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⚙️AP Physics C: Mechanics
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⚙️AP Physics C: Mechanics

FRQ 2 – Translation Between Representations
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Unit 1: Kinematics
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Practice FRQ 1 of 161/16

2. A student stands on a flatbed train car that is moving with constant velocity along a straight, horizontal track. The train car moves to the right with speed v0=12 m/sv_0 = 12 \text{ m/s}v0​=12 m/s relative to the ground, as shown in Figure 1. At time t=0t = 0t=0, when the student is at position x=0x = 0x=0 on the ground, the student throws a ball vertically upward with an initial speed u0=8.0 m/su_0 = 8.0 \text{ m/s}u0​=8.0 m/s relative to the train car. The ball is released from a height h=2.0 mh = 2.0 \text{ m}h=2.0 m above the ground. Air resistance is negligible.

Figure 1. Ball thrown straight up from a student standing on a flatbed train car moving right at v₀ = 12 m/s; release point is h = 2.0 m above the ground at ground position x = 0 at t = 0.

Figure 1

Figure 2. Frame S (Student’s reference frame): axes for drawing the ball’s velocity at t = 0.

Figure 2

Figure 3. Frame G (Ground reference frame): axes for drawing the ball’s velocity at t = 0.

Figure 3
A.

On Figure 2, draw an arrow to represent the velocity vector of the ball at t=0t = 0t=0 as observed in Frame S (the student's reference frame). On Figure 3, draw an arrow to represent the velocity vector of the ball at t=0t = 0t=0 as observed in Frame G (the ground reference frame). The length and direction of each arrow should accurately represent the magnitude and direction of the velocity vector in each reference frame. Label each arrow with its magnitude in m/s.

B.

Derive an expression for the horizontal distance Δx\Delta xΔx traveled by the ball, as measured in Frame G, from the moment it is released until it hits the ground. Express your answer in terms of v0v_0v0​, u0u_0u0​, hhh, ggg, and physical constants, as appropriate. Begin your derivation by writing a fundamental physics principle or an equation from the reference information. The ball reaches its maximum height and then falls to the ground.

Figure 4. Axes for v_y versus time t in Frame G (from release until the ball hits the ground).

Figure 4
C.

On the axes in Figure 4, sketch a graph of the vertical component of the velocity vyv_yvy​ of the ball as a function of time ttt in Frame G, from t=0t = 0t=0 until the ball hits the ground. Clearly indicate the initial value and any intercepts with the axes. The vertical component of the ball's velocity is analyzed as a function of time in Frame G.

D.

Describe one feature of the graph of the magnitude of the ball's acceleration ∣a⃗∣|\vec{a}|∣a∣ as a function of time in Frame G, from t=0t = 0t=0 until the ball hits the ground. Specifically state whether this magnitude increases, decreases, or remains constant. A second student on the ground at position x=0x = 0x=0 observes the ball's motion and records the ball's position vector as a function of time in Frame G. The position vector can be written as r⃗(t)=x(t)i^+y(t)j^\vec{r}(t) = x(t)\hat{i} + y(t)\hat{j}r(t)=x(t)i^+y(t)j^​, where x(t)x(t)x(t) and y(t)y(t)y(t) are the horizontal and vertical components of position.

Briefly justify your answer using physics principles.

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Free Response Question Practice

This practice environment simulates the AP AP Physics C: Mechanics Free Response Questions section. Here are some guidelines:

  • Read each question carefullybefore responding. Pay attention to command verbs like "identify," "explain," "analyze," or "evaluate."
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