Gr. 12 Dynamics -- A performer being lifted up by two ropes

[danielsmith123123]

Homework Statement

Performer has a gravity force of 440N and is being pulled up by 2 ropes. One rope has a tension of 430N [Up 35* L] and the other exerts a force of 280N [Up]

Relevant Equations

F= ma
Fg= mg

1. break down 430N [Up 35* L] into components
430 cos 35 = 352.2N [L]
430 sin 35 = 246.6N

2.
ΣFx= 352.2N [L]

ΣFy= 246.6N + 280N - 430N
ΣFy = 86.6

3.
Pythagorean theorem with the two will give you a magnitude of 362.7N
Then using tan you can find the angle of [L 76 U]

This method was the one I was taught but it doesn't work for this question as the answer is 310N [U 38 L]
Can anybody help me?

 

[TSny]

One rope has a tension of 430N [Up 35* L]

Does this mean that the rope makes an angle of 35^o with respect to the y-axis, or does the rope make an angle of 35^o with respect to the negative x-axis?

 

[hmmm27]

What is the actual question ?

 

[danielsmith123123]

What is the actual question ?

"Determine the net force"

 

[danielsmith123123]

Does this mean that the rope makes an angle of 35^o with respect to the y-axis, or does the rope make an angle of 35^o with respect to the negative x-axis?

I think the y axis.
If it was the negative x-axis wouldn't it be [Left 65 Up]

 

[TSny]

I think the y axis.
If it was the negative x-axis wouldn't it be [Left 65 Up]

This is somewhat of a nonstandard notation. But, yes, I would tend to think that [Up 35^o L] would mean "from the upward direction, tilt 35^o toward the left". So, the tension force of the rope makes an angle of 35^o to the y-axis. This means that you did not calculate the x and y components of the tension force correctly. Check your work. Draw an appropriate right triangle for this force showing the force as the hypotenuse and the legs as the x and y components. Label the angles inside the triangle. Use trig to find the legs.

 

[TSny]

Oh, ok thank you. Just this morning my teacher told me that cos will always give you the "x" component, an d sin will always give you the "y" component is that wrong?

cos will give you the x-component if the angle is measured relative to the x-axis. But, in this problem, the 35 degrees is measured from the y-axis. If you want to use cos to get the x-component, then you would need to use the angle that the force makes to the x-axis, rather than the y-axis.

It's best to construct your own triangle and just use trig.

 

[danielsmith123123]

cos will give you the x-component if the angle is measured relative to the x-axis. But, in this problem, the 35 degrees is measured from the y-axis. If you want to use cos to get the x-component, then you would need to use the angle that the force makes to the x-axis, rather than the y-axis.

It's best to construct your own triangle and just use trig.

Oh ok, thank you for all your help

 

[TSny]

Oh ok, thank you for all your help

You are welcome. Hope you enjoy your course.