Stay onside

Two soccer players on a breakaway start from midfield and run straight down the field with a separation of d = 5 m d=5~\mbox{m} and with the same velocity v = 7 m/s v= 7 ~\mbox{m/s} . The last defensive player of the other team approaches the player who has the ball, so that person decides to pass the ball with velocity u = 10 m/s u=10~\mbox{m/s} to their teammate. At what angle in degrees with respect to their trajectory should they pass the ball so it reaches their teammate?

Details and assumptions

  • Assume the ball won't slow down as it travels on the pass.
  • The ball travels horizontally along the ground, not via projectile motion.


The answer is 45.57.

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8 solutions

Ahaan Rungta
Aug 25, 2013

Let t t be the time it takes for the pass to occur. In this time, the receiver runs a distance 7 t 7t , in S.I. units. The component of the velocity of the ball that is parallel to the trajectories of the runners is 10 cos θ 10 \cos \theta , where θ \theta is the desired angle.

Now, the two horizontal distances run the ball and the receiver must be the same, so: 10 t cos θ = 7 t cos θ = 7 10 . 10t \cos \theta = 7t \implies \cos \theta = \dfrac {7}{10}. Thus, taking the arccosine, we have θ = 45.5 7 \theta = \boxed {45.57^\circ} .

u said with respect to the trajectory....so shouldn't it be parallel to the 5 m distance between them making it sin inverse(7/10)??????

Sayam Chakravarty - 7 years, 2 months ago
Vitaly Breyev
Aug 26, 2013

Let the ball reach the other player P P in time t t . In this time, P P would travel the distance 7 t 7t ; the ball would travel 5 m 5 \text{ m} perpendicular to P ’s P\text{'s} trajectory and 7 t 7t along P ’s P\text{'s} trajectory. Therefore, the distance l l that the ball travels can be expressed in t t by Pythagoras from a right triangle to be 25 + 49 t 2 \sqrt{25 + 49t^2} .

Since we know the velocity u u of the ball, we can determine t t from u = l t u = \frac{l}{t} as 10 = 25 + 49 t 2 t 10 = \frac{ \sqrt{25 + 49t^2}}{t} . Solving this for positive t t values gives t = 25 51 t = \sqrt{\frac{25}{51}} .

Now we can find the angle from the same right triangle we expressed l l from, since tan ( π 2 α ) = 7 t 5 = 7 25 51 5 = 0.9802 \tan{\left(\frac{\pi}{2} - \alpha\right)} = \frac{7t}{5} = \frac{7\cdot \sqrt{\frac{25}{51}}}{5} = 0.9802 . Hence, α = π 2 arctan 0.9802 = 45.5 7 \alpha = \frac{\pi}{2} - \arctan{0.9802} = \boxed{45.57^{\circ}}

This is a nice solution, although it is a bit unnecessarily complicated. See Kyle's solution (or mine).

Ahaan Rungta - 7 years, 9 months ago
Kyle Coughlin
Aug 25, 2013

Since the ball and the teamate both move at a constant rate, you know that the distance each moves will be the same ratio of 7/10. The angle you want the complement of sin(7/10), which is approximately 45.57.

45.573

Ahmed Abdallah - 7 years, 9 months ago
Ivan Sekovanić
Aug 27, 2013

The ball and the player both have got to reach a certain point after a certain time t t . Let us call this point O O and the points of where the player with the ball and his teammate are with P 1 P_{1} and P 2 P_{2} accordingly.

Now, let us form a triangle with sides P 1 P 2 = 5 , P 1 O = c \overline{P_{1}P_{2}}=5,\overline{P_{1}O}=c and P 2 O = b \overline{P_{2}O}=b ( P 1 P 2 \overline {P_{1}P_{2}} is actually the distance between the two players, which we know). This triangle is also a right one, with the right angle being P 1 P 2 O \angle P_{1}P_{2}O .

Now, we know for a fact that because v = s t v=\frac{s}{t} , where v v is the velocity, s s is the length traveled and t t is the time, the two missing sides of the triangle can be expressed as

b = 7 t b=7t and c = 10 t c=10t

The angle we are actually looking for is P 2 O P 1 \angle P_{2}OP_{1} , which we will call α \angle \alpha for better illustrative purposes. From the given triangle, we know that

cos α = b c cos α = 7 10 \cos\alpha =\frac{b}{c} \Rightarrow \cos\alpha=\frac{7}{10}

From this point on, we simply find arccos 7 10 \arccos \frac{7}{10} , which is 45 , 568 45,568 .

Thus, the answer is 45 , 57 45,57 .

Víctor Martín
Aug 26, 2013

Let * \­( x \­) * be the displacement of the ball respect the trajectory of the players. Let α \alpha be the angle we want to know. We know the ball's speed is 10 m/s. Part of that speed must go move 5 m perpendicularly to their trajectory. That will be \­(10 \times \sin \alpha \­). By the other hand, the speed parallel to their trajectory will be the same speed as the players, 7 m/s. So \­(7 = \10 \times \cos \alpha \­). We can solve the angle now. Note that \­(10 \times \sin \alpha \­) is not necessary. Finally, we have \­(\frac{7}{10} = \cos \alpha \­), so \­(alpha = 45.573 \­)

John Smith
Aug 29, 2013

It was a guess lol

Vaibhav Negi
Aug 27, 2013

consider two players. velocity of both the players are 7m/s. velocity of ball when he passes the ball = 10 m/s. now applying trigonometry ; sin (x) = 7/10. which gives x = 44.427004. bt according to ques wee should subtract our ans from 90 because we want angle with respect to their trajectory. therefore ans is 90- x i.e 45.572996

exellent

Ahmed Abdallah - 7 years, 9 months ago

45.573

Ahmed Abdallah - 7 years, 9 months ago
Michael Tang
Aug 26, 2013

Let t t be the amount of time it takes for the soccer ball to transfer between the two players. In that time, the ball travels 10 t m 10t \; \text{m} and the receiving player travels 7 t m . 7t \; \text{m}. The angle θ \theta between the trajectory of the defender and the trajectory of the ball therefore has cosine 7 / 10 7/10 (draw the diagram to see this.) Thus, we have θ = arccos ( 7 / 10 ) 45. 6 . \theta = \arccos{(7/10)} \approx \boxed{45.6^{\circ}}.

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