# Mechanics

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#1
Can someone explain how the answer is 7.5N I've attached the question and my workings.
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1 year ago
#2
(Original post by Hollymae764)
Can someone explain how the answer is 7.5N I've attached the question and my workings.
They're just using ,mass x acceleration.

I don't think there's anything subtle in the question

ETA: when they say "resultant force" they mean the sum of all forces acting, so there's no other calculation to be done!
Last edited by davros; 1 year ago
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#3
(Original post by davros)
They're just using ,mass x acceleration.

I don't think there's anything subtle in the question
So how do I know hen I need to use mass x acceleration and not Newton's First Law?
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#4
(Original post by Hollymae764)
Ignore that Stupid question
So why do I not take into account the weight of the particle and the force of gravity?
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1 year ago
#5
(Original post by Hollymae764)
So how do I know hen I need to use mass x acceleration and not Newton's First Law?
You're using Newton's Second Law

F = ma

where F is the resultant force and a is the acceleration.

There could be any number of individual forces that we don't know about (rocket motor, magnetism, tension etc) - the point is that we only want to know the resultant (i.e. total) force on the particle producing that final acceleration.
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1 year ago
#6
(Original post by Hollymae764)
So why do I not take into account the weight of the particle and the force of gravity?
I know what you mean - but you're overthinking the question!

It's the same question as "a particle of mass m moves with acceleration a - what is the applied force on it?"
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#7
(Original post by davros)
You're using Newton's Second Law

F = ma

where F is the resultant force and a is the acceleration.

There could be any number of individual forces that we don't know about (rocket motor, magnetism, tension etc) - the point is that we only want to know the resultant (i.e. total) force on the particle producing that final acceleration.
Ah that makes sense now thank you
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1 year ago
#8
(Original post by Hollymae764)
Ah that makes sense now thank you
no problem For a 1-mark question they're not going be asking anything too complicated!
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