# Acceleration in uniform circular motion is uniform or non-uniform?

• Darshit Sharma
Darshit Sharma
Homework Statement
Centripetal acceleration in uniform circular motion is uniform or non-uniform?
Which kind of force acts on the hand and its direction?
Relevant Equations
No equations
I had an exam ques which was as follows:

The first part is clear to me.....it is uniform (or constant) speed.

I am in doubt on the account of the second part as the answer key says this:

So the overall question concerning the second part is as follows:

We know that the direction of centripetal acceleration is ever-changing and since acceleration is a vector quantity we must conclude that the acceleration is non-uniform but the answer keys says the contrary.

Please don't give answers which are vague in this context like "It is uniform in magnitude but not non-uniform in direction." I truly understand it may be the case which is true but what I am seeing here is the answer to this question which has just Yes No option.

Now for the third part,
I believe that centrifugal force should not be the answer because centrifugal force acts on the body undergoing the circular motion, hence it should be the stone on which both centripetal and centrifugal forces are acting.

So does anyone have any clue about which force could be acting on the hand and its direction?

Another source (my textbook) says:
"Reaction of tension away from the centre of the circular path"

But isn't this same ass the centrifugal force? Doesn't all this contradict "centrifugal force acts on the body undergoing the circular motion"? And if not, then which force provides for the centrifugal force?

Darshit Sharma said:
what I am seeing here is the answer to this question which has just Yes No option.

Hill said:
Okay sir
And the force on hand?

Darshit Sharma said:
Okay sir
And the force on hand?
Centrifugal.

Darshit Sharma said:
Please don't give answers which are vague in this context like "It is uniform in magnitude but not non-uniform in direction." I truly understand it may be the case which is true but what I am seeing here is the answer to this question which has just Yes No option.
What is your understanding of "uniform acceleration?" How would you know it if you saw it? If you don't know, look it up. There are plenty of examples on the internet which will enable you to answer your own question.

By the way, if you found this problem here, the so called answers to the question are shown below. The answer to (ii) is wrong and maliciously misleading.

Darshit Sharma said:
I believe that centrifugal force should not be the answer because centrifugal force acts on the body undergoing the circular motion, hence it should be the stone on which both centripetal and centrifugal forces are acting.
That's nonsense. The centripetal and centrifugal forces have equal magnitudes and opposite directions. If both acted on the stone, the net force on it would be zero and the stone would move in a straight line.

Darshit Sharma said:
Please don't give answers which are vague in this context like "It is uniform in magnitude but not non-uniform in direction."
"Vague"? I think you meant to write "Please don't give answers which are correct in this context"...

kuruman
berkeman said:
"Vague"? I think you meant to write "Please don't give answers which are correct in this context"...
Sorry for my english skills.

No need to say sorry. I'm just saying that the answer key looks correct to me.

kuruman said:
That's nonsense. The centripetal and centrifugal forces have equal magnitudes and opposite directions. If both acted on the stone, the net force on it would be zero and the stone would move in a straight line.
But if we are in a non-inertial reference frame, then no net force acts on the body right, it seems as if it were in rest, isn't it?

Moreover my textbook says that centrifugal force act on the same body.....and as it has same magnitude diff direction it counterbalances the centripetal force....

berkeman said:
No need to say sorry. I'm just saying that the answer key looks correct to me.
Sorry to be in such a hurry and for such hopelessness. I just chilled on 4 days of holiday and I'm having the physics exam right after 2h. So much chaos......

But sir @kuruman says something different.....
And I too feel that he is correct because I also don't see the acceleration as uniform beacuse the direction is changing

Darshit Sharma said:
And I too feel that he is correct because I also don't see the acceleration as uniform beacuse the direction is changing
Acceleration would usually be a vector, but I suppose it could just be the magnitude of the accleration. It depends on how the question is asked.

berkeman said:
Acceleration would usually be a vector, but I suppose it could just be the magnitude of the accleration. It depends on how the question is asked.
The verbatim question is presented before you sir, what do you think would be the most apt answer....Yes or No? Yes?
and @kuruman yours, sir?

Part (ii) is questionable. Uniform acceleration implies motion in a straight line and equal changes in velocity in equal times just like uniform velocity is motion in a straight line and implies equal changes in position in equal times.

What we have here is uniform circular motion which implies motion in a circle of constant radius at constant speed.

SammyS
Hill said:
Centrifugal.
I've attached some pictures in #9 sir..please have a look

Darshit Sharma said:
I've attached some pictures sir..please have a look
Where are they attached?

kuruman said:
Part (ii) is questionable. Uniform acceleration implies motion in a straight line and equal changes in velocity in equal times just like uniform velocity is motion in a straight line and implies equal changes in position in equal times.

What we have here is uniform circular motion which implies motion in a circle of constant radius at constant speed.
Yes but as Velocity is non-uniform (my textbook says this) on the only basis of its direction......we can confidently say that acceleration is non-uniform on the very basis of its direction
(which my textbook doesn't say, it neither say uniform nor non-uniform but the question was there in the test 10 years ago)

kuruman said:
Where are they attached?
in #9

I am so much thankful to all of you for answering to such ill-formed messages and replies @kuruman @berkeman @Hill Let us reach a single final answer to (ii) and (iii) with plausible explanation.

Darshit Sharma said:
I am so much thankful to all of you for answering to such ill-formed messages and replies @kuruman @berkeman @Hill Let us reach a single final answer to (ii) and (iii) with plausible explanation.
You have the evidence before you. What is your conclusion?

Darshit Sharma said:
in #9
Yes, I see them now but I cannot read sideways.

kuruman said:
You have the evidence before you. What is your conclusion?
ii) Non-uniform

I am not sure about (iii) yet as sir you are saying that centrifugal force doen't acts on the stone, @Hill is saying that it does so and the textbook also says.....

Perplexed..

Darshit Sharma said:
we must conclude that the acceleration is non-uniform
Agreed.
Darshit Sharma said:
centrifugal force should not be the answer because centrifugal force acts on the body undergoing the circular motion, hence it should be the stone on which both centripetal and centrifugal forces are acting.
The force thats on the hand is clearly the tension in the rope.
Not sure that you meant both centripetal and centrifugal forces act on the stone at the same time. More likely, I hope, you meant that one or the other acts, depending on the reference frame.

But in my view that is not right either.
Centripetal force is a component of the net of all the forces that act on the body. Specifically, it is the component normal to the velocity. It is not another force acting on the body.
Centrifugal force is a force that acts on the body, so contributes to the net force. It is a force we have to invent when using a noninertial frame.
If we choose a frame fixed in relation to the body then in that frame there is no acceleration. The centrifugal force is necessarily equal and opposite to the net of all other forces.
What is under appreciated is that we could choose some other noninertial frame. In this case we can have a centrifugal force contribution and a nonzero centripetal resultant.

kuruman said:
Yes, I see them now but I cannot read sideways.
You mean you can't read in the orientation pictures are sent or you want me to send the pictures of the whole page?

Darshit Sharma said:
You mean you can't read in the orientation pictures are sent or you want me to send the pictures of the whole page?
That's OK. Please don't bother reposting them.

Darshit Sharma
haruspex said:
What is under appreciated is that we could choose some other noninertial frame. In this case we can have a centrifugal force contribution and a nonzero centripetal resultant.
Haha, I'' surely study it after my exams....you guys are making me love physics...
haruspex said:
The force thats on the hand is clearly the tension in the rope.

Tension is acting as the "component normal to the velocity," i.e. centripetal force and we know that centripetal force acts on the body undergoing the motion....so how can tension act on hand.....you mean to say reaction of tension?

kuruman said:
That's OK.

So sir....@haruspex and my textbook are of the view that centrifugal force acts on the body.....so sir what do you say now which force acts on the hand, it it centrifugal force? or is it the reaction force of the tension or something just completely out of the box?

@berkeman Sir do you still stand still with the answer (iii) in the answer key?

Darshit Sharma said:
Haha, I'' surely study it after my exams....you guys are making me love physics...

Tension is acting as the "component normal to the velocity," i.e. centripetal force and we know that centripetal force acts on the body undergoing the motion....so how can tension act on hand.....you mean to say reaction of tension?
haruspex said:
Centrifugal force is a force that acts on the body, so contributes to the net force. It is a force we have to invent when using a noninertial frame.
What I recalled just now sir beacuse of you is that there is only one case where we assume centrifugal force, i.e. in a non-inertial frame....so this removes any further possibility of two cases for (iii).....let's see what is the final single correct answer for iii

I would really be grateful if we could arrive the final answer for iii within 25-30 minutes. I know it might be late night somewhere in the world....but this act of generosity would surely be venerated.

Darshit Sharma said:
Haha, I'' surely study it after my exams....you guys are making me love physics...

Tension is acting as the "component normal to the velocity," i.e. centripetal force and we know that centripetal force acts on the body undergoing the motion....so how can tension act on hand.....you mean to say reaction of tension?
Tension in a rope acts on whatever is attached at each end. If the rope is massless or not accelerating, and not subject to other forces along its length, then the two exerted forces are equal and opposite.

I
Darshit Sharma said:
I would really be grateful if we could arrive the final answer for iii within 25-30 minutes. I know it might be late night somewhere in the world....but this act of generosity would surely be venerated.
I gave you my final answer.
If you insist on choosing between centripetal and centrifugal (or both), you have to define your frame first.

haruspex said:
Tension in a rope acts on whatever is attached at each end. If the rope is massless or not accelerating, and not subject to other forces along its length, then the two exerted forces are equal and opposite.
Is there anything known as "Reaction force of tension" If yes then it acts on what in this case?

haruspex said:
I gave you my final answer.
If you insist on choosing between centripetal and centrifugal (or both), you have to define your frame first.
No sir...I ain't insisting on anything....
Let me confirm my understanding of your final answer for the force on the hand"

Inertial reference frame: "Tension"
Non-inertial reference frame: "Tension"

If I am not wrong this is what you mean to convey sir, right? And where is the reaction of tension force?

Darshit Sharma said:
Is there anything known as "Reaction force of tension" If yes then it acts on what in this case?
Tension (likewise compression) is not exactly a force. It is more like pairs of equal and opposite forces acting all along the rope. Only at the two ends, where there is no "pair", does it manifest as a force.
The reaction to that is the force exerted by the object attached to it. But you are not asked what force the hand exerts; you are asked what force is exerted on the hand.

haruspex said:
Tension (likewise compression) is not exactly a force. It is more like pairs of equal and opposite forces acting all along the rope. Only at the two ends, where there is no "pair", does it manifest as a force.
The reaction to that is the force exerted by the object attached to it. But you are not asked what force the hand exerts; you are asked what force is exerted on the hand.
Ohk sir.........So the hand's exerted force and the rotating stone's weight are the reaction to tension.

So, here we are finally concluding the answer as "Tension."

I had one last doubt, It appears from the diagram I drew that "Tension" is not only the centripetal force here but also the centrifugal force, is it right? and the this "tension" that is in my diagram should exist in inertial reference frame also......so centrifugal force in inertial refernce frame, it doesn't seems to make any sense?

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