# Rigid body force and torque

1. Nov 29, 2008

### jimbo71

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
I have no idea what the solution to this problem is and would appreciate if someone pointed me in the right direction.

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2. Nov 29, 2008

### horatio89

Re: torque

Consider a rigid body, with a force, F, acting at a point 2R from an arbitrary point on the body, and another force 2F, acting in the same direction as the previous force and a distance R from the aforementioned arbitrary point. Notice that there is a net force of 3F in one direction, while the torque is zero. What does Newton's 2nd Law tell you about the motion of the object?

I will help if you review the conditions for translational and rotational equilibrium.

3. Nov 30, 2008

### jimbo71

Re: torque

The object will accelerate as a whole but not start rotating. Rigid body equilibrium requires vector sum of forces to be zero and sum of torques at any point be zero. Translational equlibrium means the object is either at rest or moving with constant velocity.

4. Nov 30, 2008

### horatio89

Re: torque

Exactly right, so now review the question, if the torque is zero, the object, like in the example cannot be turning, but it may or may not be in translational equil.

5. Nov 30, 2008

### jimbo71

Re: torque

It cannot turn so choice D is out. It may move at constant velocity or may acclerate so choice A and C are out. Choice E is the best choice because while choice B is a true statment is does not best describe all aspects of an object with zero torque.

6. Nov 30, 2008

### horatio89

Re: torque

You are absolutely correct!

7. Nov 30, 2008

### jimbo71

Re: torque

thanks for you help