# Uniform circular motion

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1. Aug 7, 2015

### faiziqb12

1. The problem statement, all variables and given/known data
i was going through uniform circular but a confusion popped out of my head .
as i have read before the Rutherford atomic model was only rejected due to
the fact that electrons moving in circular orbits need to accelerate constantly
to maintain a uniform speed but the acceleration causes electrons to radiate
the energy and they would eventually lose all their energy and fall into the
nucleus ..... so my question is in uniform circular motion there is a constant
acceleration which keeps a uniform speed..is it centripretal acceleration?
how?............ and how to calculate the magnitude and direction of this
acceleration? ...... and is it a constant?

2. Aug 7, 2015

3. Aug 7, 2015

### faiziqb12

i am empty
i was trying to add up the the gravitation,friction and centripretal acceleration vector
but it proved of no use

4. Aug 7, 2015

### faiziqb12

i have ran out of my mind
and here are no websites that can help me

5. Aug 8, 2015

### Qwertywerty

What is centripetal acceleration ? What causes it ?

6. Aug 8, 2015

### faiziqb12

NO MY QUESTION ACTUALL IS THE SAME AS I HAVE POSTED
the centripetal force just allows a body to be fixed in a circular orbit but what causes the object to keep a fixed velocity in that circular orbit

7. Aug 8, 2015

### Qwertywerty

Is the velocity constant ? Does uniform circular motion maintain constant speed , or constant velocity ?

8. Aug 8, 2015

### faiziqb12

oh sorry constant speed

9. Aug 8, 2015

### Qwertywerty

Okay , so it is now clear that velocity of the object is changing .

Now , for constant mass systems , if velocity of object is changing , then what does that say about object's acceleration - is it zero ?

10. Aug 8, 2015

### faiziqb12

yes

11. Aug 8, 2015

### Qwertywerty

No , it isn't !

What causes change in velocity ?

12. Aug 8, 2015

### faiziqb12

What causes change in velocity ?
now you come on the question
i can see two types of acceleration
one that keeps the object the object in orbit{centripedal acceleration}

my question is about the second one ..that one which keeps a uniform speed or say a uniform magnitude of velocity

13. Aug 8, 2015

### faiziqb12

you are right

14. Aug 8, 2015

### Qwertywerty

Consider a pebble tied to a string , rotated about a fixed point . As it starts to spin , tension acts as a centripetal force , consequently causing an acceleration - which is the centripetal acceleration .

This acceleration is responsible for circular motion . For uniform circular motion , this is the pebble's only acceleration .
It causes change in velocity of the pebble . As this is perpendicular to initial velocity of pebble , it only causes a change in the pebble's direction of motion of the pebble , working like this throughout the circular motion .

Thus , my question to you is this - how is the acceleration changing the velocity ( we have already seen that velocity is changing ) ?
Also , how is velocity ( a vector ) related to speed ( a scalar ) ?

15. Aug 8, 2015

### faiziqb12

THE centripetal force is only responsible for keeping the body in orbit not for increasing,decreasing or maintaining its speed.
in uniform circular motion there is constant speed due to the force applied by us repeatedly....and this force is the second type of acceleration
now my question is how can i calulate the amount of such acceleration needed to maintain uniformity of speed
i hope you would have understood my question now

16. Aug 8, 2015

### Qwertywerty

Please understand , I have got your question . What my problem is that you seem to be making mistakes in your basic concepts .

17. Aug 8, 2015

### faiziqb12

velocity has direction along with magnitude so it is a vector and speed has only a magnitude so it is a scalar
also acceleration is changing the velocity by changing its direction

18. Aug 8, 2015

### Qwertywerty

So , in the entire motion , only direction of the velocity is changing , and not it's magnitude . So if it's magnitude isn't changing , speed isn't changing .

So , to finally answer your question , no acceleration is causing uniform speed . Speed was never changing , and hence no acceleration was ever required to do so .

19. Aug 8, 2015

### faiziqb12

what you wanna say is if we stop rotating the string in the example , but not let the string go.then the ball will again be in uniform motion.

no, it wont

we revolve our hands to give the desired force always to cancel the effect of gravitational force and the frictional force in the example given

and this force is actually the another accleration i was talking about

20. Aug 8, 2015

### Qwertywerty

???

How exactly would you do this ?