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Ch 06: Dynamics I: Motion Along a Line
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Non è quello che usi tu?Cambia libro di testo
Capitolo 6, Problema 63a

A ball is shot from a compressed-air gun at twice its terminal speed. What is the ball's initial acceleration, as a multiple of g, if it is shot straight up?

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Step 1: Understand the forces acting on the ball. When the ball is shot straight up, two forces act on it: gravity (mg, where m is the mass of the ball and g is the acceleration due to gravity) and air resistance. The air resistance is proportional to the square of the velocity, given by F_air = kv², where k is a constant and v is the velocity.
Step 2: Recall the concept of terminal velocity. At terminal velocity, the force of air resistance equals the force of gravity, i.e., kv_terminal² = mg. From this, you can express the constant k as k = mg / v_terminal².
Step 3: Substitute the initial velocity into the air resistance formula. The ball is shot at twice its terminal velocity, so the initial velocity is v_initial = 2v_terminal. The air resistance at this velocity is F_air_initial = k * (v_initial)² = k * (2v_terminal)² = 4k * v_terminal².
Step 4: Combine the forces to find the net force. The net force acting on the ball is F_net = F_air_initial - mg = 4k * v_terminal² - mg. Substitute k = mg / v_terminal² into this equation to get F_net = 4(mg / v_terminal²) * v_terminal² - mg = 4mg - mg = 3mg.
Step 5: Relate the net force to acceleration. Using Newton's second law, F_net = ma, the acceleration of the ball is a = F_net / m = (3mg) / m = 3g. Therefore, the ball's initial acceleration is 3 times the acceleration due to gravity.

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Terminal Velocity

Terminal velocity is the constant speed an object reaches when the force of gravity pulling it down is balanced by the drag force acting against it. For a ball shot from a compressed-air gun, its terminal speed is the maximum speed it would achieve if it were falling freely through the air, where the acceleration due to gravity is countered by air resistance.
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Escape Velocity

Acceleration

Acceleration is the rate of change of velocity of an object with respect to time. In this context, when the ball is shot upwards, it experiences an initial acceleration due to the force exerted by the compressed air, which can be expressed as a multiple of gravitational acceleration (g). This initial acceleration will decrease as the ball rises and air resistance increases.
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Intro to Acceleration

Newton's Second Law of Motion

Newton's Second Law states that the force acting on an object is equal to the mass of the object multiplied by its acceleration (F = ma). This principle is crucial for calculating the initial acceleration of the ball, as the net force acting on it when shot upwards is the difference between the force from the compressed air and the gravitational force acting downwards.
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Intro to Forces & Newton's Second Law
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