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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: run-bandit on February 11, 2010, 10:08:31 pm

Title: centripetal and centrifugal forces
Post by: run-bandit on February 11, 2010, 10:08:31 pm: with central petal force why don't things fall into the centre?
i can understand for a short time as it would work but the net force would eventually cause it to come in right???
why does it keep going round then!

and whatever happened to centrefugal force that I used to hear about..
Title: Re: centripetal and centrifugal forces
Post by: appianway on February 11, 2010, 10:11:36 pm: Centrifugal is only from the frame of reference experiencing the motion due to inertia. The centripetal force changes the direction of the velocity, but not the magnitude = velocity's a vector, and F/m = a = dv/dt, and hence a change in direction and not magnitude can occur when a force acts.
Title: Re: centripetal and centrifugal forces
Post by: superflya on February 11, 2010, 10:20:40 pm: centrifugal force is the reaction force of the centripetal force. centripetal= towards centre, centrifugal= originate from centre and move out.
Title: Re: centripetal and centrifugal forces
Post by: appianway on February 11, 2010, 10:23:05 pm: It's not the reaction force, as both the centripetal and the centrifugal are on the same body. The reaction force of the centripetal force would be the force that the object accelerating (ie moving in a circle) exerts on what makes it do this. An example of this would be the gravitational force that the moon exerts on the earth.

When you think of the centrifugal force, think of inertia.
Title: Re: centripetal and centrifugal forces
Post by: superflya on February 11, 2010, 10:26:57 pm: holds true for a non-inertial reference frame :P
Title: Re: centripetal and centrifugal forces
Post by: TrueTears on February 11, 2010, 10:27:13 pm: superflya u pr0 kent
Title: Re: centripetal and centrifugal forces
Post by: appianway on February 11, 2010, 10:37:33 pm: In that case, the centrifugal force would refer to the force exerted on the object at which it's rotating around (ie on the moon) or, well, the opposite object to which is being considered. It's not really the scenario which was inferred in which the individuals in the reference frame experience a pseudoforce due to inertia - it's really just the force that they exert on whatever's causing them to rotate... it's a different definition.

In regards to you, run-bandit, it won't affect the mass, but it will provide a constant acceleration which changes the direction and not the magnitude of the velocity. dv/dt is just the rate of change of velocity over time.
Title: Re: centripetal and centrifugal forces
Post by: QuantumJG on February 11, 2010, 11:33:12 pm: Quote from: run-bandit on February 11, 2010, 10:08:31 pm
with central petal force why don't things fall into the centre?
i can understand for a short time as it would work but the net force would eventually cause it to come in right???
why does it keep going round then!

and whatever happened to centrefugal force that I used to hear about..

With centripal force let's look at the formula:

$F_{cent} = \dfrac{m v^{2} }{r}$

Let's look at Earth, and we launch a rocket at speed v off a wall at height r (above the centre).

$v = \sqrt{r a_{cent} }$

now if:

$v < \sqrt{r a_{cent} }$ - The object will fall as a projectile.

$v = \sqrt{r a_{cent} }$ - The object will orbit the earth.

$v > \sqrt{r a_{cent} }$ - The object will fly off into space.

I hope this explaination kind of helps.

Centrifugal force:

Let's first look at something different, a person is in an accelerating car (just moving in a straight line) and drops a tennis ball and in their frame they see the ball 'accelerate' back, but to someone from the outside the ball is just stationary and the car is moving.

This is basically similar to the centrifugal force on a ball droped in say a car turning a corner. The person in the car will think the ball is being pushed out, but to someone from outside the ball is stationary and the car is moving away.

The centrifugal force is basically from looking at physics in non-inertial frames where Newtons laws don't actually work. The centrifugal force is called a ficticious force, because there is no actual medium providing a force on it.