Tension in rope for 30.8 kg child swinging at 4.2 m/s

[Jtappan]

Homework Statement

A 30.8 kg child swings on a rope with a length of 6.28 m that is hanging from a tree. At the bottom of the swing, the child is moving at a speed of 4.2 m/s. What is the tension in the rope?
______ N

Homework Equations

?

The Attempt at a Solution

I know you are suppose to draw a FBD but I get stuck from there.

 

[bob1182006]

what forces are acting on the child?

and what equations have you learned so far that deal with something that resembles swinging. if you draw out the swinging motion it's actually part of a larger circular motion.

 

[Jtappan]

Gravity is acting on the child. I think that's it. I don't know which equation woulud apply...thats why i put the ? haha

 

[learningphysics]

Gravity is acting on the child. I think that's it. I don't know which equation woulud apply...thats why i put the ? haha

you have tension and gravity acting on the child. think centripetal.

$$\Sigma F = ma$$

what is a here? put the forces in the left side of the equation...

 

[TMM]

We find the centripetal acceleration at the bottom:

a = (v^2)/r

so

F= (mv^2)/r

Plug stuff in:

F = 86.51N

Now we add that to the gravitational force, which it must counteract:

F = m*9.80

F = 301.84N

And find the tension is:

3.9x10^2 N

(388.35 w/o sigfigs)