cianfa72
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- Homework Statement
- The problem setting is given in the thread itself
- Relevant Equations
- ##v_{cm} = R \omega##
##I\dot {\omega} = M##
##M a_{cm} = F_{ext}##
Hi, I started this thread to address the solution of this problem.
Problem
A yo-yo of radii ##R_1=R## and ##R_2=\frac{7}{5}R## is acted upon by forces ##F## and ##\kappa F~~~(0<\kappa<\infty)## as shown in the figure on the right. The yo-yo rolls without slipping on the horizontal surface. The mass of the yo-yo is ##M## and its moment of inertia about its center of mass is ##I=qMR_2^2##.
Given quantities are ##R##, ##F##, ##\kappa##, ##M## and ##q##.
(a) Find the linear acceleration of the center of mass of the yo-yo in terms of the given quantities.
(b) Find the force of static friction acting on the yo-yo in terms of the given quantities.
(c) Find the value of κ such that the yo-yo rolls at constant velocity.
(d) Find the value of κ such that the force of static friction is zero.
(The ratio R2/R1 is given a numerical value to match the drawing to scale)
My attempt
Let's take the yo-yo as the "system". Vertically acting forces balance out, so they aren't part of the problem.
a) to calculate the yo-yo CoM's linear acceleration w.r.t. horizontal plane rest frame, one can proceed as follows: at any given time ##t##, going in the (inertial) system's CoM momentarily comoving reference frame, conservation of angular momentum gives: $$I\dot \omega = FR_1 + R_2(f_s - \kappa F)$$ From rolling without slipping condition ##v_{cm} = R_2 \omega## follows ##\dot v_{cm} = R_2 \dot \omega##.
Furthermore in the horizontal plane's rest frame: $$MR_2 \dot \omega = F(1 - \kappa) - f_s$$
Solving the above linear system for ##\dot \omega## $$\dot \omega = \frac {5F (12 - 14 \kappa)} {49MR(1 + q)}$$ hence $$\dot v_{cm} = \frac {F(12 - 14 \kappa)} {7M (1+q)}$$
b) solving the system for ##f_s## gives: $$f_s = \frac {F[7(1+q)(1- \kappa) -12 + 14 \kappa]} {7 (1+q)}$$
c) rolling without slipping at constant system's CoM velocity means ##\dot v_{cm} = \dot \omega = 0## hence $$\kappa = \frac {6} {7}$$ d) From b) setting ##f_s=0## one gets: $$\kappa = \frac {5 - 7q} {7(1-q)}$$
Problem
A yo-yo of radii ##R_1=R## and ##R_2=\frac{7}{5}R## is acted upon by forces ##F## and ##\kappa F~~~(0<\kappa<\infty)## as shown in the figure on the right. The yo-yo rolls without slipping on the horizontal surface. The mass of the yo-yo is ##M## and its moment of inertia about its center of mass is ##I=qMR_2^2##.
Given quantities are ##R##, ##F##, ##\kappa##, ##M## and ##q##.
(a) Find the linear acceleration of the center of mass of the yo-yo in terms of the given quantities.
(b) Find the force of static friction acting on the yo-yo in terms of the given quantities.
(c) Find the value of κ such that the yo-yo rolls at constant velocity.
(d) Find the value of κ such that the force of static friction is zero.
(The ratio R2/R1 is given a numerical value to match the drawing to scale)
My attempt
Let's take the yo-yo as the "system". Vertically acting forces balance out, so they aren't part of the problem.
a) to calculate the yo-yo CoM's linear acceleration w.r.t. horizontal plane rest frame, one can proceed as follows: at any given time ##t##, going in the (inertial) system's CoM momentarily comoving reference frame, conservation of angular momentum gives: $$I\dot \omega = FR_1 + R_2(f_s - \kappa F)$$ From rolling without slipping condition ##v_{cm} = R_2 \omega## follows ##\dot v_{cm} = R_2 \dot \omega##.
Furthermore in the horizontal plane's rest frame: $$MR_2 \dot \omega = F(1 - \kappa) - f_s$$
Solving the above linear system for ##\dot \omega## $$\dot \omega = \frac {5F (12 - 14 \kappa)} {49MR(1 + q)}$$ hence $$\dot v_{cm} = \frac {F(12 - 14 \kappa)} {7M (1+q)}$$
b) solving the system for ##f_s## gives: $$f_s = \frac {F[7(1+q)(1- \kappa) -12 + 14 \kappa]} {7 (1+q)}$$
c) rolling without slipping at constant system's CoM velocity means ##\dot v_{cm} = \dot \omega = 0## hence $$\kappa = \frac {6} {7}$$ d) From b) setting ##f_s=0## one gets: $$\kappa = \frac {5 - 7q} {7(1-q)}$$
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