Anticlockwise Moment & Force Direction: Why the Change?

[goldfish9776]

Homework Statement

in figure b , based on the direction of moment of anticlockwise is positive , i can say that -Tcos20 (7 sin 30 ) is a clockwise moment , so that the tension of rope should be acted in upward direction , so that the horizontal component of the force acted to the left to produce clockwise moment .

My question is why the author change to direction of tension or rope is acting downwards in figure c ?

Homework Equations

The Attempt at a Solution

 

[CWatters]

in figure b , based on the direction of moment of anticlockwise is positive , i can say that -Tcos20 (7 sin 30 ) is a clockwise moment

Yes. If you draw the free body diagram for the steps then T acts at point B and points downwards and to the right. The horizontal component of T is Tcos20 points to the right and so acts clockwise. That's why it is -Tcos20 when he sums the torques.

...so that the tension of rope should be acted in upward direction , so that the horizontal component of the force acted to the left to produce clockwise moment.

No that bit is wrong. At B, the horizontal component of T acts to the right and that produces a clockwise moment.

If in doubt draw the free body diagram for the steps only. Post that here so we can see you have that right.

 

[CWatters]

My question is why the author change to direction of tension or rope is acting downwards in figure c ?

He didn't change. It acts downwards and to the right in b and c.

 

[goldfish9776]

Yes. If you draw the free body diagram for the steps then T acts at point B and points downwards and to the right. The horizontal component of T is Tcos20 points to the right and so acts clockwise. That's why it is -Tcos20 when he sums the torques.
No that bit is wrong. At B, the horizontal component of T acts to the right and that produces a clockwise moment.

If in doubt draw the free body diagram for the steps only. Post that here so we can see you have that right.

Ok, can you explain why the tension of rope is acting downwards??

 

[SteamKing]

Ok, can you explain why the tension of rope is acting downwards??

The tension in the rope is what is keeping the truck ramp from dragging on the ground behind the truck.

 

[goldfish9776]

The tension in the rope is what is keeping the truck ramp from dragging on the ground behind the truck.

Is it okay if I assume the tension of rope to act upwards at first??

 

[SteamKing]

Is it okay if I assume the tension of rope to act upwards at first??

No. Why do you want to do that?

The rope runs from the truck ramp to the attachment point as shown on the diagram.

 

[goldfish9776]

No. Why do you want to do that?

The rope runs from the truck ramp to the attachment point as shown on the diagram.

It's because I don't know how to determine the direction of tension at most of the time...if I gt the tension is in negative value tat means the direction of tension is opposite to the direction that I have initially made??

 

[SteamKing]

It's because I don't know how to determine the direction of tension at most of the time...if I gt the tension is in negative value tat means the direction of tension is opposite to the direction that I have initially made??

You don't need to determine the direction of tension in this case; it is clearly shown in the example.

Sometimes, you have to live with the geometry of the problem.

 

[goldfish9776]

You don't need to determine the direction of tension in this case; it is clearly shown in the example.

Sometimes, you have to live with the geometry of the problem.

If I gt the tension of rope is negative value, then can I say than the direction of tension is opposite to the direction that I have initially made?

 

[SteamKing]

If I gt the tension of rope is negative value, then can I say than the direction of tension is opposite to the direction that I have initially made?

You could, but I would hesitate to say that as a general rule, especially without seeing any of your work.

Ropes only act one way, unlike bars, beams or trusses. You can't push on a rope.

 

[CWatters]

Gravity causes the steps to rotate anti clockwise about the bottom end so the tension must act clockwise or the moments won't sum to zero. The geometry means that the latter also acts downwards. You could move the bottom end of the rope up and to the right but that would be a different problem.

 

[CWatters]

Can you show us your working so we can see how you get a negative tension?

 

[goldfish9776]

Gravity causes the steps to rotate anti clockwise about the bottom end so the tension must act clockwise or the moments won't sum to zero. The geometry means that the latter also acts downwards. You could move the bottom end of the rope up and to the right but that would be a different problem.

Can I use the sum of forces =0 to determine the direction of tension? Since you said that the gravity forces causes the road to turn in anticlockwose, then the direction of tension must be downwards to create clockwise movement ...
So I determine the direction of torque caused by gravitational force first, then I would know the direction of moment should be opposed to the direction of gravity force to counter the force?

 

[CWatters]

Yes. Many statics problems require you to use net force = 0, or net torque = 0, or both to solve them.