4. In Scenario 1, a student stands on a stationary sidewalk and observes a battery-powered cart moving in a straight line to the east, as shown in Figure 1. The cart's velocity relative to the ground is constant with magnitude 2.0 m/s. At time t = 0 s, the cart is at position x = 0 m. Ignore air resistance and friction so the cart's acceleration is zero.
In Scenario 2, the same cart moves on the sidewalk exactly as in Scenario 1, but the motion is now observed by a runner moving to the east at a constant speed of 1.5 m/s relative to the ground. At time t = 0 s, the runner is at the same location as the cart. Ignore air resistance and friction so the cart's acceleration is zero in the ground frame.
Figure 1. Two observers describing the same cart motion along an east–west sidewalk (ground frame vs runner frame).
Refer to Figure 1. Indicate whether the magnitude of the cart's average velocity measured by the runner in Scenario 2, , is greater than, less than, or equal to the magnitude of the cart's average velocity measured by the student on the ground in Scenario 1, , by writing one of the following in your answer booklet.
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Justify your answer by describing, in words, the vector velocities involved and the one-dimensional vector sum (or difference) needed to determine the cart's velocity relative to the runner. Use qualitative reasoning beyond referencing equations.
Starting with a fundamental definition of average velocity and the definition of relative position, derive an expression for the cart's average velocity relative to the runner, , in terms of and . Express your final answer as a one-dimensional vector equation that includes appropriate direction (sign). Begin your derivation by writing a fundamental physics principle or an equation from the reference information. Consider the general case in which an object (the cart) moves with constant velocity relative to the ground, and an observer (the runner) moves with constant velocity relative to the ground. Both velocities are along the same straight line (east-west), and east is the positive direction.
Indicate whether the expression you derived in part B is or is not consistent with the claim made in part A. Briefly justify your answer by referencing how the magnitudes compare when and , and by stating the direction of .
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