4. In Scenario 1, an observer standing on the ground next to a level track (ground reference frame) watches a cart move to the right at a constant speed of 2.0 m/s, as shown in Figure 1. At the instant the cart passes the observer, a student on the cart throws a ball straight upward relative to the cart with an initial speed of 6.0 m/s. The ball is released from a height of 1.2 m above the ground. Air resistance is negligible, and the acceleration due to gravity has magnitude 9.8 m/s^2 downward. The ground observer measures the ball's initial velocity components as vₓ,1 and vᵧ,1 and measures the ball's average velocity over the first 0.50 s after release as .
In Scenario 2, an observer riding on the cart (cart reference frame) analyzes the same throw. The cart observer measures the cart as at rest, and measures the ball's initial velocity components as vₓ,2 and vᵧ,2 and measures the ball's average velocity over the first 0.50 s after release as . Air resistance is negligible, and the acceleration due to gravity has magnitude 9.8 m/s^2 downward.
Figure 1. Cart moving at constant speed; ball thrown straight upward relative to the cart at the instant the cart passes a ground observer.
Refer to Figure 1. Indicate whether the magnitude of the average velocity measured over the first 0.50 s after release is greater in Scenario 1 than in Scenario 2 by writing one of the following in your answer booklet.
•
•
•
Justify your answer by discussing the change in the ball's position in the horizontal and vertical directions during the 0.50 s interval as measured in each reference frame. Use qualitative reasoning beyond referencing equations.
Starting with the definition of average velocity, derive an expression for the magnitude of the ground-frame average velocity of the ball over the interval . Express your answer in terms of , , , and . Begin your derivation by writing a fundamental physics principle or an equation from the reference information. Consider the general case in which, in the cart reference frame, the cart moves at a constant speed to the right relative to the ground and the ball is thrown straight upward relative to the cart with initial speed . Over a time interval after release, the acceleration of the ball is in both inertial frames.
Indicate whether the expression you derived in part B is or is not consistent with the claim you made in part A. Briefly justify your answer by referencing how affects in your expression from part B and comparing it to the cart-frame average velocity over the same interval.
00:00