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The bungee jumper, an 80 -kg man, falls from the bridge at \(A\) with the bungee cord secured to his ankles. He falls \(20 \mathrm{m}\) before the 17 -m length of elastic bungee cord begins to stretch. The \(3 \mathrm{m}\) of rope above the elastic cord has no appreciable stretch. The man is observed to drop a total of \(44 \mathrm{m}\) before being projected upward. Neglect any energy loss and calculate \((a)\) the stiffness \(k\) of the bungee cord (increase in tension per meter of elongation) (b) the maximum velocity \(v_{\max }\) of the man during his fall, and \((c)\) his maximum acceleration \(a_{\max } .\) Treat the man as a particle located at the end of the bungee cord.

Short Answer

Expert verified
(a) The stiffness \(k\) is approximately 601.67 N/m. (b) The maximum velocity \(v_{\max}\) is approximately 19.8 m/s. (c) The maximum acceleration \(a_{\max}\) is approximately 190.11 m/s².

Step by step solution

01

Analyze the Situation

An 80 kg man falls from a bridge. He falls 20 m before the full length of the elastic bungee cord begins to stretch. In total, he drops 44 m before reversing direction.
02

Calculate Elongation of Bungee Cord

The total drop is 44 m, and the non-elastic length before the cord stretches is 20 m. Therefore, the elongation of the bungee cord is 24 m (44 m - 20 m).
03

Apply Energy Conservation for Stiffness

Using the conservation of mechanical energy: the potential energy at maximum fall is equal to the elastic potential energy of the stretched bungee.\[ mgh = \frac{1}{2} k x^2 \]where \(m = 80 \, kg\), \(g = 9.81 \, m/s^2\) (acceleration due to gravity), \(h = 44 \, m\), and \(x = 24 \, m\) (elongation). Solve for \(k\):\[ 80 \times 9.81 \times 44 = \frac{1}{2} k (24)^2 \]\[ k = \frac{80 \times 9.81 \times 44 \times 2}{24^2} \approx 601.67 \, N/m \]
04

Find Maximum Velocity Using Energy Conservation

The maximum velocity occurs just before the bungee cord starts stretching. Using conservation of energy:\[ \text{Gravitational potential energy} = \text{Kinetic energy} \mgh_1 = \frac{1}{2} mv^2 \]where \(h_1 = 20 \, m\):\[ 80 \times 9.81 \times 20 = \frac{1}{2} \times 80 \times v^2 \]Solving for \(v\):\[ v = \sqrt{2 \times 9.81 \times 20} = \sqrt{392.4} \approx 19.8 \, m/s \]
05

Calculate Maximum Acceleration

The maximum acceleration occurs when the bungee cord is fully stretched. To find it, use Hooke's law:\[ F = kx \]so the net force is:\[ F_{ ext{net}} = mg + kx = ma_{\text{max}} \]\[ a_{\text{max}} = g + \frac{kx}{m} \]Substitute known values:\[ a_{\text{max}} = 9.81 + \frac{601.67 \times 24}{80} \approx 190.1145 \, m/s^2 \]

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Key Concepts

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

Mechanical Energy Conservation in Bungee Jumping
Understanding mechanical energy conservation is key when analyzing bungee jumping dynamics. Conservation of mechanical energy states that the total mechanical energy (sum of potential energy and kinetic energy) in a system remains constant, provided no external work is done, or energy losses occur (such as through air resistance).

In the case of a bungee jumper, the gravitational potential energy is converted into kinetic energy as the jumper falls and then into elastic potential energy as the bungee cord stretches. Initially, the jumper possesses potential energy due to their height above the ground. As they descend, this is converted to kinetic energy, bringing the jumper up to their maximum speed just before the bungee cord begins to elongate.

When the cord stretches, the mechanical energy is transformed once more, from kinetic to elastic potential energy. Through these energy transformations, we can use equations to calculate various physical quantities, such as the stiffness of the bungee cord, maximum velocity, and acceleration during the jump.
Hooke's Law and Bungee Cord Dynamics
Hooke’s Law is crucial for understanding how materials stretch or compress. It's particularly relevant to bungee jumping as it describes the behavior of the elastic bungee cord.

According to Hooke's Law, the force required to stretch or compress a spring is directly proportional to the displacement of the spring from its resting (unstretched) position. This relationship is expressed by the formula:
  • \( F = kx \)
where \( F \) is the force exerted, \( k \) is the stiffness (spring constant), and \( x \) is the displacement (or elongation) of the cord.

In the context of a bungee jump, this law helps calculate how much force acts on the jumper as the cord stretches. The stiffness of the bungee cord is a measure of how much tension increases with stretch, and it is determined using energy conservation principles by equating gravitational potential energy to the stored elastic potential energy. A correctly calculated stiffness ensures a safe and thrilling jump!
Maximum Velocity Calculation During a Jump
Determining the maximum velocity of the bungee jumper is a fascinating part of analyzing the jump dynamics. Maximum velocity occurs right before the bungee cord begins to stretch. At this point, the jumper's body is moving only under the influence of gravitational force.

Using energy conservation principles, we equate the initial potential energy to the kinetic energy at this critical point:
  • \( mgh = \frac{1}{2} mv^2 \)
Here, \( h \) is the height at which the cord starts to stretch (20 meters in this case). By rearranging the equation, the formula for velocity \( v \) becomes:
  • \( v = \sqrt{2gh} \)
Plug in the numbers, and you can determine that the jumper reaches a maximum speed of approximately 19.8 m/s. Understanding this helps in planning and safety checks for bungee jumping activities.
Acceleration Analysis at Maximum Stretch
Calculating the maximum acceleration of a bungee jumper can reveal interesting insights into the jump's most intense moment. This acceleration occurs when the cord is fully stretched, a crucial point in the jump.

At this stretch point, two main forces act on the jumper:
  • The gravitational force \( mg \)
  • The elastic restoring force \( kx \)
According to Newton's second law of motion, the net force \( F_{\text{net}} \) is the sum of these two forces.

The formula for maximum acceleration \( a_{\text{max}} \) is:
  • \[ a_{\text{max}} = g + \frac{kx}{m} \]
With this equation, you can calculate the maximum acceleration felt by the jumper, which can be quite significant at approximately 190.11 m/s². This not only contributes to the exhilaration but also underscores why ensuring proper equipment and techniques is essential for safety in bungee jumping.

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Most popular questions from this chapter

A car with a mass of \(1500 \mathrm{kg}\) starts from rest at the bottom of a 10 -percent grade and acquires a speed of \(50 \mathrm{km} / \mathrm{h}\) in a distance of \(100 \mathrm{m}\) with constant acceleration up the grade. What is the power \(P\) delivered to the drive wheels by the engine when the car reaches this speed?

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