# Homework Help: Boy swings from tree

1. Oct 17, 2007

### Jtappan

1. The problem statement, all variables and given/known data

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

2. Relevant equations

?????

3. The attempt at a solution

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

2. Oct 17, 2007

### 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.

3. Oct 17, 2007

### Jtappan

Gravity is acting on the child. I think thats it. I dont know which equation woulud apply....thats why i put the ??? haha

4. Oct 18, 2007

### learningphysics

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...

5. Oct 18, 2007

### 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)