Please ,
Explain me this situation
A spider is attached to two threads of equal length that make a 120o angle with each other. The spider is motionless and its mass is m=30mg.
What is the tension in each thread?

According to the Newton first Law, the sum of the force must be 0.

T1+T2+Fspider=0
since the threads are the same length and symetrical, T1=T2.

Fspider=-3.0 *10-4N

so I found T1=1.5 *10-4N

I am surprised to see that the answer is 3.0 *10-4N on each thread

please now I am confused! help

 

Originally posted by braddy@Sep 17 2005, 08:02 PM
Please ,
Explain me this situation
A spider is attached to two threads of equal length that make a 120o angle with each other. The spider is motionless and its mass is m=30mg.
What is the tension in each thread?

According to the Newton first Law, the sum of the force must be 0.

T1+T2+Fspider=0
since the threads are the same length and symetrical, T1=T2.

Fspider=-3.0 *10-4N

so I found T1=1.5 *10-4N

I am surprised to see that the answer is 3.0 *10-4N on each thread

please now I am confused! help

[post=10408]Quoted post[/post]​

Remember you are dealing with forces in 2 axes. X and Y not in 1. I think that is where you made ur mistake.

Fyspider=mspider*gravity

Since spider is not moving back and forth then Fx=0
which means that |Fxthread1|=|Fxthread2|

Since the angle is 120 between them and symectrical we have 30-60-90 triangles created for each thread.

Fyspider-2Fythread=0
therefore Fythread=Fyspider/2

Now we need Fxthread, since both at equal, we solve for either. Knowing Fythread, we use the law of sines.

Fythread/sin 30 = Fxthread/sin 60
Fxthread=Fythread*sin60/sin30

Fthread=sqrt(Fxthread^2+Fythread^2)

Hope that helps.

 

Originally posted by Brandon@Sep 17 2005, 08:39 PM
Remember you are dealing with forces in 2 axes. X and Y not in 1. I think that is where you made ur mistake.

Fyspider=mspider*gravity

Since spider is not moving back and forth then Fx=0
which means that |Fxthread1|=|Fxthread2|

Since the angle is 120 between them and symectrical we have 30-60-90 triangles created for each thread.

Fyspider-2Fythread=0
therefore Fythread=Fyspider/2

Now we need Fxthread, since both at equal, we solve for either. Knowing Fythread, we use the law of sines.

Fythread/sin 30 = Fxthread/sin 60
Fxthread=Fythread*sin60/sin30

Fthread=sqrt(Fxthread^2+Fythread^2)

Hope that helps.

[post=10412]Quoted post[/post]​

Thanks I got it now