Small Salah

UEFA Champions League Quarter Final. Anfield. Liverpool vs Manchester City. 89th minute. Liverpool corner.

Jordan Henderson is taking the corner on the football pitch at Anfield, with dimensions shown above. Manchester City have cloned Vincent Kompany ( 1.93 1.93 m \text{m} tall), all around the penalty area as shown in blue. Mohamed Salah ( 1.75 1.75 m \text{m} tall) is standing on the penalty spot as shown in red.

Henderson wants to kick the ball to deliver the perfect cross to Salah for him to score a goal (and avoiding any blocks by Kompany). What is the least possible angle of elevation (to the nearest degree) above the horizontal pitch surface needed?

Details and assumptions:

  • The ball is a particle
  • Gravity acts uniformly downwards at 9.81 9.81 ms 2 \text{ms}^{-2}
  • The players have no thickness
  • The pitch is perfectly symmetrical / geometrically formed
  • None of the players jump
  • There is no sideways curve on the ball


The answer is 11.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Brian Moehring
Jul 10, 2018

The height of the ball above a point on the ground x m x \text{ m} from the corner is given by f ( x ; a ) = a x 2 + tan ( θ ) x f(x;a) = -ax^2 + \tan(\theta) x where a > 0 a > 0 is some parameter and θ \theta is the angle of elevation of the pitch.

Along the line segment from the corner to the penalty spot, we find that it is 1 1 2 + 3 4 2 = 1277 m \sqrt{11^2 + 34^2} = \sqrt{1277} \text{ m} to the penalty spot and [by similar triangles] 14 34 1277 = 7 17 1277 m \frac{14}{34}\sqrt{1277} = \frac{7}{17}\sqrt{1277} \text{ m} to the edge of the penalty area. Therefore, in order to make it over Kompany and to Salah, we must have f ( 1277 ; a ) 1.75 , f ( 7 17 1277 ; a ) 1.93 f\left(\sqrt{1277};a\right) \leq 1.75, \qquad f\left(\frac{7}{17}\sqrt{1277};a\right) \geq 1.93

In order to make the following steps easier, we will uniformly scale everything by a factor of 17 1277 \frac{17}{\sqrt{1277}} . This changes a > 0 a>0 to some other A > 0 A > 0 , but it doesn't change tan θ . \tan\theta\text{.} The result is f ( 17 ; A ) 17 1.75 1277 , f ( 7 ; A ) 17 1.93 1277 f\left(17;A\right) \leq \frac{17 \cdot 1.75}{\sqrt{1277}}, \qquad f\left(7;A\right) \geq \frac{17\cdot 1.93}{\sqrt{1277}} and by writing these out: 1 7 2 A + 17 tan θ 17 1.75 1277 17 A + tan θ 1.75 1277 49 A + 7 tan θ 17 1.93 1277 -17^2A + 17\tan\theta \leq \frac{17 \cdot 1.75}{\sqrt{1277}} \implies -17A + \tan\theta \leq \frac{1.75}{\sqrt{1277}} \\ -49A + 7\tan\theta \geq \frac{17\cdot 1.93}{\sqrt{1277}} Multiplying the first inequality by 49 -49 and the second by 17 17 gives 833 A + 49 tan θ 49 1.75 1277 833 A + 119 tan θ 289 1.93 1277 833A + -49\tan\theta \geq \frac{-49 \cdot 1.75}{\sqrt{1277}} \\ -833A + 119\tan\theta \geq \frac{289\cdot 1.93}{\sqrt{1277}} Adding them: 70 tan θ 289 1.93 49 1.75 1277 70\tan\theta \geq \frac{289\cdot 1.93 - 49 \cdot 1.75}{\sqrt{1277}} and then solving for θ \theta : θ tan 1 ( 289 1.93 49 1.75 70 1277 ) 10.6 9 \theta \geq \tan^{-1}\left(\frac{289\cdot 1.93 - 49 \cdot 1.75}{70\sqrt{1277}}\right) \approx 10.69^\circ

Rounded to the nearest degree, this gives an answer of 11 \boxed{11} .

In all honesty, I can't remember how I exactly solved it but that's the exact answer from looking at my notes that I made! Looks good to me. Upvoted

Stephen Mellor - 2 years, 11 months ago

Two things to note:

  • As a matter of theoretical nonsense, exactly one of the original inequalities probably should have been a strict inequality, but which one it would be depends on whether the players are closed or not at their top. If you follow such a change through, we'll find that the final inequality giving the solution set for θ \theta should also be strict, but this is the only change and it doesn't change the final answer.
  • Implicitly, the only assumption we used about gravity was that it is constant--of course it's not true on large scales, but on small scales, it's a harmless assumption. In particular we never used the given value for g g anywhere; however, if you wanted to find the range for the initial velocity v 0 v_0 of the ball, it would depend on both θ \theta and g g .

Brian Moehring - 2 years, 11 months ago

Log in to reply

I had included the gravity assumption in the question from the beginning. I had used a strict inequality, going on the premise that a defender touching the ball would be enough to send it off course, but the rounding to the nearest degree removes this issue

Stephen Mellor - 2 years, 11 months ago

Log in to reply

Oh, I'm not saying that the assumption on gravity isn't in the problem. My main point about that is that the angle has absolutely no dependency on the actual value of g g , so even on an planet where the force of gravity is, say, 1 billion times the force we feel on earth, we would still have the exact same result for the angle. I just found that interesting.

As for the inequalities, it really depends on how we model the defenders as well as Salah. For instance, if we model the defender as the segment [ 0 , 1.93 ] [0,1.93] , then a particle passing through 1.93 1.93 would be blocked by the defender, but if we model the defender as the segment [ 0 , 1.93 ) [0,1.93) , then a particle passing through 1.93 1.93 would pass right over the defender. The fact, however, that this reduces the problem to a debate on whether the defender and Salah are closed at their tops makes it what I called "theoretical nonsense". We agree, of course, that the final answer isn't affected by the result of this nonsense ;-) .

Brian Moehring - 2 years, 11 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...