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A rugby player kicks the ball with an initial velocity of 40 m/s at an angle of 10 degrees. The goal posts are 40 m away and 2.2 m high. Will the ball go over the posts?

Physics

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Step-by-step solution

4 steps

Here's the breakdown of the problem:

Given:

  • Distance to goal post, x=40.0 mx = 40.0 \text{ m}
  • Height of goal post, hgoal=2.2 mh_{goal} = 2.2 \text{ m}
  • Initial velocity, v0=40 m/sv_0 = 40 \text{ m/s}
  • Launch angle, θ=10∘\theta = 10^\circ
  • Acceleration due to gravity, g=9.8 m/s2g = 9.8 \text{ m/s}^2 (assumed)

Part a· 4 steps

Show by calculation whether she hits the target or not.

  1. Resolve the initial velocity into horizontal and vertical components.

    v0x=v0cos⁡θ=40cos⁡(10∘) m/sv_{0x} = v_0 \cos \theta = 40 \cos(10^\circ) \text{ m/s} v0y=v0sin⁡θ=40sin⁡(10∘) m/sv_{0y} = v_0 \sin \theta = 40 \sin(10^\circ) \text{ m/s}

    Using a calculator:

    v0x≈40×0.9848=39.392 m/sv_{0x} \approx 40 \times 0.9848 = 39.392 \text{ m/s} v0y≈40×0.1736=6.944 m/sv_{0y} \approx 40 \times 0.1736 = 6.944 \text{ m/s}
  2. Calculate the time (tt) it takes for the ball to reach the horizontal distance of the goal post.

    The horizontal motion is uniform (constant velocity).

    x=v0xtx = v_{0x} t 40.0 m=(39.392 m/s)t40.0 \text{ m} = (39.392 \text{ m/s}) t t=40.039.392 st = \frac{40.0}{39.392} \text{ s} t≈1.0154 st \approx 1.0154 \text{ s}
  3. Calculate the vertical height (yy) of the ball at this time tt.

    The vertical motion is uniformly accelerated.

    y=v0yt−12gt2y = v_{0y} t - \frac{1}{2}gt^2 y=(6.944 m/s)(1.0154 s)−12(9.8 m/s2)(1.0154 s)2y = (6.944 \text{ m/s})(1.0154 \text{ s}) - \frac{1}{2}(9.8 \text{ m/s}^2)(1.0154 \text{ s})^2 y=7.050 m−(4.9 m/s2)(1.0310 s2)y = 7.050 \text{ m} - (4.9 \text{ m/s}^2)(1.0310 \text{ s}^2) y=7.050 m−5.052 my = 7.050 \text{ m} - 5.052 \text{ m} y=1.998 my = 1.998 \text{ m}
  4. Compare the calculated height with the goal post height.

    The height of the ball when it reaches the goal post is 1.998 m1.998 \text{ m}.
    The height of the goal post is 2.2 m2.2 \text{ m}.
    Since 1.998 m<2.2 m1.998 \text{ m} < 2.2 \text{ m}, the ball passes under the crossbar. It is also above the ground (0 m0 \text{ m}). Therefore, the ball hits the target (the goal area).

    The ball hits the target.\boxed{\text{The ball hits the target.}}

Part b

Explain why the ball is parabolic.

The trajectory of the ball is parabolic because it undergoes two independent motions simultaneously:

  • Horizontal motion: The ball moves with a constant velocity in the horizontal direction (assuming no air resistance).
  • Vertical motion: The ball experiences a constant downward acceleration due to gravity.

The combination of these two motions—one uniform and one uniformly accelerated—results in a path that is a segment of a parabola.

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