How Does Particle B's Movement Depend on Particle A's Velocity Direction?

[frozen7]

6. Two particles, each of mass m, are connected by a light inextensible string of length 2l. Initially they lie on a smooth horizontal table at points A and B distant l apart. The particle at A is projected across the table with velocity u. Find the speed with which the second particle begins to move if the directions of u is.
(a) along AB,
(b) at an angle of 120 degree with AB,
(c) perpendicular to AB.


How to solve for the b and c part? What`s the usage of the 2 given lengths?

 

[vaishakh]

I assume the string is flexible.
For the b part you just imagine or draw atleast a free diagram to get a pictue of what I am saying. The mommentum will slowly betransferred to B. After sometime they will start moving in the same direction and the string becomes straight. The tension starts to act when distance apart become 2l. Proceed from this information.

 

[daniel_i_l]

Particle B only starts moveing when the distance between the particles is 2l. Fist figure out the angle between the string and AB when this happens. Since particle B is pulled only by the string, you need to find the component of A's momentum along the string (using the angle) and then calculate how much gets transferred to B using momentum conservation.