The center of mass of a pitched baseball or
radius 3.91 cm moves at 33.6 m/s. The ball
spins about an axis through its center of mass
with an angular speed of 52.1 rad/s.
Calculate the ratio of the rotational energy
to the translational kinetic energy. Treat the
ball as a uniform sphere.

Respuesta :

The ratio between rotational energy and translational kinetic energy is [tex]1.47\cdot 10^{-3}[/tex]

Explanation:

The translational kinetic energy of the ball is given by:

[tex]KE_t = \frac{1}{2}mv^2[/tex]

where

m is the mass of the ball

v is the speed of the ball

The rotational kinetic energy of the ball is given by

[tex]KE_r = \frac{1}{2}I\omega^2[/tex]

where

I is the moment of inertia

[tex]\omega[/tex] is the angular speed

The moment of inertia of a solid sphere through its axis is given by

[tex]I=\frac{2}{5}mR^2[/tex]

where

m is the mass of the ball

R is its radius

Substituting into the previous equation,

[tex]KE_r = \frac{1}{2}(\frac{2}{5}mR^2)\omega^2 = \frac{1}{5}mR^2 \omega^2[/tex]

The ratio between the two energies is

[tex]\frac{KE_r}{KE_t}=\frac{\frac{1}{5}mR^2 \omega^2}{\frac{1}{2}mv^2}=\frac{2R^2 \omega^2}{5v^2}[/tex]

And substituting:

R = 3.91 cm = 0.0391 m

v = 33.6 m/s

[tex]\omega=52.1 rad/s[/tex]

we find:

[tex]ratio = \frac{2(0.0391)^2(52.1)^2}{5(33.6)^2}=1.47\cdot 10^{-3}[/tex]

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The ratio of the rotational energy to the translational kinetic energy. Treat the ball as a uniform sphere should be [tex]1.47.10^-3[/tex].

Calculation of the ratio:

Since

The translational kinetic energy of the ball should be

[tex]KE_t = 1/2mv^2[/tex]

here

m is the mass of the ball

v is the speed of the ball

Now

The rotational kinetic energy of the ball should be

[tex]KE_r = 1/2Iw^2[/tex]

Here

I is the moment of inertia

w is the angular speed

Now

The moment of inertia of a solid sphere through its axis should be

[tex]I = 2/5mR^2[/tex]

Here

m is the mass of the ball

R is its radius

So,

[tex]KE_r = 1/2(2mR^2)w^2\\\\= 1/5mR62w^2[/tex]

Now

the ratio be

[tex]KE_r/KE_t = (1/2mR^2w^2) / (1/2mv^2)\\\\= (2R^2 w^2) / (5v^2)\\\\= 290.0391)^2 (52.1)^2 / 5(33.6)^2\\\\= 1.47*10^-3[/tex]

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