Why do we use negative signs for clockwise moments in torque calculations?

[mechanicstudy]

I'm stuck on this problem at the moment.

The answer is 19.08 nm clockwise but i just don't see how it can be.

Check this.

2.8x(0.8+5.5)= 17.64 nm

If I use the 4.6 it ends up
4.6x(0.8+5.5) = 28.98 nm

I don't get how it can be 19.08 clockwise when there is only one force going clockwise or anticlockwise. That moment from that force isn't 19 though.

 

[HallsofIvy]

The moment due to the 2.8 N force is (2.8)(5.5)= 15.4 Nm (The force is multiplied by the length of the arm perpendicular to the force). The moment due to the 4.6 N force is (4.6)(0.8)= 3.68 Nm for a total of 15.4+ 3.68= 19.08 Nm

 

[mechanicstudy]

The moment due to the 2.8 N force is (2.8)(5.5)= 15.4 Nm (The force is multiplied by the length of the arm perpendicular to the force). The moment due to the 4.6 N force is (4.6)(0.8)= 3.68 Nm for a total of 15.4+ 3.68= 19.08 Nm

I don't get how that would work. The 2.8 Force would have to go through 0.8m distance to get to point A.

 

[Doc Al]

I don't get how that would work. The 2.8 Force would have to go through 0.8m distance to get to point A.

No, only the perpendicular distance matters. (The distance perpendicular to the line of force.) As far as the 2.8 N force is concerned, that 0.8m distance could be 800m and it would make no difference. Read this: http://hyperphysics.phy-astr.gsu.edu/hbase/torq.html" (Torque is another name for moment.)

 

[mechanicstudy]

Finally i get it. Ok i got a questions to ask.

1: When does clockwise become negative? When i look at the notes i have been given. Sometimes its negative and sometimes it's not. Does it only become negative when an Anticlockwise force is present?

thanks by the way.

 

[Doc Al]

1: When does clockwise become negative? When i look at the notes i have been given. Sometimes its negative and sometimes it's not.

Calling clockwise moments negative is just the standard convention. Often you only care about the magnitude of the torque, so the direction doesn't matter.

Does it only become negative when an Anticlockwise force is present?

You must distinguish anticlockwise from clockwise torques when both are present, and using the standard sign convention is a great way to do that.

 

[mechanicstudy]

Calling clockwise moments negative is just the standard convention. Often you only care about the magnitude of the torque, so the direction doesn't matter.

You must distinguish anticlockwise from clockwise torques when both are present, and using the standard sign convention is a great way to do that.

thank you. My notes just seem to neglect information like that and it has left me confused.