Math Problem Statement

A block with mass mb = 1.4 kg is connected by a rope across a 50-cm -diameter, 2.0 kg pulley, as shown in (Figure 1). There is no friction in the axle, but there is friction between the rope and the pulley; the rope doesn't slip and the pulley can be modeled as a solid cylinder. The weight is accelerating upward at 1.2 m/s2 . Figure1 of 1 The figure shows a block with a mass of m subscript b that hangs vertically on a rope. The rope goes through the pulley. A vertical force T directed downward is applied to the free end of the rope. Part A What is the tension in the rope on the right side of the pulley? Express your answer to three significant figures and include the appropriate units.

Solution

To solve for the tension in the rope on the right side of the pulley, we need to apply Newton's second law to both the block and the pulley.

Given:

  • Mass of the block mb=1.4kgm_b = 1.4 \, \text{kg}
  • Acceleration of the block ab=1.2m/s2a_b = 1.2 \, \text{m/s}^2 (upward)
  • Mass of the pulley mp=2.0kgm_p = 2.0 \, \text{kg}
  • Radius of the pulley R=0.50mR = 0.50 \, \text{m}
  • Moment of inertia of the pulley (solid cylinder) I=12mpR2I = \frac{1}{2} m_p R^2
  • Gravitational acceleration g=9.81m/s2g = 9.81 \, \text{m/s}^2

Step 1: Block's equation of motion

For the block, we use Newton’s second law: Fnet=mbabF_{\text{net}} = m_b \cdot a_b The net force on the block is the difference between the downward gravitational force and the upward tension in the rope on the left side (T1T_1): mbgT1=mbabm_b \cdot g - T_1 = m_b \cdot a_b Solving for T1T_1: T1=mbgmbabT_1 = m_b \cdot g - m_b \cdot a_b T1=(1.4kg)(9.81m/s2)(1.4kg)(1.2m/s2)T_1 = (1.4 \, \text{kg}) \cdot (9.81 \, \text{m/s}^2) - (1.4 \, \text{kg}) \cdot (1.2 \, \text{m/s}^2) T1=13.734N1.68NT_1 = 13.734 \, \text{N} - 1.68 \, \text{N} T1=12.054NT_1 = 12.054 \, \text{N}

Step 2: Pulley’s equation of motion

Now, let's apply Newton's second law for rotation to the pulley. The torque on the pulley comes from the tension difference on the left and right sides of the rope. This torque causes angular acceleration of the pulley.

The moment of inertia for a solid cylinder is: I=12mpR2=12(2.0kg)(0.50m)2=0.25kgm2I = \frac{1}{2} m_p R^2 = \frac{1}{2} \cdot (2.0 \, \text{kg}) \cdot (0.50 \, \text{m})^2 = 0.25 \, \text{kg} \cdot \text{m}^2

The net torque is: τ=(T1T2)R\tau = (T_1 - T_2) \cdot R The angular acceleration α\alpha of the pulley is related to the linear acceleration aba_b of the block by α=abR\alpha = \frac{a_b}{R}. The rotational form of Newton's second law gives: τ=Iα\tau = I \cdot \alpha Substitute for τ\tau and α\alpha: (T1T2)R=IabR(T_1 - T_2) \cdot R = I \cdot \frac{a_b}{R} (T1T2)=IabR2(T_1 - T_2) = \frac{I \cdot a_b}{R^2} (T1T2)=(0.25kgm2)(1.2m/s2)(0.50m)2(T_1 - T_2) = \frac{(0.25 \, \text{kg} \cdot \text{m}^2) \cdot (1.2 \, \text{m/s}^2)}{(0.50 \, \text{m})^2} (T1T2)=0.30Nm0.25m2(T_1 - T_2) = \frac{0.30 \, \text{N} \cdot \text{m}}{0.25 \, \text{m}^2} T1T2=1.2NT_1 - T_2 = 1.2 \, \text{N}

Step 3: Solve for T2T_2

Now that we know T1T2=1.2NT_1 - T_2 = 1.2 \, \text{N}, we can solve for T2T_2: T2=T11.2NT_2 = T_1 - 1.2 \, \text{N} T2=12.054N1.2NT_2 = 12.054 \, \text{N} - 1.2 \, \text{N} T2=10.854NT_2 = 10.854 \, \text{N}

Final Answer:

The tension on the right side of the rope is 10.9N\boxed{10.9 \, \text{N}}.

Would you like to go over any details or explore another aspect of the problem? Here are some related questions:

  1. How would the tension change if the block’s acceleration were greater?
  2. How would the moment of inertia of the pulley affect the tension in the rope?
  3. What happens if the pulley had a different shape, like a hollow cylinder?
  4. How would friction between the rope and the pulley affect the solution?
  5. How can we calculate the angular velocity of the pulley after a certain time?

Tip: When dealing with rotating objects, remember that torque τ\tau is the rotational equivalent of force, and angular acceleration α\alpha corresponds to linear acceleration through α=a/R\alpha = a / R.

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Math Problem Analysis

Mathematical Concepts

Newton's Second Law
Rotational Dynamics
Moment of Inertia
Torque
Force and Acceleration

Formulas

T1 = mb * g - mb * ab
I = 1/2 * mp * R²
τ = (T1 - T2) * R
τ = I * α
α = ab / R
(T1 - T2) = (I * ab) / R²

Theorems

Newton's Second Law (linear and rotational forms)
Moment of Inertia for a solid cylinder

Suitable Grade Level

Grades 11-12