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Homework Statement
A platform rotates with ##\omega=10## rad/s around ##z##-axes. A ball is connected, with a yarn to ##z##. Its distance to the axes is 15 cm and it rotates with ##\omega=10## rad/s. There isn't friction between platform and ball. Suddenly, the angular velocity of the platform is reduced to ##\omega'=2## rad/s. Find velocity and acceleration of the ball in the system of the platform.
Homework Equations
##\vec{a_0}=\vec{a'}+\vec{a_{cc}}+\vec{a_c}##
where ##\vec{a'}##=acceleration calculted in a non inertial reference frame, ##\vec{a_{cc}}=2\vec{\omega} \times \vec{v'} ##, ##\vec{a_c}=-\omega ^2r \vec{u_r}##.
So ##\vec{a'}=\vec{a_0}-\vec{a_{cc}}-\vec{a_c}##
The Attempt at a Solution
I have written ##a'=\omega^2 r-2 w_r v'+w_r^2r##, where ##\omega_r=\omega-\omega'##
But the correct formula is ##a'=\omega^2r-2\omega'v'-\omega'^2r##
I dont't understand
1) why ##a_c## has to have opposite sign
2) why in ##a_c## and ##a_{cc}## the angular velocity is the angular velocity of the platform insted of relative angular velocity
Thanks for the help