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# Moments (part 2)

2 more moment problems. Created by Sal Khan.

## Want to join the conversation?

• Why aren't the clockwise forces marked with a negative sign when Sal does the algebra? As always, thanks a bunch. • The reason why there is no negative sign in the equation is because He is setting it as in equilibrium, the rightwise is equal to the counter-clockwise. This is like up is equal to down, it is implied that the forces are already acting in opposite directions so the negative sign is not needed. Another way to look at this would be The net movement of the table equals zero; 0 = clockwise force minus counter-clockwise force. Add counter-clockwise force on each side and you get. clockwise force = counter-clockwise force.
• I'm not sure whether or not I may have missed a title, but is there a video that addresses the parallel axis theorem? I have a textbook that briefly addresses the theorem, but it gives a poor example and moves right along. • I don't think there is one specifically for that topic, but the parallel axis theorem as I know states that the moment of inertia of a body about any axis is equal to the sum of [the moment of inertia of the body about an axis (parallel to the axis taken) at its center of mass] and [the product of the mass of the body and the square of the distance between the two parallel axes].
That is : I = Icm + (m).(d^2)
{where I is the moment of inertia about the axis taken; Icm is the moment of inertia about the center of mass and parallel to the axis taken; And m is the mass of the body, d is the distance between the two parallel lines}

I hope the statement wasn't so confusing...
• please can give me answer of this ques , 10N weight can be lifted in one end of a thin rod by applying a 5N force on the
other end. If the length of the rod is 1.5 meter, where is the fulcrum situated? and please tell me how to measure • when calculating the counterclockwise forces, why isn't the weight of the right leg counted? only the force up is counted not the one going down? why is that ? • Wouldn't the axis of rotation be at the base of the left leg? If you removed the right leg, the table would rotate on the base of the left leg. If the axis was at the top, that left leg would slip off the floor and rotate clockwise. That would be weird. • just with the table in general, i was wondering why you didn't have to calculate the effect of the weight of the table on the outside of the pivots? • I didn't get how the forces of 20 N and and the force exerted by the book are clockwise ? • is it because he takes the leg as the fulcrum that the moment is 1*100, why would the moment of the block not be anticlockwise and be 3*100??   