A2 Physics | Post your doubts here

[waleedsmz]

And the effect of centripetal acceleration is to cause a rate of change of angular velocity.

Look bro, centripetal acceleration causes body to move in a circle, the angular velocity is ONLY a property of circular motion.

What I was trying to tell was An object can't move in a uniform circle with a constant angular velocity.

This is the whole problem. It's quiet the opposite! If the angular velocity actually changes and the force remains the same, then the radius would constantly decrease ( F = mw^2r ) which means that the motion will no longer be uniform!

Let's understand this by a simple example, if I tie a ball to the end of a rope and keep moving it in a circle. By your claim, it is impossible to move in a circular motion without the rope being cut. Why? Because radius and mass are constant and the omega "should" be changing thus the force should be increasing! It will increase to a certain point that the rope will not be able to provide the necessary tension. so it will be cut.

This is of course not true...

And no , when I wrote when the velocity is constant, there's no acceleration, I wasn't referring to rotational motion, but I was trying to refer to translatory motion, and later analyse both the motions in order to get to the right point.

Whichever motion you're referring to, you'll get the same answer. Let's think about something, if you have a ball moving in a circle with constant angular speed and velocity, and you gradually increase the magnitude of the velocity. What will happen to the angular speed? It will ALSO increase both practically and technically since v = wr. This means that if the speed is constant, then the angular velocity should be also constant, again both practically and technically.

However, to keep moving in a circular motion, a force is needed to cause a constant change in the direction of the velocity. This force causes centripetal acceleration which does NOT change the magnitude of the velocity rather its direction ( By definition ). If there is no change in magnitude, there is no change in angular velocity BUT there is a centripetal acceleration.

If either the velocity or the angular velocity somehow change ( they're actually connected so if one does change then the other will change as well ) .... Then the motion will be distorted and won't be a uniform circular motion... Unless the force varies as well and to some extinct the circular motion will be distorted as the radius would gradually change...

Hope that helped...

 

[waleedsmz]

View attachment 7516 Question about the electromagnetic forces between parallel conductors ...
Can anyone explain the questions in the figure with the Fleming's Rule ?
Also, in a long straight current carrying wire, the magnetic field is circular right ? But what is the direction of that magnetic field when trying to
put it into Fleming's Left Hand Rule ( as the first finger- the external magnetic field ?

To identify the magnetic field of a wire, you should use fleming's right hand grip rule.

There: http://sciencecity.oupchina.com.hk/npaw/student/glossary/right_hand_grip_rule.htm

Notice that the field of Y is actually coming out of the paper on wire X in it's circular shape. By using Fleming's left hand, middle finger ( current ) upwards, index finger ( out of paper ), the thumb would point to the right. Pretty much the same for the force on Y.

Hope that helped. : )

 

[Mustehssun Iqbal]

I haven't read the whole post yet but let's look at this text now:" This is the whole problem. It's quiet the opposite! If the angular velocity actually changes and the force remains the same, then the radius would constantly decrease ( F = mw^2r ) which means that the motion will no longer be uniform!

Let's understand this by a simple example, if I tie a ball to the end of a rope and keep moving it in a circle. By your claim, it is impossible to move in a circular motion without the rope being cut. Why? Because radius and mass are constant and the omega "should" be changing thus the force should be increasing! It will increase to a certain point that the rope will not be able to provide the necessary tension. so it will be cut.

This is of course not true... "
Let's do this simple and straight;
In translatory motion;
A constant force produces a constant acceleration.
A constant acceleration causes a change in velocity.
Now, zero force means zero acceleration caused.
zero acceleration caused means no change of velocity.
Now read it like this.
constant velocity means no acceleration.
no acceleration caused means no force providing the acceleration.
And yes, in your example, omega should be changing. You might have to produce some extra force to tweak the motion so it doesn't look like omega is changing. When you provide a constant centripetal force, omega changes continously throughout its motion. However, I did not say that the wire has to be cut.
Have you ever been in a ride called crazy bus, where the bus revolves round and round in circles?? You might've noticed that the omega of the bus changes constantly. It works on the principles of centripetal force.
Here's another thing;
Constant centripetal force causes constant centripetal acceleration.
Constant acceleration produces a change in velocity.
Acceleration is constant, but angular velocity isn't. Centripetal acceleration is the rate of change of angular velocity
( I get the part about velocity also has direction). Now please read this slowly. Centripetal acceleration is the rate of change of angular velocity. When someone says, angular velocity is constant, how can it then change?? Then how can there be any rate of change of angular velocity?? And in other words, how can there be a centripetal acceleration when there's no rate of change of angular velocity?? There probably can't.
Back to Kinematic of AS, you might find it helpful to recall acceleration graphs. When there was constant velocity-time graph, there was no sketch on the corresponding acceleration-time graph.

 

[Mustehssun Iqbal]

To identify the magnetic field of a wire, you should use fleming's right hand grip rule.

There: http://sciencecity.oupchina.com.hk/npaw/student/glossary/right_hand_grip_rule.htm

Notice that the field of Y is actually coming out of the paper on wire X in it's circular shape. By using Fleming's left hand, middle finger ( current ) upwards, index finger ( out of paper ), the thumb would point to the right. Pretty much the same for the force on Y.

Hope that helped. : )

oh wait. I guess I did something wrong there. I should revise the chapter.
Xtreme apologies for meddling with concepts

 

[Mustehssun Iqbal]

http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_s07_ms_4.pdf
Pls explain Q.1,b,part 1 , Q.3b, Q.4,c,part 2 and Q.7c.

 

[Mustehssun Iqbal]

Look bro, centripetal acceleration causes body to move in a circle, the angular velocity is ONLY a property of circular motion.



This is the whole problem. It's quiet the opposite! If the angular velocity actually changes and the force remains the same, then the radius would constantly decrease ( F = mw^2r ) which means that the motion will no longer be uniform!

Let's understand this by a simple example, if I tie a ball to the end of a rope and keep moving it in a circle. By your claim, it is impossible to move in a circular motion without the rope being cut. Why? Because radius and mass are constant and the omega "should" be changing thus the force should be increasing! It will increase to a certain point that the rope will not be able to provide the necessary tension. so it will be cut.

This is of course not true...



Whichever motion you're referring to, you'll get the same answer. Let's think about something, if you have a ball moving in a circle with constant angular speed and velocity, and you gradually increase the magnitude of the velocity. What will happen to the angular speed? It will ALSO increase both practically and technically since v = wr. This means that if the speed is constant, then the angular velocity should be also constant, again both practically and technically.

However, to keep moving in a circular motion, a force is needed to cause a constant change in the direction of the velocity. This force causes centripetal acceleration which does NOT change the magnitude of the velocity rather its direction ( By definition ). If there is no change in magnitude, there is no change in angular velocity BUT there is a centripetal acceleration.

If either the velocity or the angular velocity somehow change ( they're actually connected so if one does change then the other will change as well ) .... Then the motion will be distorted and won't be a uniform circular motion... Unless the force varies as well and to some extinct the circular motion will be distorted as the radius would gradually change...

Hope that helped...

thanks for the explanations.

 

[waleedsmz]

constant velocity means no acceleration.
no acceleration caused means no force providing the acceleration.
You might've noticed that the omega of the bus changes constantly.

I believe you're mistakenly describing omega for angular displacement, in the bus ride, the angular displacement changes constantly not the omega... The omega changes in the beginning of the ride when the bus starts to move... After that it becomes constant as far as I'm concerned and thus the angular displacement changes constantly.... Omega is the rate of change of angular displacement after all...

You might have to produce some extra force to tweak the motion so it doesn't look like omega is changing. When you provide a constant centripetal force, omega changes continously throughout its motion. However, I did not say that the wire has to be cut.

The example was pretty straight with some solid concepts so I don't believe that there should be any tweaking to have a circular motion... Please read again why I said the wire will be cut...

Centripetal acceleration is the rate of change of angular velocity. When someone says, angular velocity is constant, how can it then change?? Then how can there be any rate of change of angular velocity?? And in other words, how can there be a centripetal acceleration when there's no rate of change of angular velocity?? There probably can't.

The problem is that the centripetal acceleration is not the rate of change of angular velocity... The centripetal acceleration is the result of the centripetal force, which changes the direction of the velocity not it's magnitude... The definition has pretty much nothing to do with angular velocity... Because if the force is "translatory" then it's simply no longer centripetal as by definition the centripetal force does NOT change the magnitude of a velocity of a body... Google it or check the link that I've provided much earlier...

If it was truly the rate of change of angular velocity, then its rule will be C.A. = w/t ... But it is w^2/r

Back to Kinematic of AS, you might find it helpful to recall acceleration graphs. When there was constant velocity-time graph, there was no sketch on the corresponding acceleration-time graph.

The sketches in the AS have nothing to do with the centripetal acceleration... They're all based in a change of SPEED which is only the magnitude of the velocity... You can't present a circular motion on a Velocity-Time graph as far as I think.

thanks for the explanations.

You're welcome and I hope you trust me on that one since I did study it like that in the physics and the mathematics. : )

 

[zenyatales]

To identify the magnetic field of a wire, you should use fleming's right hand grip rule.

Okay, so the circular field lines are all we need for the magnetic field of a wire ? This I can find with the right-hand-grip rule.

I thought, since there is the attraction effect between the two parallel conducting wires, there must be some North-South things at work.
But couldn't figure out how it is possible to find the norths and souths of CIRCULAR magnetic field lines of the two wires.

Think I get it now.
So, what we need to know is , for the Fleming's left hand rule, whether the circular field is coming out of the page (or going into the page) and place the index finger accordingly ?

Notice that the field of Y is actually coming out of the paper on wire X in it's circular shape.

Thanks, it was very helpful

 

[doc.ahmed]

please help me

can somebody please explain me Q6c m/j 09

 

[hassam]

http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_w03_qp_4.pdf
q1 part c

 

[doc.ahmed]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Physics (9702)/9702_s07_ms_4.pdf
Pls explain Q.1,b,part 1 , Q.3b, Q.4,c,part 2 and Q.7c.

For question number 3b u need to find the area under the graph but just change the unit of x-axis i.e. cm to m . as PD= E*d
For 4c u will draw the graph with same peaks but u will be reducing the ripple voltage as when u increase the resistance the ripple will reduce and voltage gets smoother and smoother

q7c talks about resonance hence you have to draw a peak at 1.o(omega), then a little low at 0.7omega and 1.3 omega almost equal to 0.7

 

[doc.ahmed]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w03_qp_4.pdf
q1 part c

Hey what i think this is the value you calculated is the differnce... this is because in newtons 2nd law of motion Fnet= acc * mass

and over here acc= Fnet/mass

 

[candid24hours]

i have problem in the second part.
weirdly, the answer to this is one channel is required for each bit.
please help!

 

[waleedsmz]

Okay, so the circular field lines are all we need for the magnetic field of a wire ?
Think I get it now.
So, what we need to know is , for the Fleming's left hand rule, whether the circular field is coming out of the page (or going into the page) and place the index finger accordingly ?

Umm This is partially correct. The main idea is to check the direction of the magnetic field of a wire on the other wire.Which is in this case coming out of the paper... Sometimes it may get into the paper. I don't know what the examiner could put in there so I just wanted to give you a heads up.

 

[hassam]

yea anybody got an idea on why a multichannel cable needed,,,,

 

[hassam]

i hope u find this useful
plus
why is UHF USED?in mobile phone signals
ms
. short aerial so easy to handle
short range so less interference between base stations
larger waveband so more carrier frequencies
cn smbody explain the last point

 

[hassam]

Hey what i think this is the value you calculated is the differnce... this is because in newtons 2nd law of motion Fnet= acc * mass

and over here acc= Fnet/mass

there is a problem wid dis explanation cos net force shud be the centripetal force.......(cos centripetal force is not a force on its own)it shud be like Fg-N=Fc........so
Fg-Fc=N ......And N=mg where g is the acceleration of free falll........wat do u say

 

[pratikdahal]

9702/04/M/J/05 question number 6(b)
i got stuck with this one

 

[hassam]

One condition for a satellite to be in geostationary orbit is that the orbit of the
satellite must lie in the plane containing the equator. Explain why this is
necessary?

 

[hassam]

9702/04/M/J/05 question number 6(b)
i got stuck with this one View attachment 7551

flux will graph wwill be in phase with current in primary coil graph...........wats the cnfusion here?