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A spacecraft is in free fall toward the surface of the moon at a speed of \(1000 \mathrm{mph}\) (mi/h). Its retrorockets, when fired, provide a constant deceleration of \(20,000 \mathrm{mi} / \mathrm{h}^{2}\). At what height above the lunar surface should the astronauts fire the retrorockets to insure a soft touchdown? (As in Example 2, ignore the moon's gravitational field.)

Short Answer

Expert verified
Fire the retrorockets at 25 miles above the lunar surface.

Step by step solution

01

Understand the Problem

We need to determine the distance required for the spacecraft to decelerate from its initial velocity of 1000 mph to 0 mph using its retrorockets, which provide a deceleration of 20,000 mi/h². This will also be the height above the lunar surface where the retrorockets should be fired.
02

Use the Kinematic Equation

Use the kinematic equation that relates initial velocity, final velocity, acceleration, and distance: \[ v_f^2 = v_i^2 + 2a d \] where \( v_f \) is the final velocity (0 mph for soft landing), \( v_i \) is the initial velocity (1000 mph), \( a \) is the acceleration (which is -20,000 mi/h² due to deceleration), and \( d \) is the distance (height) we need to find.
03

Plug in the Known Values

Substitute the known values into the kinematic equation:\[ 0 = (1000)^2 + 2(-20,000) d \] This simplifies to: \[ 0 = 1,000,000 - 40,000 d \]
04

Solve for Distance

Rearrange the equation to solve for \( d \):\[ 40,000 d = 1,000,000 \] Divide both sides by 40,000:\[ d = \frac{1,000,000}{40,000} \] \[ d = 25 \]
05

Interpret the Result

The calculation shows that the retrorockets should be fired when the spacecraft is at a height of 25 miles above the lunar surface to ensure a soft landing.

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Key Concepts

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

Deceleration
Deceleration refers to the reduction in speed or velocity of an object. It is essentially the opposite of acceleration. In the context of spacecraft landing, deceleration is crucial to ensure that a spacecraft reduces its speed in a controlled manner to achieve a safe and gentle touchdown. For a spacecraft in free fall toward a surface, the deceleration process must be carefully calculated to avoid a crash. In our example, the spacecraft decelerates from an initial velocity of 1000 mph to 0 mph using its retrorockets. These retrorockets provide a constant negative acceleration, or deceleration, of 20,000 mi/h². To fully understand deceleration, it's important to note:
  • Deceleration is represented by a negative value in kinematic equations, indicating the reduction of velocity.
  • Proper control of deceleration is vital to match the desired final velocity, which, in the case of landing, is typically 0 mph for a soft touchdown.
  • In space missions, deceleration needs to account for various factors, including initial speed and available time for velocity reduction.
Understanding deceleration helps predict when and how the spacecraft's retrorockets should be employed to ensure smooth landings.
Kinematic Analysis
Kinematic analysis involves the study of motion without considering the forces that cause it. It is a fundamental aspect of physics that helps understand how objects behave when subjected to motion. In our example, kinematic analysis is used to calculate the height above the lunar surface at which the spacecraft should initiate deceleration for a safe landing.By employing the kinematic equation:\[ v_f^2 = v_i^2 + 2a d \]we can determine the required distance, or height, by substituting known values:- Final velocity, \(v_f\), is 0 mph (target velocity for landing)- Initial velocity, \(v_i\), is 1000 mph- Acceleration, \(a\), is \(-20,000\) mi/h² (indicative of deceleration)Solving this equation ultimately reveals the required distance, \(d\), which is 25 miles for achieving a complete stop just above the moon's surface. 😊 This highlights how kinematic equations serve as powerful tools in planning and executing space missions effectively.
Spacecraft Landing
Spacecraft landing is a complex process that requires precise calculations and careful planning. Achieving a soft landing involves reducing the velocity of a spacecraft to zero just before impact to avoid damage. In our exercise, this is accomplished through effective use of deceleration technologies like retrorockets. When preparing for a spacecraft landing:
  • It's crucial to calculate the appropriate point to begin deceleration to allow enough distance for the spacecraft to slow down.
  • Engineers must monitor initial speed and available braking time closely to ensure the spacecraft can reach the target velocity before landing.
  • The retrorockets' capacity to provide consistent deceleration is key to managing the spacecraft's descent rate.
In our scenario, calculating the ignition point for retrorockets at 25 miles above the lunar surface ensures the spacecraft lands smoothly, illustrating the importance of precision in space travel.

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