# Homework Help: A problem in mechanics

1. Sep 24, 2014

### Daniel95

1. The problem statement, all variables and given/known data
A child is sitting on a wooden stick, hanging from a 3 m long blue rope,
attached to the top of a 4 m tall metal pole. The child is performing uniform
rotation around the pole, in such a way that the angle between pole and rope
is . The horizontal distance between child and pole is called r.

The centripetal acceleration resulting
from the combined forces of rope and gravity is given by
ac = g tan
where g = 9.80 m/s2 is the gravitational acceleration.

If the period of rotation is T = 3 s, what is:
a) What is the angle ? The speed of rotation? r ? The angular speed? The
centripetal acceleration?​

Now the nasty big brother taunts the child into going faster and faster, and
keeps pushing him until his uniform motion has an angle with vertical of
= 60 degrees.
b) How fast is he now going, and what is the period of rotation? How high
up in the air is the child now zooming around?​

As the helpless child is spinning overhead, the nasty big brother ducks underneath,
reaches up and cuts the rope with a hedge-trimmer.
c) How far does the child fly off before hitting the ground? You may assume
that the child has the initial height and speed computed in b).​

2. Relevant info
So I have done a) and b) and I need some help with the c).

I have found out (in a)) that the angle is 41.9 degrees, r =2m, angular speed=2,09m/s, centripetal acceleration=8.79m/s^2, and the speed of rotation=4.19m/s
In b) I found that the speed is 6.65m/s, the period og rotation=2.46seconds and that the child is 2.5m up in the air.

How can I solve the c). I had an idea, but I've found that it was wrong.

2. Sep 24, 2014

### nasu

So what was your idea? How do you know is wrong?

3. Sep 24, 2014

### Simon Bridge

Welcome to PF;
... what was the idea?

For (c) - consider: what normally happens to an object in circular motion when it is released?
Do you know any physics involving objects falling under gravity?

4. Sep 24, 2014

### Daniel95

Okey, so I have uploaded two pictures which shows how I have done (a) and (b).

And for (c) I first thought about using x(t)= xi + vit + 1/2 at^2 but that does not include the angle so it feels wrong..

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5. Sep 24, 2014

### Simon Bridge

What normally happens to an object in circular motion when it is released? It flys off in a straight line right: but which straight line? How does it move wrt the initial circle?

What were you planning to use for vi? or a or t?
Do you know any physics involving objects falling under gravity?

This is not a problem where you can just look up an equation and hope for the best, you have to understand the physics.

6. Sep 24, 2014

### Daniel95

Okey, the child will clearly not fall in a straight line on to the ground. It has a velocity whitch is a tangent of the horizontal circle.

For vi I was planning to use 6,65m/s that I found in (b). And I don't know about t.

So now I am maybe thinking I can use the range equation R = vi^2 * sin(2*q) / g
If that does not work, than I don't have any idea...

7. Sep 24, 2014

### Orodruin

Staff Emeritus
So ask yourself the following questions:
What forces are acting on the kid after the rope has been cut?
How high above the ground is the kid at the time the rope is cut?
How long will it take the kid to hit the ground?

8. Sep 24, 2014

### Daniel95

Well, aside from the air resistance, there is no other forces than gravitation acting on the child after the rope has been cut. And at that point the child is 2.5m up in the air. But still I am not sure how to find how long it will take the child to hit the ground or the distance it will fly before hitting the ground.

I feel a little ashamed, it's my first month with physics.

9. Sep 24, 2014

### Orodruin

Staff Emeritus
So let us ignore air resistance for the time being.

Based on that you are given this problem, I believe you will already have covered basic parabolic motion under the influence of a gravitational field, yes? What is the initial velocity in the vertical direction and how does this change with time?

10. Sep 24, 2014

### Daniel95

Okey, I think I've sorted it ou based on your last question. Does this look right to you?

#### Attached Files:

• ###### 2014-09-24 19.44.20.jpg
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11. Sep 24, 2014

### Orodruin

Staff Emeritus
Your approach looks reasonable, I have not checked the actual numbers.

12. Sep 24, 2014

### Daniel95

Wow, ok! Thank you very much, both Orodruin and Simon. Now I can sleep:)

13. Sep 25, 2014

Well done :)