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Ch 04: Kinematics in Two Dimensions
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 4, Problem 6a

A rocket-powered hockey puck moves on a horizontal frictionless table. FIGURE EX4.6 shows graphs of vx and vy, the x- and y-components of the puck's velocity. The puck starts at the origin. In which direction is the puck moving at t = 2s? Give your answer as an angle from the x-axis.

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Step 1: Analyze the graph for the x-component of velocity (v_x). From the graph, we observe that v_x is constant at -3 m/s for all times, including t = 2s. This indicates that the puck is moving in the negative x-direction with a constant velocity.
Step 2: Analyze the graph for the y-component of velocity (v_y). At t = 2s, the graph shows that v_y is decreasing linearly. By reading the graph, we find that v_y at t = 2s is approximately 3 m/s. This indicates that the puck is moving in the positive y-direction at this instant.
Step 3: Combine the x- and y-components of velocity to determine the direction of motion. The puck's velocity vector at t = 2s can be represented as v = (-3 m/s) î + (3 m/s) ĵ, where î and ĵ are unit vectors in the x- and y-directions, respectively.
Step 4: Calculate the angle of the velocity vector relative to the x-axis. The angle θ can be found using the formula tan(θ) = v_y / v_x. Substituting the values, tan(θ) = 3 / -3.
Step 5: Solve for θ using the inverse tangent function. Since v_x is negative and v_y is positive, the angle will be in the second quadrant. Use θ = arctan(v_y / v_x) and adjust for the quadrant to find the direction of motion relative to the x-axis.

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Key Concepts

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

Velocity Components

Velocity can be broken down into its components along the x and y axes. The x-component (vx) indicates motion in the horizontal direction, while the y-component (vy) indicates motion in the vertical direction. Understanding these components is crucial for determining the overall direction of an object's motion, especially when analyzing two-dimensional motion.
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Graph Interpretation

Graphs of velocity versus time provide visual representations of how an object's velocity changes over time. In this case, the x-component graph shows a constant velocity of -3 m/s, indicating motion to the left, while the y-component graph shows a linear decrease from 5 m/s to 0 m/s, indicating a change in vertical motion. Analyzing these graphs helps in determining the object's trajectory at specific time intervals.
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Direction of Motion

The direction of motion can be expressed as an angle relative to a reference axis, typically the x-axis. This angle can be calculated using the arctangent function, which relates the y-component to the x-component of velocity. By determining the signs and magnitudes of these components at a given time, one can find the angle of the puck's motion, which is essential for understanding its trajectory.
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