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Falling with a parachute: If an average-size man jumps from an airplane with a properly opening parachute, his downward velocity \(v=v(t)\), in feet per second, \(t\) seconds into the fall is given by the following table. $$ \begin{array}{|c|c|} \hline \begin{array}{c} t=\text { Seconds } \\ \text { into the fall } \end{array} & v=\text { Velocity } \\ \hline 0 & 0 \\ \hline 1 & 16 \\ \hline 2 & 19.2 \\ \hline 3 & 19.84 \\ \hline 4 & 19.97 \\ \hline \end{array} $$ a. Explain why you expect \(v\) to have a limiting value and what this limiting value represents physically. b. Estimate the terminal velocity of the parachutist.

Short Answer

Expert verified
The terminal velocity is about 20 ft/s, which is when the velocity stops increasing significantly.

Step by step solution

01

Understanding Terminal Velocity

Terminal velocity is the constant velocity reached by an object when the force of gravity is balanced by the drag force of the air. For a parachutist, after some time into the fall, the velocity no longer increases significantly because the forces are balanced.
02

Identifying a Pattern in Velocity Data

Given the table, observe how the velocity increases substantially from 0 to 1 second, then incrementally from 2 to 4 seconds. The changes in velocity (9;s increase) become smaller as time progresses.
03

Recognizing the Approaching Limit

As time increases, the increase in velocity diminishes. From 3 to 4 seconds, the change in velocity is minimal (19.84 to 19.97 ft/s). This indicates the velocity is nearing a plateau.
04

Estimating the Limiting Value

From the data, the velocity changes approach zero as time progresses: from 19.84 to 19.97 ft/s is a small change. Thus, the limiting value can be estimated as the velocity when changes are negligible, approximately 20 ft/s.

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Key Concepts

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

Parachute Physics
Parachute physics is fascinating as it combines concepts from fluid dynamics and mechanics to explain how a parachutist descends safely. When a parachute deploys, it significantly increases air resistance, also known as drag force, acting against gravity. This increased air resistance is due to the large surface area that the parachute provides. The force of gravity tries to pull the parachutist downward, while the drag force works in the opposite direction, slowing down the fall.

A key concept in parachute physics is how this balance between gravity and drag determines the descent speed. Initially, when the parachute opens, there’s a sudden increase in air resistance. This causes a rapid deceleration from free fall, bringing the descent to a more controllable speed. Understanding how these forces balance is crucial in predicting how a parachutist will reach terminal velocity.
Velocity Patterns
Velocity patterns during a parachute descent show how speed changes over time. Initially, right after jumping, the parachutist accelerates quickly, as seen from the increase from 0 to 16 feet per second in the first second. This rapid increase is due to gravitational acceleration, briefly unopposed by sufficient drag.

Observing the table from the problem, the velocity's increase slows significantly after the parachute opens. From 2 seconds onward, the parachute's drag force negates much of gravity's pull, reducing the rate of acceleration. By examining the slower increases, such as 19.2 to 19.84 and then 19.84 to 19.97 feet per second, we see that each subsequent change is smaller. This pattern suggests that the parachutist’s velocity is approaching a steady limit, where change is negligible, indicating the reaching of terminal velocity.
Force Balance
The concept of force balance involves understanding how different forces interact during a parachute descent. The main forces at play are gravity and air resistance, or drag. Initially, gravity dominates, and the parachutist speeds up quickly. However, as the parachute opens, it dramatically increases drag, eventually balancing gravity.

When these forces equalize, no net force acts on the parachutist. This is when the descent becomes stable, and velocity barely changes. Analyzing the balance between these forces helps us grasp why the descent slows and eventually stabilizes. Understanding this balance is crucial for estimating when terminal velocity is reached. It portrays the perfect equilibrium where gravity and drag exactly counter each other, leading to a constant fall speed.
Limiting Behavior
Limiting behavior refers to how an object behaves as it reaches a constant state over time, like the parachutist reaching terminal velocity. Initially, high-speed changes occur, but as time progresses, these changes reduce and stabilize. In this scenario, the consistent decrease in velocity changes from 19.84 to 19.97 feet per second hints at this limiting behavior.

This behavior shows that eventually, the parachutist's acceleration ceases, and the velocity levels out to around 20 feet per second. Understanding this final state is important for ensuring safe parachuting. It means the parachutist can prepare for landing at a predictable and manageable speed. By comprehending limiting behavior, one can better plan for real-world applications, ensuring safety and control during activities involving similar physics principles.

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