity. Assuming positve is up, you'd mark an arbitrary positive acceleration a as upwards, g (9.8) acting downwards and youd get an equivalent diagram to what they have here. Then newton 2 gives

a = -9.8

so it is decelerating (upwards) or equivalently accelerating downwards.

(edited 1 year ago)

Reply 2

1 year ago

Original post by mqb2766

Friction opposes motion (opposite of velocity), it doesnt matter whether the object is accelerating or decelerating. In this scenario it is the sole cause the deceleration so the frictional force is in the opposite direction to positive velocity..

For their diagram, Id guess the positive direction etc is to the left as friction is acting to the right. Similarly, a represents an arbitrary (positive) acceleration. For this example it turns out to be negative as it must equal the deceleration caused by the friction. When they use v-u+at, ithe sign of a is correctly worked out and it is decelerating as the question states. Note for a force diagram, once the positive direction is clearly stated, you'd mark on the force directions (and their magnitudes) relative to that.

Edit - as a related example, imagine you threw a ball up in the air and its motion was solely determined by gravity. Assuming positve is up, you'd mark an arbitrary positive acceleration a as upwards, g (9.8) acting downwards and youd get an equivalent diagram to what they have here. Then newton 2 gives

a = -9.8

so it is decelerating (upwards) or equivalently accelerating downwards.

For their diagram, Id guess the positive direction etc is to the left as friction is acting to the right. Similarly, a represents an arbitrary (positive) acceleration. For this example it turns out to be negative as it must equal the deceleration caused by the friction. When they use v-u+at, ithe sign of a is correctly worked out and it is decelerating as the question states. Note for a force diagram, once the positive direction is clearly stated, you'd mark on the force directions (and their magnitudes) relative to that.

Edit - as a related example, imagine you threw a ball up in the air and its motion was solely determined by gravity. Assuming positve is up, you'd mark an arbitrary positive acceleration a as upwards, g (9.8) acting downwards and youd get an equivalent diagram to what they have here. Then newton 2 gives

a = -9.8

so it is decelerating (upwards) or equivalently accelerating downwards.

Ahh yes I got it now thank you! I actually put v and a in different directions which caused the problem. If I am taking right as positive then even a and v should be positive to the right

Original post by Goldenknight

Ahh yes I got it now thank you! I actually put v and a in different directions which caused the problem. If I am taking right as positive then even a and v should be positive to the right

The direction that you assume is positive on your sketch is obviously arbitrary. If you assumed positive (displacement) is right, then a positive velocity increases displacement and that is interpreted as pointing right. Similarly for positive acceleration. Drawing the frictional force muR would then be to the left as it acts to reduce a positive velocity and hence produces a negative acceleration (deceleration).

If the velocity was negative, friction would produce a positive acceleration and the velocity and friction arrows would be obviously reversed. They must be in opposing directions.

In your sketch in the OP, you have this, the only thing that might cause confusion is your "a". Im sure you mean that its the direction of the deceleration, so the acceleration is negative in the opposite direction to the current velocity. In the book, Im assuming they're referring to the direction of an arbitrary positive acceleration. For more complex problems, you may not be sure of whether the resultant force will accelerate or decelerate, so the main thing is to make sure the positive direction is clearly marked so the sign of individual forces can be determined correctly.

(edited 1 year ago)

- Maths (Mechanics) : How to practise resolving forces on a diagram?
- A-LEVEL MATHS HELP! Asap
- A level maths mechanics
- Help with resolving non perpendicular force on weirdly shaped rod
- Further Mechanics 1 Question
- A-Level Mechanics Help
- Mechanics Question Help
- AS Maths and Mechanics
- Maths Mechanics Moments Question
- A level maths - resolving forces question
- A level maths Mechanics help!!!
- A-level Maths Mechanics
- A/S level math question.
- resolving forces
- maths a level mechanics question
- Do I need to resolve these forces? A level maths mechanics
- A level maths mechanics moments and forces question
- standard ladder force diagram
- A Level Mechanics
- Why is tension not equal to weight in moments questions?

- Possessiveness
- RCA 2024 Applicants
- Official Thread: Graduate Entry Medicine 2024 Entry
- dermatologist as a career
- Official Oxford 2024 Postgraduate Applicants Thread
- offers for Leeds Uni
- What unis should I apply to
- Why did France ban abaya ban in schools
- alevel languages
- Law offers - 2024 entry
- TSR Study Together: Spanish Version!
- EY graduate scheme 2024
- The Cambridge College Hurt/Heal Game [part 2]
- Not liking a friend anymore
- Make it More Sauce-ey !!
- Oxford Creative Writing MSt 2024
- Religious education teachers, your experiences?
- Family issues
- Official UCL Offer Holders Thread for 2024 entry
- Solicitor apprenticeship applications 2024

- Home Office Customer Services Group - HEO
- gcse physics
- Cambridge Applications statistics page showing 2024 applicants??
- UEA or Sheffield University?
- UCL Medicine A100 2024
- Keele University A104 2024 Entry applicants
- What do guys find attractive and how can i glow up
- Official: University of Bristol A100 2024 Entry Applicants
- The Official King's College London Applicants for 2024 Entry Thread
- Official KCL Offer Holders Thread for 2024 entry
- Mpharm 2024
- Official: University of Birmingham A100 2024 Entry Applicants
- Steps to becoming a fire fighter
- Official Dental Hygiene and Therapy (Oral Health Science) 2024 Entry Thread
- English literature paper 1
- Does anyone ever feel like their mother hates them
- Imperial Mechanical Engineering Interview
- CPS Pupillage/Legal Trainee Scheme 2024 (2025 Start)
- What YouTube tutorial channels do you watch?
- Unsure of where to go to, any insight if possible would be greatly appreciated!

- GCSE Mathematics Study Group 2023-2024
- A-level Mathematics Study Group 2023-2024
- Mock set 4 paper 2 q14 a level maths (4 distinct points)
- UKMT Intermediate Math Challenge 2024 - Discussion
- Help with complex summation further maths a levels
- Could I have some help with this suvat question?
- MAT practice
- Alevel Maths Question
- weird cosine question
- Series function not differentiable at a point

- hyperbolic function catenary problem
- can anyone answer this A level vectors question (very challenging)
- STEP foundation module help pls
- Ukmt IMC 2024
- ukmt imc
- This maths question is driving me crazy
- Maths Mechanics
- Confused on surds question
- HNC MATHS A2 Task 3 (Radio Transmitters)
- Senior Maths Challenge 2023

- GCSE Mathematics Study Group 2023-2024
- A-level Mathematics Study Group 2023-2024
- Mock set 4 paper 2 q14 a level maths (4 distinct points)
- UKMT Intermediate Math Challenge 2024 - Discussion
- Help with complex summation further maths a levels
- Could I have some help with this suvat question?
- MAT practice
- Alevel Maths Question
- weird cosine question
- Series function not differentiable at a point

- hyperbolic function catenary problem
- can anyone answer this A level vectors question (very challenging)
- STEP foundation module help pls
- Ukmt IMC 2024
- ukmt imc
- This maths question is driving me crazy
- Maths Mechanics
- Confused on surds question
- HNC MATHS A2 Task 3 (Radio Transmitters)
- Senior Maths Challenge 2023

The Student Room and The Uni Guide are both part of The Student Room Group.

© Copyright The Student Room 2024 all rights reserved

The Student Room and The Uni Guide are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: Imperial House, 2nd Floor, 40-42 Queens Road, Brighton, East Sussex, BN1 3XB