Physics: Post your doubts here!

[i_try9621]

View attachment 61545

Why should the stone weight matter here? Shouldn't the centripetal force and the Tension be equal?

 

[DragonCub]

Why should the stone weight matter here? Shouldn't the centripetal force and the Tension be equal?

When talking about circular motion, the centripetal force should always be the net force on the object. This is why we need to take into account every single force on the stone, including its weight.
Why necessarily net force? Because Newton's Second Law demands that the F in F = ma be the net force, in every case; and in circular motion, centripetal acceleration and centripetal force also obeys Newtons Second Law.

 

[i_try9621]

When talking about circular motion, the centripetal force should always be the net force on the object. This is why we need to take into account every single force on the stone, including its weight.
Why necessarily net force? Because Newton's Second Law demands that the F in F = ma be the net force, in every case; and in circular motion, centripetal acceleration and centripetal force also obeys Newtons Second Law.

So the weight also acts towards the centre?

 

[DragonCub]

So the weight also acts towards the centre?

Not always.

The weight always acts downward.
And since:
1. the centripetal force (F_c) is constantly changing direction as the object changes position
2. F_c = F + F_g ( F is the force exerted by the glue)
So...
The required magnitude of F is also changing.
And the largest F required corresponds to the "bottom" instant, where F_c is upwards and F_g is downwards (opposite to each other).

 

[i_try9621]

Not always.

The weight always acts downward.
And since:
1. the centripetal force (F_c) is constantly changing direction as the object changes position
2. F_c = F + F_g ( F is the force exerted by the glue)
So...
The required magnitude of F is also changing.
And the largest F required corresponds to the "bottom" instant, where F_c is upwards and F_g is downwards (opposite to each other).

As you mentioned above in the first reply, that the gravity of the stone is exerted on the glue. Could that be the reason why the weight should be included here ? Or am I looking at it the wrong way?

 

[DragonCub]

As you mentioned above in the first reply, that the gravity of the stone is exerted on the glue. Could that be the reason why the weight should be included here ? Or am I looking at it the wrong way?

I think I explained that a bit vaguely at first.

Let's start it again from the beginning. First, two things hold true at any moment:
1. F_c = F + F_g
2. F_g always acts downward

Now, since we know that a force has directions, that may affect its sign (positive / negative), let's suppose that, on the vertical axis, upward is positive, downward is negative.
F_g always acts downward, so we can say that F_g = - 3.0 N.
I know the question says that the stone's gravity is 3.0 N, without the negative sign. It actually means that the gravity has a magnitude of 3.0 N.

Here, the magnitude of something means the modulus, or absolute value, or that thing. For gravity, its magnitude means | F_g |.
We can also say that the magnitude of a force does not consider the force's direction, so it doesn't have a +/- discretion.

But once we start to consider directions, we need to use +/- signs to designate them. And surely you can designate downward as positive - either way is okay. But let's take upward as "+" for this time.

We know that F_c always acts towards the centre. And let's suppose that F_c has a magnitude of 10 N.
Then, when the stone is at the top, F_c is acting downward (centre is below the stone). At this moment, F_c = -10 N.
Based on F_c = F + F_g, we can get: -10 = F + (-3) --> F = -7 N then F has a magnitude of 7 N.

When the stone is at the bottom, F_c is acting upward (centre is above the stone). Then F_c = +10 N.
Again, using F_c = F + F_g, --> +10 = F + (-3) --> F = +13 N --> magnitude of F is 13 N.

You can see, at the top and the bottom, the gravity has opposite effects on the required magnitude of F.
This is why we need to take into account the stone's gravity.



P.S. We can also discuss on the moments when the stone is horizontally level with the centre (centripetal force acting horizontally), but that will involve the horizontal axis as well, and make things more complicated.
But if you are curious enough, we can discuss this in a separate chat.

 

[i_try9621]

I think I explained that a bit vaguely at first.

Let's start it again from the beginning. First, two things hold true at any moment:
1. F_c = F + F_g
2. F_g always acts downward

Now, since we know that a force has directions, that may affect its sign (positive / negative), let's suppose that, on the vertical axis, upward is positive, downward is negative.
F_g always acts downward, so we can say that F_g = - 3.0 N.
I know the question says that the stone's gravity is 3.0 N, without the negative sign. It actually means that the gravity has a magnitude of 3.0 N.

Here, the magnitude of something means the modulus, or absolute value, or that thing. For gravity, its magnitude means | F_g |.
We can also say that the magnitude of a force does not consider the force's direction, so it doesn't have a +/- discretion.

But once we start to consider directions, we need to use +/- signs to designate them. And surely you can designate downward as positive - either way is okay. But let's take upward as "+" for this time.

We know that F_c always acts towards the centre. And let's suppose that F_c has a magnitude of 10 N.
Then, when the stone is at the top, F_c is acting downward (centre is below the stone). At this moment, F_c = -10 N.
Based on F_c = F + F_g, we can get: -10 = F + (-3) --> F = -7 N then F has a magnitude of 7 N.

When the stone is at the bottom, F_c is acting upward (centre is above the stone). Then F_c = +10 N.
Again, using F_c = F + F_g, --> +10 = F + (-3) --> F = +13 N --> magnitude of F is 13 N.

You can see, at the top and the bottom, the gravity has opposite effects on the required magnitude of F.
This is why we need to take into account the stone's gravity.



P.S. We can also discuss on the moments when the stone is horizontally level with the centre (centripetal force acting horizontally), but that will involve the horizontal axis as well, and make things more complicated.
But if you are curious enough, we can discuss this in a separate chat.

Okay, I now did understand the significance of including the weight but the equation that you have given here F_c = F + F_g and the equation you used in the first reply where the tension is 18N, and is equal to the sum of the stone's gravity and the centripetal force. My confusion is that are these two the same?

From the question
Tension(F) = 18N
Centripetal force(net force)(F_c) = ??
Weight of the stone ( here it's F_g) = 3.0N

Net force = F - F_g = 18 - 3 = 15N

So is it "F_c = F - F_g" OR "F_c = F + F_g" ?

 

[krishnapatelzz]

http://cieoandalevelnotes.blogspot.in/

mereko to kabhi bola hi nahi

 

[Thought blocker]

mereko to kabhi bola hi nahi

teko already pata tha be -_-

 

[DragonCub]

Okay, I now did understand the significance of including the weight but the equation that you have given here F_c = F + F_g and the equation you used in the first reply where the tension is 18N, and is equal to the sum of the stone's gravity and the centripetal force. My confusion is that are these two the same?

From the question
Tension(F) = 18N
Centripetal force(net force)(F_c) = ??
Weight of the stone ( here it's F_g) = 3.0N

Net force = F - F_g = 18 - 3 = 15N

So is it "F_c = F - F_g" OR "F_c = F + F_g" ?

Forget about that statement for a moment.
F_c = F + F_g is always the case, when the forces here are seen as vectors.
When the stone is at the bottom, centripetal force is upward, and gravity is downward. So F_c > 0, F_g = -3.0 N
So, plug in the value, we can see that F_c = F + (-3) --> F_c = F - 3

In the question,
- "tension is 18 N" actually means the tension's magnitude is 18 N, or |F| = 18 N
- "weight of the stone is 3 N" means |F_g| = 3 N

Now we know that F_g acts downward, but what's the direction of F?
F_c = F - 3 and F_c > 0 --> F - 3 > 0 --> F > 0
So F is upward. And since |F| = 18 N, we can say that F = +18 N

Then, at last, F_c = (+18) - 3 = +15 --> |F_c| = 15 N
/
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At this point, we have obtained the following:
|F| = 18 N
|F_g| = 3 N
|F_c| = 15 N

It's clear that |F| = |F_g| + |F_c|, or |F_c| = |F| - |F_g|
And this is actually what I meant by "tension is equal to the sum of the stone's gravity and the centripetal force", when we only look at the magnitude.

 

[shahmir khan]

Hello guys need help with this question?

 

[DragonCub]

View attachment 61555

Hello guys need help with this question?

1. Suppose one uses the same heater to:
- heat a cup of water to boil
- heat a full pot of water to boil
Are they going to take the same time? The answer is no.

Different time required means different thermal energy into the two portions of water. However, their starting and ending temperatures are the same. So temperature doesn't measure the thermal energy level.

2. When ice is melting, it's gaining thermal energy, but is its temperature rising? No.
Temperature stays the same, but thermal energy amount is getting larger. Again this proves the statement wrong.

 

[i_try9621]

Forget about that statement for a moment.
F_c = F + F_g is always the case, when the forces here are seen as vectors.
When the stone is at the bottom, centripetal force is upward, and gravity is downward. So F_c > 0, F_g = -3.0 N
So, plug in the value, we can see that F_c = F + (-3) --> F_c = F - 3

In the question,
- "tension is 18 N" actually means the tension's magnitude is 18 N, or |F| = 18 N
- "weight of the stone is 3 N" means |F_g| = 3 N

Now we know that F_g acts downward, but what's the direction of F?
F_c = F - 3 and F_c > 0 --> F - 3 > 0 --> F > 0
So F is upward. And since |F| = 18 N, we can say that F = +18 N

Then, at last, F_c = (+18) - 3 = +15 --> |F_c| = 15 N
/
-----------------------------------------------------------------------------------------------------------
At this point, we have obtained the following:
|F| = 18 N
|F_g| = 3 N
|F_c| = 15 N

It's clear that |F| = |F_g| + |F_c|, or |F_c| = |F| - |F_g|
And this is actually what I meant by "tension is equal to the sum of the stone's gravity and the centripetal force", when we only look at the magnitude.

I now finally understand.
You sir, are a genius. Thank you!

 

[shahmir khan]

Aoa guys
I need advice on how to prepare paper 5 ? Could you just send me links of the resources to learn paper 5 and tips on how to prepare. I would greatly appreciate it !
Thanks in advance

 

[Thought blocker]

Aoa guys
I need advice on how to prepare paper 5 ? Could you just send me links of the resources to learn paper 5 and tips on how to prepare. I would greatly appreciate it !
Thanks in advance

You should know all the basic knowledge of As and A level to apply it in paper 5.
If you notice, there is a pattern in paper 5 for question 1. And queston 2 is so easy.
go through some papers with markscheme and slowly after 5 - 6 papers u will get idea howto solve
http://cieoandalevelnotes.blogspot.in/
the site above has solved papers, u can refer it for diagrams and u can read as well how to write,
Good luck.

 

[silver slick]

Please can u solve this step by step, i cant seem to have the correct value, maybe my calculating method wrong,or i am doing some silly mistake- help will be highly appreciated-

 

[areeba240]

the ans of (ii) is no force ...is this because motion of particle is parallel to magnetic field?

 

[areeba240]

(i) no change
(ii) upward
(iii) no change
can anyone plzz explain me the reason for these ans

 

[Thought blocker]

View attachment 61563
(i) no change
(ii) upward
(iii) no change
can anyone plzz explain me the reason for these ans

Fb = bqv and Fe = qV
(i) no change coz both the forces equation includes charge (q) and thus no change.
(ii) upwards coz velocity is included in Fb equation but not in Fe equation, and as Fb is upward thus upwards
(iii) no change as both equation dont include mass so it doest affect anything.
Mass is independent in both.

 

[Thought blocker]

View attachment 61562
the ans of (ii) is no force ...is this because motion of particle is parallel to magnetic field?

yup.