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The compressive strength of our bones is important in everyday life. Young's modulus for bone is about 1.4 \(\times\) 10\(^{10}\) Pa. Bone can take only about a 1.0% change in its length before fracturing. (a) What is the maximum force that can be applied to a bone whose minimum cross-sectional area is 3.0 cm\(^2\)? (This is approximately the crosssectional area of a tibia, or shin bone, at its narrowest point.) (b) Estimate the maximum height from which a 70-kg man could jump and not fracture his tibia. Take the time between when he first touches the floor and when he has stopped to be 0.030 s, and assume that the stress on his two legs is distributed equally.

Short Answer

Expert verified
a) 4.2 x 10^4 N, b) Maximum safe height is practically negligible with given conditions.

Step by step solution

01

Understanding Young's Modulus

Young's modulus \( E \) is the ratio of stress to strain. It helps us determine the force a material can withstand without deforming. Young's modulus for bone is given as \( E = 1.4 \times 10^{10} \, \text{Pa} \). A bone can handle a maximum strain of 1.0%, or 0.01 in decimal form (strain = change in length/original length). We use this information to calculate maximum stress and force.
02

Calculating Maximum Stress

Stress is defined as force per unit area, \( \sigma = \frac{F}{A} \). From Young's modulus, \( \sigma = E \times \text{strain} \). Substitute \( E = 1.4 \times 10^{10} \, \text{Pa} \) and strain = 0.01. Thus, maximum stress \( \sigma = 1.4 \times 10^{10} \, \text{Pa} \times 0.01 = 1.4 \times 10^{8} \, \text{Pa} \).
03

Calculating Maximum Force

To find the maximum force \( F \), use the relation \( F = \sigma \times A \). Here, \( \sigma = 1.4 \times 10^{8} \, \text{Pa} \) and \( A = 3.0 \, \text{cm}^2 = 3.0 \times 10^{-4} \, \text{m}^2 \). Therefore, \( F = 1.4 \times 10^{8} \, \text{Pa} \times 3.0 \times 10^{-4} \, \text{m}^2 = 4.2 \times 10^{4} \, \text{N} \).
04

Determining Safe Jump Height

To estimate the maximum jump height without fracturing, use the impulse-momentum relation. The impulse \( F \times t \) equals the change in momentum \( m \times v \) of the man. Calculate \( v_f \) using \( v_f = g \times t \), where \( g = 9.81 \, \text{m/s}^2 \) and \( t = 0.030 \, \text{s} \). Thus, \( v_f = 9.81 \, \text{m/s}^2 \times 0.030 \, \text{s} = 0.2943 \, \text{m/s} \).
05

Using Energy Conservation for Jump Height

Assume the man jumps from height \( h \), converting potential energy to kinetic energy: \( mgh = \frac{1}{2} mv_f^2 \). Simplifying, \( gh = \frac{1}{2} v_f^2 \). Substituting \( v_f = 0.2943 \, \text{m/s} \) results in \( 9.81 \, \text{m/s}^2 \times h = 0.5 \times (0.2943 \, \text{m/s})^2 \). Solve for \( h = 0.0044 \, \text{m} \), which is very small, indicating that practically any jump requiring these conditions is negligible or requires "improved mechanics".

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Key Concepts

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

Compressive Strength
Compressive strength is a vital aspect when considering the durability and performance of materials under stress. For bones, it's crucial since they bear the weight of the body and withstand various forces throughout daily activities. When a force compresses a material, it could either deform or fracture it. The compressive strength is the maximum force a material like bone can withstand without compromising its structure. In our scenario, the tibia, being a load-bearing bone, should not exceed the determined maximum force to prevent fracturing. In practical terms, if a force higher than the compressive strength is applied, the material will likely fail or break. This concept is instrumental in fields like construction and biology, ensuring that designs and materials can meet the demands placed on them throughout their usage. Understanding and calculating compressive strength helps in designing safer structures and understanding how biological entities withstand everyday forces.
Maximum Stress
Maximum stress is another critical concept, representing the highest stress a material can endure before it starts to deform or fail. Stress is what determines how a material behaves under the application of an external force and is calculated using stress formula: \[ \sigma = \frac{F}{A} \]Where \( F \) is the force applied, and \( A \) is the area over which the force is applied.It's tightly linked to Young's modulus, which provides a relationship between stress and strain (the relative change in shape or size due to applied stress). In the problem described, we calculate the maximum stress by multiplying Young's modulus by the maximum strain the bone can bear, giving us a specific figure in pascals. This stress is distributed over the cross-sectional area of the tibia, leading to an objective analysis of how much force it can safely handle. Recognizing these limits ensures the materials are used within their safe operational capacities and helps predict the conditions leading to potential failure.
Kinetic Energy Conservation
Understanding kinetic energy conservation is vital in predicting how objects move and interact in systems. When a person jumps, their potential energy is converted into kinetic energy as they descend, accelerating under gravity. The principle of energy conservation implies that the energy before the jump (potential energy) transforms into kinetic energy at the point of landing.The potential energy can be expressed as \[ PE = mgh \]and kinetic energy as \[ KE = \frac{1}{2} mv^2 \].In a jump scenario, when the man impacts the ground, the kinetic energy developed is absorbed by his muscles and bones. Proper conservation and conversion of these energies help avoid fractures or injuries, highlighting how we naturally use energy efficiently during simple actions like jumping or walking. Understanding this concept is crucial not only in physics but also in physical therapy and sports science, where energy mechanics are pivotal to performance and injury prevention.
Impulse-Momentum Relation
The impulse-momentum relation interlinks the concepts of force, time, and momentum. It finds extensive application in understanding how forces change the momentum of objects over time—highlighting its importance in dynamic scenarios like jumping or collisions.The impulse is defined as the product of force and time (\( Ft \)), impacting momentum, which is the product of mass and velocity \( (mv) \). The relation is given as: \[ Ft = \Delta(mv) \],indicating how the momentum change \( (\Delta mv) \) of an object is influenced by the impulse applied. In our exercise, when a person lands from a jump, the time to halt plays a crucial role in cushioning the impact force. Knowing the impulse helps in determining the necessary mechanical stress and its distribution over time to remain safe without over-straining the bones.This relation is integral to designing safe sports equipment and understanding the biomechanics of human movement, ensuring that impacts are absorbed effectively by the body.

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