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\(\mathrm{A}_{\text {gunncr wants to firc a gun at a targct at a horixontal dis- }}\) tance of \(12500 \mathrm{~m}\) from his position. (a) If hii gun firos with a murzle spoed of \(700 \mathrm{~m} / \mathrm{s}\) and if \(g=9.81 \mathrm{~m} / \mathrm{M}^{2}\), what in the correet clevation anglc? Pretend that therc is no air resistence. (b) If the gunser mistaluenly astumes \(g=9.80 \mathrm{~m} / \mathrm{s}^{2}\), by how many meten will he nits the target?

Short Answer

Expert verified
(a) \( 7.24^\circ \); (b) 15.74 meters.

Step by step solution

01

Understand the Problem

We are looking at projectile motion where a gun is fired, and we need to find two things: (a) the correct elevation angle for hitting a target 12500 m away with a muzzle speed of 700 m/s, considering gravity as 9.81 m/s², and (b) the miss distance if gravity is mistakenly assumed to be 9.80 m/s².
02

Set Up Projectile Motion Equations

The horizontal distance (range) formula for projectile motion is \( R = \frac{v_0^2 \sin(2\theta)}{g} \), where \( v_0 \) is the initial velocity, \( \theta \) is the angle of elevation, and \( g \) is the gravitational acceleration.
03

Solve for the Elevation Angle (Correct g)

We are given \( R = 12500 \), \( v_0 = 700 \), and \( g = 9.81 \). Substitute into the equation: \[ 12500 = \frac{700^2 \sin(2\theta)}{9.81} \] Solve for \( \sin(2\theta) \) and then \( \theta \). \( \sin(2\theta) = \frac{12500 \times 9.81}{700^2} \). Calculate \( 2\theta \) using \( \sin^{-1} \) and then find \( \theta \).
04

Calculate the Sin Inverse and Find\( \theta \)

Compute \( \sin(2\theta) \) as \( \sin(2\theta) = \frac{12500 \times 9.81}{490000} = 0.25 \). Thus, \( 2\theta = \sin^{-1}(0.25) \approx 14.48^\circ \). Therefore, \( \theta = \frac{14.48}{2} = 7.24^\circ \).
05

Solve for Miscalculation in Case of Wrong g

Find the range with the incorrect gravity: \( g = 9.80 \), use the same angle \( \theta = 7.24^\circ \). Recalculate \( R' \) as \[ R' = \frac{700^2 \sin(2 \times 7.24)}{9.80} \approx 12515.74 \text{ m} \].
06

Calculate the Miss Distance

The miss distance is the difference between the calculated range at incorrect \( g \) and the target range: \( \text{Miss distance} = R' - R = 12515.74 - 12500 = 15.74 \text{ m} \).

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Key Concepts

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

Elevation Angle
In projectile motion, the elevation angle, represented as \( \theta \), is the angle at which an object, like a projectile or missile, is launched relative to the horizontal. This angle significantly influences the range and trajectory of the projectile.
  • The angle determines how high and how far the projectile will travel. A steeper angle results in a higher but shorter trajectory, while a shallower angle may not achieve the desired height or distance.
  • The goal is typically to calculate this angle for a projectile to hit a specific target.
To solve for the elevation angle, we use the projectile motion equation: \[ R = \frac{v_0^2 \sin(2\theta)}{g} \]where \( R \) is the range, \( v_0 \) is the initial velocity, and \( g \) is the gravitational acceleration. Solving for \( \theta \) involves rearranging this formula and finding \( \sin(2\theta) \) using the desired range and known values of initial velocity and gravity. This gives us the correct launch angle to hit a target precisely at a specific horizontal distance.
Initial Velocity
Initial velocity, noted as \( v_0 \), plays a crucial role in determining the range and path of a projectile. It is the speed at which the projectile is fired or launched.
  • High initial velocity allows the projectile to travel farther and can be achieved through increased force at launch.
  • This speed influences both the horizontal range and the time the projectile remains in the air.
For horizontal motion in projectile problems, the initial velocity is particularly important because it affects the calculation of how far and how long the projectile can travel. Given \[ R = \frac{v_0^2 \sin(2\theta)}{g} \],increasing \( v_0 \) will increase the range \( R \), assuming \( \theta \) and \( g \) remain constant.
In practical scenarios, engineers and scientists calculate this carefully to ensure that the projectile reaches its intended target efficiently.
Gravitational Acceleration
Gravitational acceleration, denoted by \( g \), is the acceleration due to gravity experienced by an object in freefall. It affects the vertical motion of a projectile while exerting a consistent downward force.
  • On Earth, standard gravitational acceleration is approximately \( 9.81 \, \mathrm{m/s^2} \).
  • This value can slightly vary depending on geographical location.
  • It's a critical factor in calculating the projectile's trajectory and range.
In projectile motion equations, gravity acts on the object, slowing it down as it ascends and accelerating it back towards the ground on descent. When computing the distance a projectile will travel with \( R = \frac{v_0^2 \sin(2\theta)}{g} \), gravity reduces the horizontal distance by pulling the projectile downward. Even a small change, like misassuming \( g = 9.80 \, \mathrm{m/s^2} \) instead of \( 9.81 \, \mathrm{m/s^2} \), can result in a significant miss over long distances, as shown in the original exercise.

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