# Homework Help: Centripetal force and velocity help

1. Dec 20, 2008

### physicsman2

centripetal force (urgent)

1. The problem statement, all variables and given/known data
what should a centripetal force vs. velocity(speed) graph look like.
what should a velocity(speed) vs. radius(of the circle) look like.
what should a velocity(speed) vs. stopper mass graph look like.

2. Relevant equations
To find centripetal force, i mulitiplied the mass of the string by 9.80m/s^2
To find velocity, i divided the circumference by the period

The centripetal force in the first trial was (.1 kg)*9.80, second trial was (.2 kg)*9.80, third trial was (.3 kg)*9.80, the fourth trial (.4 kg)*9.80

3. The attempt at a solution
i am supposed to use pwreq for the first graph and pwrReg for the other two on my TI 84 to get the shape but they don't seem right.
For me, the first one was an upside down parabola, the second and third a straight line

Thank you for the help

2. Dec 20, 2008

### rock.freak667

Re: centripetal force (urgent)

Where were these values obtained?

But if the centripetal force,Fc is related to velocity,v, by the formula:

$$F_c =\frac{mv^2}{r} \Rightarrow F_c= (\frac{m}{r})v^2$$

Assuming m and r are constants, then Fc=kv2, which is a simple parabola for you to plot.

3. Dec 20, 2008

### physicsman2

Re: centripetal force (urgent)

i am supposed to find the centripetal force and the velocity in a lab using information i got.
the centripetal force was the mass of the string multiplied by 9.80m/s^2(i have no idea why that is what our teacher gave us) and we were asked to make graphs for homework

i hope this helps a little, i just need to know the relationship illustrated in the three graphs and what shape each graph will be. i'm still trying to figure it out

thank you

4. Dec 20, 2008

### PhanthomJay

Re: centripetal force (urgent)

rock.freak has given you the general equation for centripetal force. For part 1, with the mass and radius held constant, you get a parabolic curve when plotting F vs. V, as noted, when you rotate the stopper at various speeds. For part 2, with the the mass and force held constant, use the same centripetal force equation , and determine the type curve you should get, when rotating the stopper with a changing radius (by slowly lowering the mass) . Same again for part 3, only this time, the radius is constant, and you use different masses (which changes also the applied force, the(mass times 9.8) weight you are inquiring about, and plot the curve. You must see how closely your experimental results agree with the theoretical results.

5. Dec 20, 2008

### physicsman2

Re: centripetal force (urgent)

thank you very much