Physics, 29.11.2019 06:31, doggylover8655

Amotor drives a disk initially at rest through 25.9 rotations in 5.0 s. assume the vector sum of the torques caused by the force exerted by the motor and the force of friction is constant. the rotational inertia of the disk is 4.0 kg⋅m2. when the motor is switched off, the disk comes to rest in 12 s.
a.)what is the magnitude of torque created by the force of friction?
b.)what is the magnitude of torque caused by the force exerted by the motor?

Answers: 3

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answered: robert7248

Answer

given,

rotation of disk is equal to(θ) = 25.9

time taken = 5 s

moment of inertia of disk = 4 Kg.m²

disk come to rest in = 12 sec

a) θ = 2 π x 25.9

θ = 162.73 rad

using equation of rotational motion

θ = ω₀t + 0.5 α t²

162.79 = 0 + 0.5 x α x 5²

α = 13.02 rad/s²

total torque acting

$\tau_{frictional} + \tau_{rotational} = I \alpha$

$\tau_{frictional} + \tau_{rotational} = 4\times 13.02$

$\tau_{frictional} + \tau_{rotational} =52.08$

again using the rotational equation

$\omega_f - \omega_i = \alpha\ t$

$\omega_f -0 = 13.02 \times 5$

$\omega_f = 65.1\ rad/s$

when only friction is acting angular acceleration

$\omega_f - \omega_i = \alpha\ t$

$0 -65.1 =\alpha\ 12$

$\alpha= -5.425\ rad/s^2$

now torque due to friction

$\tau_{frictional} =- 4 \times 5.425$

$\tau_{frictional} =-21.7\ Nm$

now,

$-21.7+ \tau_{rotational} =52.08$

$\tau_{rotational} =73.78\ Nm$

answered: Guest

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answered: Guest

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

so, refractive index of medium = 300000/200000 = 1.5

1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

∞ refractive index is usually for opaque mediums. so third medium is opaque medium.