Physics or Math
posted by Natalie .
A force F = 0.8 i + 2.5 j N is applied at the point x = 3.0 m, y = 0 m. Find the torque about the point x = 1.3 m, y = 2.4 m.
Hmm, (this is essentially just a vector problem) I know you're supposed to find the position vector r and take the cross product of r x F, the force vector, for the torque vector. What I seem to be having trouble with is understanding how to come up with the position vector. So I thought that you're applying the force at point (3, 0, 0), but you want to put the rotation axis at (1.3, 2.4, 0), and thus the position vector r would be the vector "from" the point (1.3, 2.4, 0) "to" the point (3, 0, 0) (instead of just say (3, 0, 0 if it the center was at the origin). I'm pretty sure you can simply subtract: (1.3, 2.4, 0)  (3, 0, 0) = (4.3, 2.4, 0), and that's your position vector r. And so the cross product should be:
r x F =
i j k
(4.3, 2.4, 0)
(0.8, 2.5, 0)
r x F = 0i + 0j + (4.3*2.5  0.8*2.4)k = 12.67k = (0, 0, 12.67)
I'm not sure that this is right though, I tried it in a homework program and it says this is incorrect, any tips, I'm sure I'm just missing some simple thing :(.

Physics or Math 
Jennifer
The torque is about the point (1.3, 2.4), so treat this as your origin,. . .
This makes the r vector (4.3, 2.4, 0) 
Physics or Math 
Natalie
I thought maybe I mixed up the order, and that I should have done (3,0,0)  (1.3,2.4,0), point minus "effective origin" (the rotation axis) which makes more sense, this changes it to 12.67k, is that correct?

Physics or Math 
Natalie
Yay thanks, thought it was simple thank you.
Respond to this Question
Similar Questions

physics torque
1. Find the magnitude of the torque produced by a 3.0 N force applied to a door at a perpendicular distance of 0.25 m from the hinge. 2. A simple pendulum consists of a 3.0 kg point mass hanging at the end of a 2.0 m long light string … 
Physics
A body is pivoted at a Point O. Three forces act on it in the directions shown on the figure. FA = 11 N at point A, 8.0 m from O. FB = 15 N at point B, 4.0 m from O. FC = 20 N at point C, 3.0 m from O. What is the net torque about … 
physics
In the figure, forces A, B, C and D each have magnitude 50.0 N and act at the same point on the object. What torque (magnitude and direction) does A force exert on the object about point P? 
physics
A force, F = (2x + 5y) N, is applied to an object at a point whose position vector with respect to the pivot point is r = (2x + 4y + 5z) m. Calculate the torque created by the force about that pivot point. 
Physics
A force, F = (3x + 4y) N, is applied to an object at a point whose position vector with respect to the pivot point is r = (3x + 3y + 3z) m. Calculate the torque created by the force about that pivot point. I know that torque = 12x … 
physics
I am studying for a physics exam and this is the pretest. There are some that I don't understand. If someone could help me get to the answer (show all the work so I can look it all over) that would be AMAZING! Thank you very much! … 
physics
A force in the +y direction applied at the point x=2.3m,y=1.4m gives rise to a torque of 39n*m about the origin. Find the magnitude of the force. 
College Physics
A force, F = (2x + 3y) N, is applied to an object at a point whose position vector with respect to the pivot point is r = (5x + 4y + 2z) m. Calculate the torque (in units on Nm) created by the force about that pivot point. 
Physics
A 49N force is applied perpendicular to the portion BC of the bent bar. Determine the moment of P about point B and about point A. The image shows a bar that is straight from point A to B but then bends at a 42 degree angle to the … 
Physics
A force, F = (6x̂ + 7y hat) N, is applied to an object at a point whose position vector with respect to the pivot point is r = (7x̂ + 7y hat + 7ẑ) m. Calculate the torque created by the force about that pivot point.