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A person is standing on a level floor. His head, upper torso, arms, and hands together weigh \(438 \mathrm{N}\) and have a center of gravity that is \(1.28 \mathrm{m}\) above the floor. His upper legs weigh \(144 \mathrm{N}\) and have a center of gravity that is \(0.760 \mathrm{m}\) above the floor. Finally, his lower legs and feet together weigh \(87 \mathrm{N}\) and have a center of gravity that is \(0.250 \mathrm{m}\) above the floor. Relative to the floor, find the location of the center of gravity for his entire body.

Short Answer

Expert verified
Center of gravity is at approximately 1.034 m above the floor.

Step by step solution

01

Understand Center of Gravity Calculation

To find the center of gravity for the entire body, we need to take a weighted average of the centers of gravity of each part of the body. This involves multiplying the weight of each body part by its center of gravity height, summing these products, and then dividing by the total weight.
02

Identify Each Component

The weights and their respective centers of gravity are: Head and torso: 438 N at 1.28 m, Upper legs: 144 N at 0.760 m, Lower legs and feet: 87 N at 0.250 m.
03

Calculate the Total Weight

Add the weights of all the components of the body. \[ W_{total} = 438 \, \text{N} + 144 \, \text{N} + 87 \, \text{N} = 669 \, \text{N} \]
04

Calculate Weighted Heights Sum

Multiply each component's weight by its center of gravity height, and sum these values. \[ (438 \, \text{N} \times 1.28 \, \text{m}) + (144 \, \text{N} \times 0.760 \, \text{m}) + (87 \, \text{N} \times 0.250 \, \text{m}) \] This equals: \[ (560.64 \, \text{Nm}) + (109.44 \, \text{Nm}) + (21.75 \, \text{Nm}) = 691.83 \, \text{Nm} \]
05

Calculate the Overall Center of Gravity

Divide the weighted heights sum by the total weight to find the center of gravity for the person's entire body. \[ \text{CG}_{total} = \frac{691.83 \, \text{Nm}}{669 \, \text{N}} \approx 1.034 \, \text{m} \]
06

Interpret the Result

The center of gravity for the entire body means the point about which the body would balance perfectly on a fulcrum.

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Key Concepts

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

Weighted Average
A weighted average is a fundamental concept often used when we have different components that contribute to a total in varying magnitudes. It allows one to calculate an average that takes into account the relative importance or contribution of each component. For finding the center of gravity in our exercise, we considered each body part's weight as its significance factor. We then multiplied each weight by its center of gravity, effectively giving more influence to heavier components.

To calculate the weighted average for this problem involves:
  • Multiplying the weight of each body part by its respective center of gravity height.
  • Summing all these products to account for each part's contribution.
  • Dividing this sum by the total body weight, resulting in the overall center of gravity height.
This method gives us a definitive point on which the body's gravitational effects would perfectly balance out. Recognizing that not all parts weigh the same is crucial for accurate results. This mathematical approach finds applications not just in physics, but also in various fields where decisions must consider varying factors, such as finance and engineering.
Human Body Mechanics
Human body mechanics is the study of how our bodies move and stabilize themselves under different forces. When assessing the center of gravity, as presented in the exercise, it involves understanding how distributed mass and forces affect the stability and balance of our body.

In this context:
  • The head, torso, and arms constitute the upper part of the body, significantly impacting the overall center of gravity due to their weight.
  • The legs provide support and contribute to the body's lower section weight distribution, balancing the upper mass parts.
Each body part's position and mass distribution affect how we move and maintain balance, standing or walking. Understanding these mechanics is vital for fields like physiotherapy and ergonomics, primarily focusing on designing better ergonomic furniture or understanding gait patterns. Knowing the center of gravity helps in enhancing performance, ensuring stability, and preventing injuries during physical activities.
Physics Problem Solving
Physics problem solving often involves several systematic steps to break down a complex problem into simpler parts. Understanding and identifying each component's contribution is key, as shown in calculating the overall center of gravity in the exercise.

Here’s a simplified approach to solving similar physics problems:
  • Identify all components involved and their respective properties, such as weight and center of gravity.
  • Apply relevant formulas accurately, like using weighted averages for calculating centers.
  • Perform necessary mathematical calculations, systematically and accurately, ensuring units and measurements are consistent.
  • Always interpret the results in context to understand what they mean physically or practically.
This structured method applies to many areas within physics, like mechanics, kinematics, and dynamics. It helps build a strong foundation in analyzing real-world problems by breaking them down into manageable parts, ensuring that students and practitioners alike can find meaningful solutions with confidence and accuracy.

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Most popular questions from this chapter

The parallel axis theorem provides a useful way to calculate the moment of inertia \(I\) about an arbitrary axis. The theorem states that \(I=I_{c m}+M h^{2},\) where \(I_{c m}\) is the moment of inertia of the object relative to an axis that passes through the center of mass and is parallel to the axis of interest, \(M\) is the total mass of the object, and \(h\) is the perpendicular distance between the two axes. Use this theorem and information to determine an expression for the moment of inertia of a solid cylinder of radius \(R\) relative to an axis that lies on the surface of the cylinder and is perpendicular to the circular ends.

A stationary bicycle is raised off the ground, and its front wheel \((m=1.3 \mathrm{kg})\) is rotating at an angular velocity of 13.1 rad/s (see the drawing). The front brake is then applied for \(3.0 \mathrm{s},\) and the wheel slows down to \(3.7 \mathrm{rad} / \mathrm{s}\). Assume that all the mass of the wheel is concentrated in the rim, the radius of which is \(0.33 \mathrm{m}\). The coefficient of kinetic friction between each brake pad and the rim is \(\mu_{\mathrm{k}}=0.85 .\) What is the magnitude of the normal force that each brake pad applies to the rim?

A hiker, who weighs 985 N, is strolling through the woods and crosses a small horizontal bridge. The bridge is uniform, weighs \(3610 \mathrm{N}\), and rests on two concrete supports, one at each end. He stops one-fifth of the way along the bridge. What is the magnitude of the force that a concrete support exerts on the bridge (a) at the near end and (b) at the far end?

Just after a motorcycle rides off the end of a ramp and launches into the air, its engine is turning counterclockwise at 7700 rev/min. The motorcycle rider forgets to throttle back, so the engine"s angular speed increases to 12.500 rev/min. As a result, the rest of the motorcycle (including the rider) begins to rotate clockwise about the engine at 3.8 rev/min. Calculate the ratio \(I_{\mathrm{L}} / I_{\mathrm{M}}\) of the moment of inertia of the engine to the moment of inertia of the rest of the motorcycle (and the rider). Ignore torques due to gravity and air resistance.

A clay vase on a potter's wheel expericnces an angular acceleration of \(8.00 \mathrm{rad} / \mathrm{s}^{2}\) due to the application of a \(10.0-\mathrm{N} \cdot \mathrm{m}\) net torque. Find the total moment of inertia of the vase and potter's wheel.

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