Register to reply 
Bar with masses on ends 
Share this thread: 
#1
Oct611, 06:05 PM

P: 10

1. The problem statement, all variables and given/known data
A machine part consists of a thin 35.1cmlong bar with small 1.81kg masses fastened by screws to its ends. The screws can support a maximum force of 70.3 N without pulling out. This bar rotates about an axis perpendicular to it at its center. 2. Relevant equations Newton's 2nd law, a_rad=v^2/r 3. The attempt at a solution (What am I doing wrong?) The centripetal acceleration is v^{2}/r=v^{2}/0.351 m/s^2, so the net force on each mass is 1.81v^2/0.351 N ≤ 70.3 N Solving we get v ≤ ~3.69226 m/s, so the largest v is that. However, I'm being told this is wrong. However, the same problem with very similar numbers but different numbers had an answer of ~3.61 which is pretty close. 


#2
Oct611, 07:39 PM

P: 2

Are you sure centripetal force is the right force?
Gravity is attempting to pull the weight down, which would turn the bar. If the bar turned, the weight would get closer to the axis of rotation. Is there a force that might be against the weight getting any closer to the axis of rotation? ;) 


#3
Oct611, 07:47 PM

Emeritus
Sci Advisor
HW Helper
PF Gold
P: 7,794

usamo42j,
What you have done seems to be correct, that is, if you were asked the maximum speed of the masses. Are you sure it doesn't ask for the maximum rotational speed (angular velocity) ? 


Register to reply 
Related Discussions  
Sticky ends and blunt ends in DNA cloning?  Biology  14  
A rod constrained at it's ends  Advanced Physics Homework  8  
Lorentz transformed pressure for a rod with two masses at its ends  Special & General Relativity  6  
Your hole ends here  General Discussion  20  
At wits ends please someone help  Introductory Physics Homework  4 