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#1
I've been looking over my GCSE Physics notes and there are some very simple things I just don't get.

Number one. Newton's 3rd Law... forces occur in pairs which are equal and opposite. It is probably just the simplified definition that is making this confusing but what I don't get is, when forces are unbalanced they are obviously not equal. So how does this work? Can someone explain this law better? Also, how does this work with gravity?

Number two. Why do planets have elliptical orbits if they are being kept in their orbit by the Sun's gravitational field?

Number three. I have a diagram of a comet's orbit round the Sun and it states that the comet's GPE is at a maximum when it is nearest the Sun and at a minimum when it is furthest away. However, an object (e.g. a ball) held up high on Earth has more GPE than one lower down, so I would have assumed that the further the object (ie the comet) from the gravitational field (ie the Sun), the greater the GPE... but this is not so. Since GPE = m*g*h, is the effect of the g when closer to the Sun greater than the effect of the h?

As you can see I am very !!! Any help very greatly appreciated and will be rewarded with rep
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16 years ago
#2
(Original post by dinkymints)
I've been looking over my GCSE Physics notes and there are some very simple things I just don't get.

Number one. Newton's 3rd Law... forces occur in pairs which are equal and opposite. It is probably just the simplified definition that is making this confusing but what I don't get is, when forces are unbalanced they are obviously not equal. So how does this work? Can someone explain this law better? Also, how does this work with gravity?

Number two. Why do planets have elliptical orbits if they are being kept in their orbit by the Sun's gravitational field?

Number three. I have a diagram of a comet's orbit round the Sun and it states that the comet's GPE is at a maximum when it is nearest the Sun and at a minimum when it is furthest away. However, an object (e.g. a ball) held up high on Earth has more GPE than one lower down, so I would have assumed that the further the object (ie the comet) from the gravitational field (ie the Sun), the greater the GPE... but this is not so. Since GPE = m*g*h, is the effect of the g when closer to the Sun greater than the effect of the h?

As you can see I am very !!! Any help very greatly appreciated and will be rewarded with rep
That's GCSE Physics... Now I'm glad I did Double Award Science
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16 years ago
#3
Okay for the first one:
Newton's third says that forces come in pairs with the following properties:
(a) the 2 forces have equal magnitudes
(b) they have opposite directions
(c) they act on different bodies
So, for example the third law pair force to your weight (ie. gravitational force you feel due to the Earth) is the gravitational force the Earth feels due to you - the Earth is attracted towards you with the same force but because it's so massive this hardly makes a difference so you don't move the Earth.
(c) is the important thing many people initially usually overlook. The fact that one body is in equilibrium or not hasn't got much to do with Newon's Third since the pair doesn't cancel each other out as they act on different bodies.
Eg. the gravitational force on the Earth doesn't cancel out your weight since one acts on the Earth and the other acts on you.

I'll try answer the other two if I get round to it
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16 years ago
#4
(Original post by dinkymints)
I've been looking over my GCSE Physics notes and there are some very simple things I just don't get.

Number one. Newton's 3rd Law... forces occur in pairs which are equal and opposite. It is probably just the simplified definition that is making this confusing but what I don't get is, when forces are unbalanced they are obviously not equal. So how does this work? Can someone explain this law better? Also, how does this work with gravity?

Number two. Why do planets have elliptical orbits if they are being kept in their orbit by the Sun's gravitational field?

Number three. I have a diagram of a comet's orbit round the Sun and it states that the comet's GPE is at a maximum when it is nearest the Sun and at a minimum when it is furthest away. However, an object (e.g. a ball) held up high on Earth has more GPE than one lower down, so I would have assumed that the further the object (ie the comet) from the gravitational field (ie the Sun), the greater the GPE... but this is not so. Since GPE = m*g*h, is the effect of the g when closer to the Sun greater than the effect of the h?

As you can see I am very !!! Any help very greatly appreciated and will be rewarded with rep
Forces don't have to come in pairs, Newtons third law: If a body A exerts a force on body B, body B exerts an equal and opposite force on body A.

if 2 objects are in contact they exert equal opposite forces on each other, the resultant force (net force) maybe unbalance due to outside forces, this only describes the system it is in eg the forces acting on an object.

Gravity acts on all objects with mass, if a ball is 5m above the surface, the earth will pull the ball towards itself, and the ball will pull the earth towards itself, the forces being equal and opposite.

An inverse square law applies for gravity i.e. half the distance 4 times the strength of the gravitational field. So as h doubles, g would quarter, and will result in a lower GPE.
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#5
(Original post by marcos)
Okay for the first one:
Newton's third says that forces come in pairs with the following properties:
(a) the 2 forces have equal magnitudes
(b) they have opposite directions
(c) they act on different bodies
So, for example the third law pair force to your weight (ie. gravitational force you feel due to the Earth) is the gravitational force the Earth feels due to you - the Earth is attracted towards you with the same force but because it's so massive this hardly makes a difference so you don't move the Earth.
(c) is the important thing many people initially usually overlook. The fact that one body is in equilibrium or not hasn't got much to do with Newon's Third since the pair doesn't cancel each other out as they act on different bodies.
Eg. the gravitational force on the Earth doesn't cancel out your weight since one acts on the Earth and the other acts on you.

I'll try answer the other two if I get round to it
Great, thanks. So what would the gravitational effect be if two bodies had the same mass?
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16 years ago
#6
Number 2 - Its eliptical rather than circular because there are other planets, whose gravitational field will affect the earth and cause it to be immediately non-circular as it pulls the earth one side or the other.
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16 years ago
#7
(Original post by dinkymints)
Great, thanks. So what would the gravitational effect be if two bodies had the same mass?
depends on the size of the bodies as well as the distance from them
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16 years ago
#8
depends on the size of the bodies as well as the distance from them
LOL what an immature mind I've got...
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16 years ago
#9
lol
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16 years ago
#10
(Original post by dinkymints)
Great, thanks. So what would the gravitational effect be if two bodies had the same mass?
at GCSE you don't have to know WHY, you just have to learn concepts and forumulae and APPLY it i.e. it will say " state newtons 2nd law of motion " then it will say " a ball of mass 0,5 kg is accelerating at 0.2 ms-1, apply newtons 2nd law to find the net force "
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16 years ago
#11
(Original post by Pixelfairy #1)
LOL what an immature mind I've got...
LOL
=P
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16 years ago
#12
GCSE Physics was annoying, they never explained anything properly coz they go half-way through it then say "I wont explain the rest as its not needed"...the most confusing topic at GCSE was probably interference it was sooo confusing at the time lol
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16 years ago
#13
depends on the size of the bodies as well as the distance from them
but like i say dinkie, you're asking questions people in my class would ask i.e. a level questions ......... it's definately a good thing but for GCSE it is pushing it slightly

to be more specific the net force on two bodies F = K Q1 Q2
--------
r2

where k = 1/(4pi epsilon 0 ) where ep 0 is the permittivity of free space
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16 years ago
#14
(Original post by kenkennykenken)
GCSE Physics was annoying, they never explained anything properly coz they go half-way through it then say "I wont explain the rest as its not needed"...the most confusing topic at GCSE was probably interference it was sooo confusing at the <a href="http://www.ntsearch.com/search.php?q=time&v=55">time</a> lol
agreed and things they teach you at A level physics are different to that of GCSE ( wrong basically ) LOL
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16 years ago
#15
but like i say dinkie, you're asking questions people in my class would ask i.e. a level questions ......... it's definately a good thing but for GCSE it is pushing it slightly

to be more specific the net force on two bodies F = K Q1 Q2
--------
r2

where k = 1/(4pi epsilon 0 ) where ep 0 is the permittivity of free space
That's the force between two charged particles, not just two bodies.
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16 years ago
#16
yeah lol. at the beginning of A-level they're like "Before we start the course, I'd like to clarity a few things. Most of the things you learnt at GCSE were WRONG!" ...its sooo pointless lol. But then again it'd probably be impossibly difficult to teach the real things at GCSE level
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16 years ago
#17
Number 2 - Its eliptical rather than circular because there are other planets, whose gravitational field will affect the earth and cause it to be immediately non-circular as it pulls the earth one side or the other.
Actually this is the reason that the orbits are NOT actually elliptical but just approximately so. (Interestingly it's extremely complicated to work out the path when there are three or more bodies even with calculus). Even if it were just the Sun and the Earth then it would still be possible for the Earth to move in a stable elliptical orbit - depends on the initial motion of it.
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16 years ago
#18
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16 years ago
#19
(Original post by Nylex)
That's the force between two charged particles, not just two bodies.
true thanks for that, my mistake: i meant F = G m1 m2 / r2
G = constant
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#20
Thanks all. I know I don't need to know all of this but I'm interested... my teacher refuses to answer my questions, I get at least one "I'm not telling you coz it's too hard and it's not on the syllabus" per lesson. It's kind of annoying that we have to learn that things happen and not why, for example why moving a wire through a magnetic field produces a current.

One more question... in our mock paper, one of the questions mentioned that there used to be debate as to whether light acted as a wave or particles and that now it is known that it acts as both. (Very simply put, as the question wasn't really about that at all but about ideas and evidence blah blah). So I was wondering whether light is affected by gravity, if it behaves as a particle? Somebody mentioned in this thread that gravity acts on all things with mass - light doesn't have mass, does it? So I assume that's a no?
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