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

    14
    ReputationRep:
    In the image attached, why is the change in total energy KE - GPE, and not GPE - KE, and in what case would it be GPE - KE.

    If someone can please explain this to me it would be great, thanks.

    Name:  vvv.PNG
Views: 168
Size:  135.9 KB
    Offline

    3
    ReputationRep:
    It doesn't matter which way round you write it you still get the same answer.

    gain in KE- loss in GPE = 0 so v^2 - 196 = 0 so v^ = 196 so v = 14 m/s

    OR

    loss in GPE - gain in KE = 0 so 196 - v^2 = 0 so 196 = v^2 so v = 14 m/s
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by Teenie2)
    It doesn't matter which way round you write it you still get the same answer.

    gain in KE- loss in GPE = 0 so v^2 - 196 = 0 so v^ = 196 so v = 14 m/s

    OR

    loss in GPE - gain in KE = 0 so 196 - v^2 = 0 so 196 = v^2 so v = 14 m/s
    Yes, but in this question then here, why is it GPE - KE. It won't be the same answer in this question.Name:  nnnn.PNG
Views: 140
Size:  127.9 KB
    • Community Assistant
    • Study Helper
    Offline

    20
    ReputationRep:
    Community Assistant
    Study Helper
    (Original post by Reda2)
    In the image attached, why is the change in total energy KE - GPE, and not GPE - KE, and in what case would it be GPE - KE.

    If someone can please explain this to me it would be great, thanks.

    Name:  vvv.PNG
Views: 168
Size:  135.9 KB
    Can you post the whole question and the full working?
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by notnek)
    Can you post the whole question and the full working?
    This for example, attached. You can see it is, Loss in KE - gain in GPE = work done against friction.

    Why is it that, and why is it not, gain in GPE - KE = work done against friction?
    Is it because it is going from KE to GPE?
    Thanks
    • Community Assistant
    • Study Helper
    Offline

    20
    ReputationRep:
    Community Assistant
    Study Helper
    (Original post by Reda2)
    This for example, attached. You can see it is, Loss in KE - gain in GPE = work done against friction.

    Why is it that, and why is it not, gain in GPE - KE = work done against friction?
    Is it because it is going from KE to GPE?
    Thanks
    Referring to the image from your first post:

    That's the solution but you haven't posted the question. Is the working on the left related to the working on the right?

    Please post everything.

    It's confusing since they talk about "work done against friction" but it seems to be equal to 0. I'd need to see the question before I can help you.
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by notnek)
    Referring to the image from your first post:

    That's the solution but you haven't posted the question. Is the working on the left related to the working on the right?

    Please post everything.

    It's confusing since they talk about "work done against friction" but it seems to be equal to 0. I'd need to see the question before I can help you.
    Sorry I forgot to attach them. The question in this reply.
    Name:  11.PNG
Views: 79
Size:  13.6 KBAttachment 551477551479
    Attached Images
     
    • Community Assistant
    • Study Helper
    Offline

    20
    ReputationRep:
    Community Assistant
    Study Helper
    (Original post by Reda2)
    Sorry I forgot to attach them. The question in this reply.
    Name:  11.PNG
Views: 79
Size:  13.6 KBAttachment 551477551479
    I thought your issue was with the working that you posted in your original post.

    This is a different question.

    Can you please start again, and post the question and working that confused you and tell us again why.
    Offline

    2
    ReputationRep:
    (Original post by notnek)
    Referring to the image from your first post:

    That's the solution but you haven't posted the question. Is the working on the left related to the working on the right?

    Please post everything.

    It's confusing since they talk about "work done against friction" but it seems to be equal to 0. I'd need to see the question before I can help you.
    Always use this formula..

    WORK DONE AGAINST RESISTANCE = LOSS IN ENERGY (KE OR GPE) - GAIN IN ENERGY (KE OR GPE)

    Always do whatever energy is lost - whatever energy is gained
    Offline

    3
    ReputationRep:
    In both questions (GPE at start) + (KE at start) = (GPE at finish) + (KE at finish) + (work done)

    In question 1 there is no loss in energy (e.g. no air resistance) this means that

    (GPE at start) + (KE at start) = (GPE at finish) + (KE at finish) + (work done) so (mgh) + (0) =(0) + (0.5mv^2) + (0) so 2 * 9.8 * 10 = 0.5 * 2 * v^2 so v^2 = 196 so v = 14 m/s

    In question 2 there is work done to overcome friction so this means that

    (GPE at start) + (KE at start) = (GPE at finish) + (KE at finish) + (work done) so

    (GPE at start) - (GPE at finish) + (KE at start) - (KE at finish) = (work done against friction) so

    (GPE at start - GPE at finish) + (KE at start) - (KE at finish) = (work done against friction) so

    (mgh) + (0.5 * m * initial velocity ^2) - (0.5 * m * final velocity ^2) = (work done against friction) so

    (6 * 9.98 * 2) + (0.5 * 6 * 4) - (0.5 * 6 * v^2) = (0.25 * FN * 4)

    There's a youtube video at https://www.youtube.com/watch?v=dj8Y1Pof9XU that explains this using an example of a block starting from rest sliding down a slope that might help you.
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by fpmaniac)
    Always use this formula..

    WORK DONE AGAINST RESISTANCE = LOSS IN ENERGY (KE OR GPE) - GAIN IN ENERGY (KE OR GPE)

    Always do whatever energy is lost - whatever energy is gained
    That's the answer I was looking for, thank you very much.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: June 16, 2016
Poll
Do you agree with the proposed ban on plastic straws and cotton buds?
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.