Centripetal force throughout a vertical circle

[Matthew_B]

Homework Statement

In what position in vertical circular motion is the centripetal force the greatest?
Top, Bottom, Left, or Right

Homework Equations

Can someone explain how Fc is greatest at the top?

The Attempt at a Solution

I had reasoned that since centripetal acceleration which I will assign to a is equal to v^2/r and as a result of the velocity being greatest at the bottom the centripetal acceleration (a) will also be the greatest at the bottom
Since the formula for centripetal force is Fc= ma and the mass remains constant I said that Fc will be the greatest where the centripetal acceleration (a) will be the greatest which is the bottom.
When I received my test back I was told that the centripetal force is greatest at the top rather than the bottom can someone please explain?

 

[Orodruin]

The question is ill posed as it is not stated what velocity the circular motion has. If it is just subject to gravity and a constraining force, the correct answer is at the bottom.

One could also ask where the constraining force is the greatest (with constant speed or with speed given by motion in a gravitational field - in both cases the answer is at the bottom).

 

[Matthew_B]

The question is ill posed as it is not stated what velocity the circular motion has. If it is just subject to gravity and a constraining force, the correct answer is at the bottom.

One could also ask where the constraining force is the greatest (with constant speed or with speed given by motion in a gravitational field - in both cases the answer is at the bottom).

The problem from the test had no mention of gravity or the speed at any of the given points, however I assumed that gravity is present and the velocity is changing since the that is the type of problems we have covered in my class. Do you think then that it was indeed a problem with the key? Thank you for responding to my post also.

 

[PeroK]

The problem from the test had no mention of gravity or the speed at any of the given points, however I assumed that gravity is present and the velocity is changing since the that is the type of problems we have covered in my class. Do you think then that it was indeed a problem with the key? Thank you for responding to my post also.

I wouldn't worry about it. Whoever set the test probably got their calculations mixed up.

 

[Orodruin]

My point is, regardless of whatever reasonable assumption you make about the velocity, the centripetal force will be greater at the bottom. The answer provided by your teacher is then simply wrong.

 

[Matthew_B]

My point is, regardless of whatever reasonable assumption you make about the velocity, the centripetal force will be greater at the bottom. The answer provided by your teacher is then simply wrong.

Aright thanks for the reply

 

[CWatters]

Hang on a moment. I think you are confusing the centripetal force with the tension in the string. The centripetal force is always constant. The tension in the string will vary but not the centripetal force.

The centripetal force Fc is always mv^2/r that's made up of the vector sum of the tension in the string and gravity.

 

[CWatters]

http://www.schoolphysics.co.uk/age1...n/text/Motion_in_a_vertical_circle/index.html

 

[Orodruin]

Hang on a moment. I think you are confusing the centripetal force with the tension in the string. The centripetal force is always constant. The tension in the string will vary but not the centripetal force.

The centripetal force Fc is always mv^2/r that's made up of the vector sum of the tension in the string and gravity.

He is not confusing them. You are making the assumption that the motion is with constant speed, which clearly the OP does not.

 

[CWatters]

He is not confusing them. You are making the assumption that the motion is with constant speed, which clearly the OP does not.

You're right I missed that.