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

    13
    ReputationRep:
    A particle, of mass 0.4kg, is projected vertically upwards with an initial speed of 30 ms . at time t s after projection, the speed of the particle is v ms and the magnitude of the air resistance acting on the particle is 0.2 vN. Show that -9.8-0.5v = dv/dtIt is only a 2 mark question. I know to use F = ma where a=dv/dt. I can see that 9.8 is obviously gravity but I'm unable to work it into my equation :(-0.2v = 0.4dv/dt will then get you -0.5v =dv/dt but where do I fit the 9.8 in so that it remains unchanged? (Sorry for asking a very simple question ahaha)
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by 98matt)
    A particle, of mass 0.4kg, is projected vertically upwards with an initial speed of 30 ms . at time t s after projection, the speed of the particle is v ms and the magnitude of the air resistance acting on the particle is 0.2 vN. Show that -9.8-0.5v = dv/dtIt is only a 2 mark question. I know to use F = ma where a=dv/dt. I can see that 9.8 is obviously gravity but I'm unable to work it into my equation -0.2v = 0.4dv/dt will then get you -0.5v =dv/dt but where do I fit the 9.8 in so that it remains unchanged? (Sorry for asking a very simple question ahaha)
    You are forgetting the weight of the particle which acts 0.4g Newtons downward.

    Your working is correct, but if you consider the weight in addition to the air resistance, you would get:
    (-0.4g)+(-0.2v)=0.4\cdot \frac{dv}{dt} as required.
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by RDKGames)
    You are forgetting the weight of the particle which acts 0.4g Newtons downward.

    Your working is correct, but if you consider the weight in addition to the air resistance, you would get:
    (-0.4g)+(-0.2v)=0.4\cdot \frac{dv}{dt} as required.
    Thanks I knew it was something stupid and trivial like that ahaha, I feel dumb now
    • Community Assistant
    Offline

    20
    ReputationRep:
    Community Assistant
    (Original post by 98matt)
    Thanks I knew it was something stupid and trivial like that ahaha, I feel dumb now
    Happens sometimes, I still feel dumb about forgetting something obvious when it comes to problems involving Hooke's Law (as far as M2 is concerned) so I can relate
    • Thread Starter
    Offline

    13
    ReputationRep:
    (Original post by RDKGames)
    Happens sometimes, I still feel dumb about forgetting something obvious when it comes to problems involving Hooke's Law (as far as M2 is concerned) so I can relate
    Rectilinear motion is the first (and only) topic I've looked at in M2 so far. The only bit of Hooke's Law I know is from AS physics aha
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: August 4, 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.