Say you had a ball on an elastic piece of string. If you swung it round in a circle then the elastic would stretch and the radius of the circle would increase.

http://en.wikipedia.org/wiki/Centripetal_force

The centripetal force goes towards the centre so what force is causing the ball to move away from the centre?
Centrifugal force?
The force causing it to move away from the center is its momentum. So if the centripedal force dissapeared, it would just continue in a straight line away from where it was.
Last edited by SmarterChild at Apr 3, 2008,
Quote by Gaz_m2k5
Say you had a ball on an elastic piece of string. If you swung it round in a circle then the elastic would stretch and the radius of the circle would increase.

http://en.wikipedia.org/wiki/Centripetal_force

The centripetal force goes towards the centre so what force is causing the ball to move away from the centre?

Acceleration, momentum, inertia...
Quote by SmarterChild
The force causing it to move away from the center is its velocity. So if the centrifugal force dissapeared, it would just continue in a straight line away from where it was.

Wrong. Velocity =/= Force
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Wrong. Velocity =/= Force

I know. I was just using it to explain it, and because the suitable term wasn't coming to mind.
Okay so on a free body diagram of the ball, what forces are there....
I got;
The angular acceleration in a curved path
The centripetal force towards the centre
and the centrifugal force.

Is that it?
Quote by denizenz
The thread starter just asked the question poorly.

He is right...

No.

F = ma

velocity is only a partial component of a
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Quote by gen0doom
Centrifugal force?

centripedal "sp?" force doesn't exactly exist. when you let go of the ball on a string, it flies in a straight line from where it is released [zero gravity]. technically its all line motion opposed to circular; the line only changes because of the elastic band.
Don'y bother mentioning centrifugal force it doesn't exist, centripetal force creates the centrifugal effect
Quote by Gaz_m2k5
Okay so on a free body diagram of the ball, what forces are there....
I got;
The angular acceleration in a curved path
The centripetal force towards the centre
and the centrifugal force.

Is that it?
Be careful.
The path is curved, so you only concern yourself with instantaneous acceleration.
At each tiny increment of time, the direction changes
But the vectors always point directly at the central axis.
Read the article about centripetal force carefully.
Read the article about centrifugal force MORE carefully.
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Quote by gen0doom
Centrifugal force?

No such thing.

Its the ball trying to travel in a straight line, but the string is forcing it round in a circle preventing it from travelling straight on
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1) there is no such thing as centrifugal force.
2) a free body diagram won't work because the ball isn't in equillibrium.
No such thing.

Its the ball trying to travel in a straight line, but the string is forcing it round in a circle preventing it from travelling straight on

If there's no such thing, then what is causing the string to extend?
the velocity of the ball, at any given time the ball is trying to go in a straight line, but is unable to as its tethered
well, the ball will move in a straight line as far as it can get, until the string will pull it back and there is a balance between the elesticity of the string and the force the speed of the ball makes by going straight ahead
Quote by SRVNumberOne
1) there is no such thing as centrifugal force.

you are completely wrong.

Centrifugal Force = noun: Physics; an apparent force that acts outward on a body moving around a centre, arising from the body's inertia.

centrifugal force is the opposite of centripetal force.

the hand holding the string is the centre. the forces acting away from the center are: gravity, the force applied by the hand that makes the string swing, and the ball's mass.

the forces acting towards the centre (i.e. the forces that stop the ball from flying away) are: tension in the string (although if it is a rubber band then its tension will vary in relation to how forcefully you swing the ball), and air resistance.
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Quote by Gaz_m2k5
If there's no such thing, then what is causing the string to extend?

There's two types of centrifugal force, one is an illusion, but there is the reactive centrifugal force, which is opposite to the centripedal force and stops it spiralling inwards. The other linear momentum of the object itself, which is constantly pulling it away from the centre. I'd guess that this is causing the string to extend, but I'm not certain. Does the band string extend more the faster the object is the travelling?
If you swing it around, your arm is giving it a force.
This force is the cause of the acceleration, so also the velocity, of the ball.
=> that's why the ball moves (duh)
The centripetal force on the ball makes the acceleration point towards the middle point of the circle, this happens in every single point of the circle the ball is making.
Because the ball points in a different direction in all points of the circle, the direction of the acceleration is different in all points of the circle. =>like someoneyouknow said, the vectors always point at the central axis.
This causes the ball to have a circle-shaped movement, otherwise it'd just travel in a straight line.
The force causing the ball to move away from the centre would be the force your arm is giving to the ball. As long as that force is smaller than gravity, the ball will just hobble around a bit. However, if you increase the force the ball will start accelerating (the process mentioned above takes place). The bigger the force you're giving to the ball, the more the ball will accelerate and move away from you (considering the place where your hand holds the string is the middle point of the circle). You just 'throw' the ball away harder, which causes him to travel further away from the middle point. So if it's an elastic string, it'll stretch. If you increase the force even more, the chances are that the string will break if it can't whistand that amount of force.

Hopefully this cleared it up a bit
Quote by Fzau
If you swing it around, your arm is giving it a force.
This force is the cause of the acceleration, so also the velocity, of the ball.
=> that's why the ball moves (duh)
The centripetal force on the ball makes the acceleration point towards the middle point of the circle, this happens in every single point of the circle the ball is making.
Because the ball points in a different direction in all points of the circle, the direction of the acceleration is different in all points of the circle. =>like someoneyouknow said, the vectors always point at the central axis.
This causes the ball to have a circle-shaped movement, otherwise it'd just travel in a straight line.
The force causing the ball to move away from the centre would be the force your arm is giving to the ball. As long as that force is smaller than gravity, the ball will just hobble around a bit. However, if you increase the force the ball will start accelerating (the process mentioned above takes place). The bigger the force you're giving to the ball, the more the ball will accelerate and move away from you (considering the place where your hand holds the string is the middle point of the circle). You just 'throw' the ball away harder, which causes him to travel further away from the middle point. So if it's an elastic string, it'll stretch. If you increase the force even more, the chances are that the string will break if it can't whistand that amount of force.

Hopefully this cleared it up a bit

So what would I label this force on the diagram!
on the diagram you should label the centripetal force and the tangential force. so that's atan and arad the FBD won't balance because the system isn't in equillibrium. is this for a physics class or a strength of materials class?
It's for "Machine Dynamics" module. I was never taught anything about an FBD having to be in equilibrium by any of my phyics/maths teacheers since AS level.