# physics...

• Sep 28th 2008, 03:19 PM
sarabolha
physics...
http://img.photobucket.com/albums/v6...Untitled-1.jpg

The diagram shows a small ball of mass m=0.280kg that is attached to a rotating vertical bar by means of two massless strings of equal lengths L=1.10m. The distance between the points where the strings are attached to the bar is D=1.56m. The rotational speed of the bar is such that both strings are taut and the ball moves in a horizontal circular path of radius R at constant speed v=3.65m/s.

(a) Determine the magnitude and direction of the net force that is acting on the ball when it is in the position shown in the diagram.
(b) Determine the magnitude of the tension T1 in the upper string and the magnitude of the tension T2 in the lower string.
(c) Determine the speed of the ball at which the tension T2 becomes zero.

i'm not sure how to go about solving this problem, i've never done anything like it...help?
• Sep 28th 2008, 03:34 PM
skeeter
(a) net force is the centripetal force ...

$\displaystyle F_{net} = F_c = \frac{mv^2}{R}$

(b) two equations ...

as stated in part (a), $\displaystyle F_{net} = F_c$
$\displaystyle F_c = T_1 \sin{\theta} + T_2 \sin{\theta}$

Forces in the vertical direction are in equilibrium
$\displaystyle T_1 \cos{\theta} = T_2 \cos{\theta} + mg$

solve the system for $\displaystyle T_1$ and $\displaystyle T_2$

(c) The system becomes a conical pendulum.
$\displaystyle T_1 \sin{\theta}$ will be providing $\displaystyle F_c$ ... also, $\displaystyle T_1 \cos{\theta} = mg$
• Sep 28th 2008, 04:48 PM
sarabolha
i'm not quite sure what you mean for part c? the rest was really helpful though
• Sep 28th 2008, 06:15 PM
skeeter
(c) when $\displaystyle T_2 = 0$, only $\displaystyle T_1$ and the force of gravity ($\displaystyle mg$) act on the mass. The horizontal component of $\displaystyle T_1$ provides the centripetal force and the vertical component of $\displaystyle T_1$ counteracts the weight of the mass.