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Ch 09: Work and Kinetic Energy
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 9, Problem 56

A 50 g rock is placed in a slingshot and the rubber band is stretched. The magnitude of the force of the rubber band on the rock is shown by the graph in FIGURE P9.56. The rubber band is stretched 30 cm and then released. What is the speed of the rock?

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Step 1: Analyze the graph provided. The graph shows the force exerted by the rubber band (F) as a function of the stretch distance (x). The force increases linearly from 0 N at 0 cm to 30 N at 30 cm. This indicates that the force follows Hooke's Law, where F = kx, and k is the spring constant.
Step 2: Determine the spring constant (k) using the slope of the graph. From the graph, the force increases by 30 N over a stretch of 30 cm (0.30 m). Using the formula k = F/x, calculate k = 30 N / 0.30 m.
Step 3: Calculate the elastic potential energy stored in the rubber band when stretched to 30 cm. The formula for elastic potential energy is U = (1/2)kx², where x is the stretch distance in meters. Substitute the values of k and x into the formula.
Step 4: Use the principle of energy conservation. The elastic potential energy stored in the rubber band is converted into the kinetic energy of the rock when the rubber band is released. The formula for kinetic energy is KE = (1/2)mv², where m is the mass of the rock and v is its speed. Set U = KE and solve for v.
Step 5: Convert the mass of the rock from grams to kilograms (50 g = 0.050 kg) and substitute the values of m and U into the equation to solve for v. This will give the speed of the rock.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Hooke's Law

Hooke's Law states that the force exerted by a spring or elastic material is directly proportional to the amount it is stretched or compressed, up to its elastic limit. Mathematically, it is expressed as F = kx, where F is the force, k is the spring constant, and x is the displacement from the equilibrium position. In this scenario, the rubber band behaves similarly, and the graph illustrates how the force increases with the stretch of the band.
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Work-Energy Principle

The Work-Energy Principle states that the work done on an object is equal to the change in its kinetic energy. When the rubber band is stretched, work is done on the rock, converting potential energy stored in the rubber band into kinetic energy when the band is released. This principle allows us to calculate the speed of the rock after it is released by equating the work done to the kinetic energy gained.
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Kinetic Energy

Kinetic energy is the energy an object possesses due to its motion, defined by the equation KE = 1/2 mv², where m is the mass of the object and v is its velocity. In this problem, once the rubber band releases the rock, the potential energy stored in the stretched band converts into kinetic energy, allowing us to determine the speed of the rock based on its mass and the energy transferred.
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Related Practice
Textbook Question

A spring of equilibrium length L₁ and spring constant k₁ hangs from the ceiling. Mass m₁ is suspended from its lower end. Then a second spring, with equilibrium length L₂ and spring constant k₂, is hung from the bottom of m₁. Mass m₂ is suspended from this second spring. How far is m₂ below the ceiling?

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Textbook Question

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Textbook Question

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A red ball has a mass of 250 g. A constant force pushes the red ball horizontally and launches it at a speed of 15 m/s. The same force pushes a green ball through the same distance, launching it at 25 m/s. What is the mass of the green ball?

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Textbook Question

A 30 g mass is attached to one end of a 10-cm-long spring. The other end of the spring is connected to a frictionless pivot on a frictionless, horizontal surface. Spinning the mass around in a circle at 90 rpm causes the spring to stretch to a length of 12 cm. What is the value of the spring constant?

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