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Prior to being placed in its hole, a \(5700-\mathrm{N}, 9.0\) -m-long, unjform utility pole makes some nonzero angle with the vertical. A vertical cable attached 2.0 \(\mathrm{m}\) below its upperend holds it in place while its lower end rests on the ground. (a) Find the tension in the cable and the magnitude and direction of the force exerted by the ground on the pole. (b) Why don't we need to know the angle the pole makes with the vertical, as long as it is not zero?

Short Answer

Expert verified
Tension in cable: 3657.14 N. Ground exerts: 6766.67 N at 57.5°; angle doesn't matter.

Step by step solution

01

Identify Forces and Torques

First, identify all the forces acting on the pole and their points of application. The weight of the pole acts at its center, 4.5 m from the bottom (midpoint of the 9 m length), and has a magnitude of 5700 N. The tension in the cable acts 2 m below the top, so 7 m (9 m - 2 m) from the bottom. There is also a reaction force from the ground at the base including the vertical (normal force) and horizontal components.
02

Set Up Torque Equation

Use the condition of static equilibrium for torques about a point. We'll use the base of the pole as the pivot point. The sum of torques must be zero: \[ T \cdot 7 - 5700 \cdot 4.5 = 0 \]Solving for the tension \(T\), we can balance the torques.
03

Solve Torque Equation for Tension

From the torque equation: \[ T \cdot 7 = 5700 \cdot 4.5 \]\[ T = \frac{5700 \times 4.5}{7} = 3657.14 \text{ N} \]The tension in the cable is 3657.14 N.
04

Calculate Vertical Force by Ground

Apply the equilibrium condition for forces in the vertical direction. The normal force from the ground \( N \) must balance the vertical components of the forces:\[ N = 5700 \text{ N} \]The vertical force exerted by the ground is equal to the weight of the pole.
05

Calculate Horizontal Force by Ground

Apply the equilibrium condition for forces in the horizontal direction. The horizontal reaction force \( R \) by the ground balances the tension:\[ R = T = 3657.14 \text{ N} \]The horizontal force exerted by the ground is equal in magnitude to the tension in the cable.
06

Determine Magnitude and Direction of Ground Force

Combine the vertical and horizontal components to find the magnitude of the total force from the ground:\[ F_{ground} = \sqrt{N^2 + R^2} = \sqrt{5700^2 + 3657.14^2} \approx 6766.67 \text{ N} \]The direction \( \theta \) (angle from horizontal) is given by:\[ \theta = \tan^{-1}\left(\frac{N}{R}\right) = \tan^{-1}\left(\frac{5700}{3657.14}\right) \approx 57.5^\circ \]
07

Address the Independence from Angle of Pole

The angle that the pole makes with the vertical does not affect the torques because the weight of the pole always acts downward through its center, and the tension acts at a fixed distance along the pole. Thus, the torques depend only on the pole's geometry and position of the cable, not the exact angle.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Understanding Torque in Statics
Torque is a fundamental concept in understanding the balance of forces in static systems. Imagine torque as the twist or rotation that a force induces around a pivot point. In the realm of statics, torque determines whether an object remains stable or rotates. It is calculated as the product of the force and the perpendicular distance from the pivot point to the line of action of the force. Mathematically, torque \( \tau \) is expressed as:
  • \( \tau = F \times d \)
where \( F \) is the force and \( d \) is the distance perpendicular to the direction of the force.
In the utility pole problem, torque plays a critical role in explaining how the tension in the cable and the weight of the pole are balanced. By using the pole's base as the pivot, we assess the torques created by each force to maintain an equilibrium state.
When torques produced on both sides of the pivot are equal, the system achieves a state of rotational equilibrium, meaning the object doesn't rotate.
Equilibrium: The Balance of Forces and Torques
Equilibrium in physics implies that an object is either at rest or moving with constant velocity, indicating that all forces and torques acting on it are perfectly balanced. When solving statics problems, like the utility pole scenario, understanding both translational and rotational equilibrium is vital.
  • Translational Equilibrium: All forces directly acting on the object must sum to zero.
  • Rotational Equilibrium: All torques must balance so that the sum is zero.
In the case of the utility pole, the forces acting on it include the pole's weight, the tension in the cable, and the reactions from the ground. The key conditions are:
  • The sum of vertical forces \( \sum F_y = 0 \)
  • The sum of horizontal forces \( \sum F_x = 0 \)
  • The sum of torques \( \sum \tau = 0 \)
These conditions ensure that the pole remains stationary in all directions, highlighting the fixing role of the cable and ground.
Decomposing Force Components
A force can be resolved into two components that often aid in better understanding and organizing forces in statics problems: perpendicular and parallel components. This decomposition allows analysis using well-known equations of equilibrium.
Considering a force at an angle, it can be split into:
  • Vertical Component: Influences the upward or downward actions (often the weight or normal force).
  • Horizontal Component: Affects the sideways forces (often related to tension or friction).
In the utility pole scenario, the ground's reaction force splits into horizontal (\( R \)) equaling the cable's tension and vertical (\( N \)) equaling the pole's weight, supporting the entire vertical load. By resolving these components, we achieve a clearer picture of how forces interact within the pole system.
Steps to Determine Tension in A Cable
Tension calculation is critical in determining how much force is within a cable keeping objects like the utility pole in place. It requires understanding the system's equilibrium and the role of torque. Follow these straightforward steps to calculate tension:
1. **Identify Forces and Pivot:**
- Locate all the forces (e.g., weight, ground reactions, and cable tension).
- Select a point as the pivot for calculating torques (commonly where the object contacts the ground).
2. **Set Up Torque Equation:**
- Write an equation based on the balancing of torques around the pivot. 3. **Solve For Tension:**
- Rearrange the torque equation to solve for tension \( T \).
With the utility pole, once we know torques balance around the pivot, we find the tension to be 3657.14 N. Understanding each step clearly ensures accuracy in evaluating statics scenarios and further underlines how tension maintains equilibrium.

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Most popular questions from this chapter

A petite young woman distributes her 500 \(\mathrm{N}\) weight equally over the heels of her high-heeled shoes. Each heel has an area of 0.750 \(\mathrm{cm}^{2}\) (a) What pressure is exerted on the floor by each heel? (b) With the same pressure, how much weight could be supported by two flat- bottomed sandals, each of area 200 \(\mathrm{cm}^{2} ?\)

A \(350-N,\) uniform, \(1.50-m\) bar is suspended horizontally by two vertical cables at each end. Cable \(A\) can support a maximum tension of 500.0 \(\mathrm{N}\) without breaking, and cable \(B\) can support up to 400.0 \(\mathrm{N}\) . You want to place a small weight on this bart up is the heaviest weight you can put on without breaking either cable, and (b) where should you put this weight?

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A specimen of oil having an initial volume of 600 \(\mathrm{cm}^{3}\) is subjected to a pressure increase of \(3.6 \times 10^{5} \mathrm{Pa},\) and the volume is found to decrease by 0.45 \(\mathrm{cm}^{3} .\) What is the bulk modulus of the material? The compressibility?

A \(15,000-N\) crane pivots around a friction-free axle at its base and is supported by a cable making a \(25^{\circ}\) angle with the crane (Fig. 11.29 ). The crane is 16 \(\mathrm{m}\) long and is not uniform, its center of gravity being 7.0 \(\mathrm{m}\) from the axle as measured along the crane. The cable is attached 3.0 \(\mathrm{m}\) from the upper end of the crane. When the crane is raised to \(55^{\circ}\) above the horizontal holding an \(11,000-N\) pallet of bricks by a 2.2 -in very light cord, find (a) the tension in the cable, and \((b)\) the horizontal and vertical components of the force that the axle exerts on the crane. Start with a free-body diagram of the crane.

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