Need help with a seesaw torque question.

[paperdoll]

Homework Statement

Halle and Susan, each of mass 40 kg are sitting at opposite ends of a playground seesaw. The seesaw is stationary in the horizontal position and is 4.50 m long. Susan decides to jump off. Calculate the size of the unbalanced torque that now acts on the seesaw.

Homework Equations

torque= F * r (perpendicular distance?)
F=ma

The Attempt at a Solution

I'm not sure how to find the perpendicular distance from the line of action of the force to the axis of rotation. I think F would be 40*9.8=392 N but I don't know how to find the "r" value.

 

[Doc Al]

I'm not sure how to find the perpendicular distance from the line of action of the force to the axis of rotation. I think F would be 40*9.8=392 N but I don't know how to find the "r" value.

How far is she from the pivot of the seesaw? (You're given the full length of the seesaw.)

 

[paperdoll]

How far is she from the pivot of the seesaw? (You're given the full length of the seesaw.)

It doesn't state the value in the question so I guess you are supposed to assume that she is half way. So 4.5/2=2.25 m

 

[Doc Al]

It doesn't state the value in the question so I guess you are supposed to assume that she is half way. So 4.5/2=2.25 m

Yes, that's what I would assume. The axis of the seesaw is in the middle and she is sitting at one end.

 

[asteriatic]

To find the perpendicular distance, also known as the lever arm, you need to measure the distance from the pivot point (axis of rotation) to the line of action of the force. In this case, the pivot point is the center of the seesaw and the line of action of the force is where Susan's weight is applied. Since the seesaw is 4.50 m long, the distance from the pivot point to the line of action of the force would be half of that, or 2.25 m. Therefore, the unbalanced torque would be 392 N * 2.25 m = 882 Nm. It is important to note that the direction of the torque would be clockwise, since Susan's weight is causing the seesaw to rotate in that direction. I hope this helps!