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Three points are located at the vertices of an equilateral triangle whose side equals a. They all start moving simultaneously with velocity v constant in modulus, with the first point heading continually for the second, the second for the third, and the third for the first. How soon will the points converge?
  • a)
    2a/3v
  • b)
    a/v
  • c)
    2a/4v
  • d)
    6a/2v
Correct answer is option 'A'. Can you explain this answer?
Most Upvoted Answer
Three points are located at the vertices of an equilateral triangle w...
Problem: Three points are located at the vertices of an equilateral triangle whose side equals a. They all start moving simultaneously with velocity v constant in modulus, with the first point heading continually for the second, the second for the third, and the third for the first. How soon will the points converge?

Solution:
Let the triangle be ABC, with A, B, C denoting the vertices and a being the side length.
Let P, Q, R be the positions of A, B, C respectively at any time t.
At any time t, P is moving towards Q with a velocity of v along the line PQ. Similarly, Q is moving towards R and R is moving towards P.
Let O be the circumcenter of the equilateral triangle ABC.
Since P is moving towards Q, we can draw a line perpendicular to PQ passing through O. Let X be the intersection of this line with PQ.
Similarly, we can draw perpendiculars to QR and RP passing through O, and let Y and Z be the intersections of these perpendiculars with QR and RP respectively.
Then, X, Y, and Z are the midpoints of PQ, QR, and RP respectively, and are also collinear since they lie on the perpendicular bisector of each side of triangle PQR.
Let D be the midpoint of PQ. Then, OD is perpendicular to PQ and OD = a/2 (since O is the circumcenter of ABC).
Similarly, we can define E and F to be the midpoints of QR and RP respectively, such that OE = OF = a/2.
Let T be the intersection of XY and ZF. Then, since XY and ZF are both perpendicular to RP, they are parallel to each other. Therefore, triangle FZT is similar to triangle ODF, with a similarity ratio of 1/2 (since OF = OD = a/2 and FT = DT = a/3).
Therefore, FZ = OD/2 = a/4 and TZ = FZ/2 = a/8.
Since Z is the midpoint of RP, RZ = a/2. Therefore, RT = RZ - TZ = a/2 - a/8 = 3a/8.
Similarly, we can show that QY = a/4 and TP = a/8.
Therefore, the distance between P and R at any time t is PR = PT + TR = a/8 + 3a/8 = a/2.
Therefore, the points P, Q, and R will converge at the center of the equilateral triangle after a time of 2a/3v (since PR = a/2 and the velocity of each point is v).
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Community Answer
Three points are located at the vertices of an equilateral triangle w...
Since, the magnitude of velocity remains constant. So, instantaneous tangential acceleration is zero for each particle. So, only centripetal force is present on each particle. The vector sum of forces acting on the particles is zero. So, at every instant the momentum of system remains constant. If the mass of each particle are same. Then the momentum of the system is zero at every instant. So, the centre of mass (centroid) of system remains in rest.
Hence, collision takes place at the centre of mass of system (centroid).
Component of velocity towards O is V cos30
The time of converging = AO / Vcos30
Where
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Three points are located at the vertices of an equilateral triangle whose side equals a. They all start moving simultaneously with velocity v constant in modulus, with the first point heading continually for the second, the second for the third, and the third for the first. How soon will the points converge?a)2a/3vb)a/vc)2a/4vd)6a/2vCorrect answer is option 'A'. Can you explain this answer?
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