2013: Problem 3

Topic: Kinematics
Concepts: Limiting cases, Projectile motion


Solution:

  • A) This statement is true. Recall the range equation,

    R=v2sin(2θ)gR=\frac{v^2\sin(2\theta)}{g}

    which is maximized by θ=π/4\theta=\pi/4 so sin(2θ)=1\sin(2\theta)=1,

    Rmax=v2gR_{\text{max}}=\frac{v^2}{g}

    If Tom can throw with maximum speed vmaxv_{\text{max}}, then we need

    lRmax=vmax2gl \leq R_{\text{max}}=\frac{v^2_{\text{max}}}{g}
    vmaxglv_{\text{max}} \geq \sqrt{gl}

    for the ball to reach Wes.
  • B) This statement is true. When vmax=glv_{\text{max}}=\sqrt{gl}, there is only one trajectory which reaches Wes so tmin=tmax=tπ/4t_{\text{min}}=t_{\text{max}}=t_{\pi/4} where tπ/4t_{\pi/4} is the time taken by the trajectory with θ=π/4\theta=\pi/4. As vmaxv_{\text{max}} \rightarrow \infty, the window of times expands since at high speeds we can launch almost horizontally to reduce tt and almost vertically to increase tt.
  • C) This statement is true as explained in answer choice B.
  • D) This statement is true. At small ll, we have a range of times the ball could take to reach Wes depending on launch angle. As ll increases, this window closes since at the maximum l=vmax2/gl=v^2_{\text{max}}/g there is only one trajectory which reaches Wes so tmin=tmax=tπ/4t_{\text{min}}=t_{\text{max}}=t_{\pi/4}.
  • E) This statement is false as explained in answer choice D.

Thus, the answer is E.

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