# Homework Help: Arrrgh help

1. Dec 1, 2005

### IonAphis

Arrrgh help!!

There are 3 particles all with the same mass connected by rods to a central point all making 120 degrees with each other. All the rods are conected (kinda like making a triangle with little balls on the corners). So this thing is sitting in a plane, with no friction, just smooth surface. If I apply a force perpendicular to one of these little arms (rods) the body will start a free spin, and up to that point I understand. What I don't know is: after I apply the force, does the body displace (it's center of mass) on a straight line? or does applying force into one of the rods make it not only spin but also vector out? If so, how do I calculate this vector...
What you usually think it should do is displace, but wouldn't that be thanks to the friction? So really, if there is no friction, I should go on a straing line right?
Ion Aphis

2. Dec 1, 2005

### Staff: Mentor

The applied force will cause the object to both translate and rotate:
(1) It will accelerate the center of mass of the object (balls + rods) per Newton's 2nd law: $\vec{F} = m \vec{a}$. Thus the center of mass will accelerate in the direction of the force. (If you maintain the force in a constant direction, it will move in a straight line.)

(2) It will exert a torque about the center of mass, imparting an angular acceleration per Newton's 2nd law for rotation: $\tau = I \alpha$​