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Ch 09: Work and Kinetic Energy
Chapter 9, Problem 9

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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Identify the given values for the red ball: mass (m1) = 250 g (convert this to kilograms by dividing by 1000), initial velocity (u1) = 0 m/s (assuming it starts from rest), final velocity (v1) = 15 m/s.
Use the work-energy theorem, which states that the work done on an object is equal to its change in kinetic energy (KE). Calculate the change in kinetic energy for the red ball using the formula \( KE = \frac{1}{2} m v^2 \), where m is the mass and v is the velocity.
Assume the same force acts over the same distance for both balls, meaning the work done on both balls is the same. Thus, the change in kinetic energy for the red ball equals the change in kinetic energy for the green ball.
Identify the given values for the green ball: initial velocity (u2) = 0 m/s, final velocity (v2) = 25 m/s. Use the kinetic energy formula to set up an equation for the green ball, substituting the kinetic energy found for the red ball.
Solve the equation for the mass of the green ball (m2). This involves isolating m2 on one side of the equation and solving for it using the kinetic energy values and the final velocity of the green ball.

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Key Concepts

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

Newton's Second Law of Motion

Newton's Second Law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This relationship is expressed by the formula F = ma, where F is the force, m is the mass, and a is the acceleration. Understanding this law is crucial for analyzing how different masses respond to the same force.
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Kinetic Energy

Kinetic energy is the energy an object possesses due to its motion, calculated using the formula KE = 1/2 mv², where m is the mass and v is the velocity. In this problem, the kinetic energy of both balls can be compared to understand how the different speeds relate to their masses when acted upon by the same force.
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Conservation of Energy

The principle of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. In this scenario, the work done by the constant force on both balls translates into kinetic energy, allowing us to set up equations that relate the forces, distances, and resulting speeds to find the unknown mass of the green ball.
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