Circular Motion Ball on a String

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1. Nov 18, 2014

Fredred

1. The problem statement, all variables and given/known data
https://www.dropbox.com/s/slpu1p8jplmxzmy/Screenshot 2014-11-18 20.11.58.png?dl=0

2. Relevant equations
Fnet = ma
ma = (mv^2)/r

3. The attempt at a solution
ai) Ft up, mg down
/\Ft
I
m
I
\/mg

aii) both down
m
I I
\/mg \/Ft

aiii) Ft right, mg down
m - >Ft
I
\/mg

b) Fnet = ma
-Ft - mg = mv^2/r
Ft = -11.7N (after isolation anf plugging in numbers from problem)

c) Fnet = ma
Ft = mv^2/r
Ft = 5.8N

d) Fnet = ma
Ft + mg = mv^2/r
v = 8.8m/s

2. Nov 19, 2014

Simon Bridge

Problem Statement
A ball of mass $m=0.28\text{kg}$ swings in a vertical circular path on a string length $L=0.85\text{kg}$.
[diagram showing ball, string, and a dotted circle indicating the path - pretty much as you think]

(a) Draw force diagrams for the ball when it is (i) at the bottom of the circle, (ii) when it is at the top, and (iii) when the string is horizontal.
(b) If it's speed is $5.2\text{m/s}$ at the top of the circle, what is the tension in the string?
(c) If it's speed is $4.2\text{m/s}$ when the string is horizontal, what is the tension in the string?
(d) If the string breaks when the tension exceeds $23\text{N}$, what is the maximum speed the ball can have at the bottom?

... that wasn't so hard.

Your answers seem reasonable to me - you should also provide a bit of reasoning with your working when you present them for marking.