# Newtons first Law

Joined Dec 29, 2004
83
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

#### Brandon

Joined Dec 14, 2004
306
Originally posted by braddy@Sep 17 2005, 08:02 PM
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

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

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

Hope that helps.

Joined Dec 29, 2004
83
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

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