A bicycle wheel is mounted on a fixed, frictionless axle, with a light string wound around its rim. The wheel has moment of inertia I=kmr^2, where m is its mass, r is its radius, and k is a dimensionless constant between zero and one. The wheel is rotating counterclockwise with angular speed ω0, when at time t=0 someone starts pulling the string with a force of magnitude F. Assume that the string does not slip on the wheel.
(a) Suppose that after a certain time t_L, the string has been pulled through a distance L, What is the final rotational speed ω_final of the wheel?
(b) What is the instantaneous power P delivered to the wheel via the force F at time t=0?

Respuesta :

Answer:

a) [tex]\omega_{f} = \sqrt{\omega_{o}^{2}+ \frac{2\cdot F \cdot L}{k\cdot m} }[/tex], b) [tex]\dot W = F\cdot r \cdot \omega_{o}[/tex]

Explanation:

a) The final rotational speed is:

[tex]\omega_{f} = \sqrt{\omega_{o}^{2}+ 2\cdot r \cdot \alpha\cdot L}[/tex]

But:

[tex]T = I\cdot \alpha[/tex]

[tex]\alpha = \frac{T}{I}[/tex]

[tex]\alpha = \frac{F\cdot r}{k\cdot m \cdot r^{2}}[/tex]

[tex]\alpha = \frac{F}{k\cdot m \cdot r}[/tex]

Lastly, the angular acceleration is replace in the expression for the final angular velocity:

[tex]\omega_{f} = \sqrt{\omega_{o}^{2}+ \frac{2\cdot F \cdot L}{k\cdot m} }[/tex]

b) The power is rate of change of work in terms of time:

[tex]W = F\cdot r\cdot \theta[/tex]

Instantaneous power is obtained by differentiating the previous expression with respect to time:

[tex]\dot W = F\cdot r \cdot \omega(t)[/tex]

The instantaneous power delivered to the wheel at t = 0 is:

[tex]\dot W = F\cdot r \cdot \omega_{o}[/tex]

A) The final rotational speed ω_final of the wheel is; ω_f = √((ω_i)² + 2(FL/(kmr²)

B) The instantaneous power P delivered to the wheel via the force F at time t=0 is; P = Frω_i

From rotational kinematics, the formula for the final angular velocity is;

ω_f = √((ω_i)² + 2αθ)

where;

α is angular acceleration

θ = L/r. Thus;

ω_f = √((ω_i)² + 2α(L/r))

Now, α = T/I

Where;

I is moment of inertia = kmr²

T is torque = F*r

Thus;

α = (F * r)/(kmr²)

α = F/(kmr)

ω_f = √((ω_i)² + 2(F/(kmr))(L/r))

ω_f = √((ω_i)² + 2(FL/(kmr²)

B) Formula for instantaneous power is;

P = Fv

where at t = 0; v = rω_i

Thus;

P = Frω_i

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