Math Problem Statement

A hollow sphere is attached to the end of a uniform rod. The sphere has a radius of 0.64 m and a mass of 0.40 kg. The rod has a length of 1.20 m and a mass of 0.52 kg. The rod is placed on a fulcrum (pivot) at X = 0.42 m from the left end of the rod.

(a) Calculate the moment of inertia (click for graphical table) of the contraption around the fulcrum.   kg m2

(b) Calculate the torque about the fulcrum, using CCW as positive.   N m

(c) Calculate the angular acceleration of the contraption, using CCW as positive.   rad/s2

(d) Calculate the linear acceleration of the right end of the rod, using up as positive.   m/s2

Solution

Let's calculate each part of the problem step by step:

Given:

  • Radius of sphere R=0.64mR = 0.64 \, \text{m}
  • Mass of sphere msphere=0.40kgm_{\text{sphere}} = 0.40 \, \text{kg}
  • Length of rod L=1.20mL = 1.20 \, \text{m}
  • Mass of rod mrod=0.52kgm_{\text{rod}} = 0.52 \, \text{kg}
  • Pivot distance X=0.42mX = 0.42 \, \text{m}
  • Acceleration due to gravity g=9.81m/s2g = 9.81 \, \text{m/s}^2

(a) Moment of Inertia

  1. For the rod: Irod=112mrodL2+mrodX2I_{\text{rod}} = \frac{1}{12} m_{\text{rod}} L^2 + m_{\text{rod}} X^2 where 112mrodL2\frac{1}{12} m_{\text{rod}} L^2 is the moment of inertia about its center, and the second term accounts for the distance XX from the pivot (parallel axis theorem).

  2. For the hollow sphere: The moment of inertia of the hollow sphere about its center is: Isphere-center=23msphereR2I_{\text{sphere-center}} = \frac{2}{3} m_{\text{sphere}} R^2 Using the parallel axis theorem to shift the axis to the pivot (which is at a distance (LX)(L - X) from the center of the sphere): Isphere=Isphere-center+msphere(LX)2I_{\text{sphere}} = I_{\text{sphere-center}} + m_{\text{sphere}} (L - X)^2

    Adding the contributions from both the rod and the sphere: Itotal=Irod+IsphereI_{\text{total}} = I_{\text{rod}} + I_{\text{sphere}} Result: Itotal=0.507kgm2I_{\text{total}} = 0.507 \, \text{kg} \cdot \text{m}^2


(b) Torque about the Fulcrum

Torque is calculated as τ=rFsinθ\tau = r \cdot F \sin \theta. In this case, the force is the weight due to gravity, and since the forces are vertical, sin90=1\sin 90^\circ = 1.

  1. Torque due to the rod: τrod=mrodg(Xcenter of mass distance)\tau_{\text{rod}} = m_{\text{rod}} g (X - \text{center of mass distance}) The center of mass of the rod is at L2\frac{L}{2}, so: τrod=mrodg(XL2)\tau_{\text{rod}} = m_{\text{rod}} g (X - \frac{L}{2})

  2. Torque due to the sphere: The force due to the sphere acts at distance (LX)(L - X) from the pivot: τsphere=msphereg(LX)\tau_{\text{sphere}} = m_{\text{sphere}} g (L - X)

Summing both torques: τtotal=τrod+τsphere\tau_{\text{total}} = \tau_{\text{rod}} + \tau_{\text{sphere}} Result: τtotal=2.14Nm\tau_{\text{total}} = 2.14 \, \text{N} \cdot \text{m}


(c) Angular Acceleration

Using Newton's second law for rotation, the angular acceleration α\alpha is: α=τnetItotal\alpha = \frac{\tau_{\text{net}}}{I_{\text{total}}} Substituting the values for torque and moment of inertia: α=2.140.507=4.23rad/s2\alpha = \frac{2.14}{0.507} = 4.23 \, \text{rad/s}^2


(d) Linear Acceleration of the Right End of the Rod

The linear acceleration aa of the right end of the rod is related to the angular acceleration by: a=α×La = \alpha \times L Substituting the angular acceleration and length of the rod: a=4.23×1.20=5.07m/s2a = 4.23 \times 1.20 = 5.07 \, \text{m/s}^2


Final Results:

  • (a) Moment of inertia: Itotal=0.507kgm2I_{\text{total}} = 0.507 \, \text{kg} \cdot \text{m}^2
  • **(b)

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Math Problem Analysis

Mathematical Concepts

Moment of inertia
Torque
Angular acceleration
Linear acceleration

Formulas

I_rod = (1/12) * m_rod * L^2 + m_rod * X^2
I_sphere = (2/3) * m_sphere * R^2 + m_sphere * (L - X)^2
Torque (τ) = r * F * sin(θ)
Angular acceleration (α) = τ / I_total
Linear acceleration (a) = α * L

Theorems

Parallel axis theorem
Newton's second law of motion for rotation

Suitable Grade Level

Undergraduate Physics