(II) A projectile is shot from the edge of a cliff 125 m above ground level with an initial speed of 62.0 m/s at an angle of 35.0° with the horizontal, as shown in Fig. 3–45. At the instant just before the projectile hits point P, find the angle made by the velocity vector with the horizontal.
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5. Projectile Motion
Positive (Upward) Launch
Problem 52a
Textbook Question
(II) Romeo is throwing pebbles gently up to Juliet's window, and he wants the pebbles to hit the window with only a horizontal component of velocity. He is standing at the edge of a rose garden 8.0 m below her window and 8.5 m from the base of the wall (Fig. 3–48). How fast are the pebbles going when they hit her window?
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Identify the key components of the problem: The pebble must hit the window with only a horizontal velocity component. This means the vertical velocity at the window must be zero. The vertical displacement is 8.0 m, and the horizontal displacement is 8.5 m.
Use the kinematic equation for vertical motion to determine the time it takes for the pebble to reach the window. The equation is: , where is the vertical displacement, is the initial vertical velocity, is the acceleration due to gravity (9.8 m/s²), and is the time. Since the vertical velocity at the window is zero, solve for .
Once the time is determined, use the horizontal motion equation to find the horizontal velocity. The equation is: , where is the horizontal displacement, is the horizontal velocity, and is the time. Solve for .
Combine the results from the vertical and horizontal motion equations to determine the initial velocity components. The horizontal velocity will also be the final velocity since there is no horizontal acceleration.
Verify the solution by checking that the pebble's trajectory satisfies both the vertical and horizontal displacements. Ensure that the calculated horizontal velocity allows the pebble to travel 8.5 m horizontally in the same time it takes to fall 8.0 m vertically.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Projectile Motion
Projectile motion refers to the motion of an object that is thrown or projected into the air, subject to the force of gravity. It can be analyzed in two dimensions: horizontal and vertical. The horizontal component of motion is uniform, while the vertical component is influenced by gravitational acceleration. Understanding this concept is crucial for determining the trajectory and impact velocity of the pebbles.
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Introduction to Projectile Motion
Kinematic Equations
Kinematic equations describe the motion of objects under constant acceleration. They relate displacement, initial velocity, final velocity, acceleration, and time. In this scenario, the vertical motion of the pebbles can be analyzed using these equations to find the time it takes for the pebbles to fall 8.0 m, which is essential for calculating their horizontal velocity when they reach Juliet's window.
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Kinematics Equations
Velocity Components
Velocity components break down the overall velocity of an object into its horizontal and vertical parts. In this problem, the goal is to ensure that the pebbles have only a horizontal component of velocity upon reaching the window. By understanding how to calculate and manipulate these components, one can determine the required initial speed of the pebbles to achieve the desired impact condition.
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Calculating Velocity Components
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