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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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Most popular questions from this chapter

Darcy's law: The French hydrologist Henri Darcy discovered that the velocity \(V\) of underground water is proportional to the magnitude of the slope \(S\) of the water table (see Figure 1.71). The constant of proportionality in this case is the permeability of the medium through which the water is flowing. This proportionality relationship is known as Darcy's law, and it is important in modern hydrology. a. Using \(K\) as the constant of proportionality, express Darcy's law as an equation. b. Sandstone has a permeability of about \(0.041 \mathrm{me}-\) ter per day. If an underground aquifer is seeping through sandstone, and if the water table drops \(0.03\) vertical meter for each horizontal meter, what is the velocity of the water flow? Be sure to use appropriate units for velocity and keep all digits. c. Sand has a permeability of about 41 meters per day. If the aquifer from part a were flowing through sand, what would be its velocity?

A car that gets \(m\) miles per gallon: The cost of operating a car depends on the gas mileage \(m\) that your car gets, the cost \(g\) per gallon of gasoline, and the distance \(d\) that you drive. a. How much does it cost to drive 100 miles if your car gets 25 miles per gallon and gasoline costs 289 cents per gallon? b. Find a formula that gives the cost \(C\) as a function of \(m, g\), and \(d\). Be sure to state the units of each variable. c. Use functional notation to show the cost of driving a car that gets 28 miles per gallon a distance of 138 miles if gasoline costs \(\$ 2.99\) per gallon. Use the formula from part b to calculate the cost.

A rental: A rental car agency charges \(\$ 49.00\) per day and 25 cents per mile. a. Calculate the rental charge if you rent a car for 2 days and drive 100 miles. b. Use a formula to express the cost of renting a car as a function of the number of days you keep it and the number of miles you drive. Identify the function and each variable you use, and state the units. c. It is about 250 miles from Dallas to Austin. Use functional notation to express the cost to rent a car in Dallas, drive it to Austin, and return it in Dallas 1 week later. Use the formula from part b to calculate the cost.

A stock market investment: A stock market investment of \(\$ 10,000\) was made in 1970 . During the decade of the \(1970 \mathrm{~s}\), the stock lost half its value. Beginning in 1980, the value increased until it reached \(\$ 35,000\) in 1990 . After that its value has remained stable. Let \(v=v(d)\) denote the value of the stock, in dollars, as a function of the date \(d\). a. What are the values of \(v(1970), v(1980)\), \(v(1990)\), and \(v(2000)\) ? b. Make a graph of \(v\) against \(d\). Label the axes appropriately. c. Estimate the time when your graph indicates that the value of the stock was most rapidly increasing.

Research project: Look in a textbook for another class to find a function interesting to you that is given by a formula. Identify all the variables used in the formula, explaining the meaning of each variable. Explain how this formula is used.

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