wu :: forums - Print Page


    
      
        wu :: forums
        (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi)
      

        riddles >> medium >> Maximum Separation
        
(Message started by: navdeep1771 on Aug 12^th, 2021, 10:11pm)

Title: Maximum Separation
Post by navdeep1771 on Aug 12^th, 2021, 10:11pm

Two bodies move in a straight line towards each other at initial velocities v1 and v2 and with constant accelerations a1 and a2 directed against the corresponding velocities at the initial instant. What must be the maximum initial separation Lmax between the bodies for which they meet during the motion?

Title: Re: Maximum Separation
Post by rmsgrey on Aug 13^th, 2021, 7:29am

Answer: [hide]L_max=(v₁ + v₂)²/(2(a₁ + a₂)) [/hide]

Working:
::[hideb]
Working in a frame where one of the bodies is stationary throughout (and ignoring relativistic corrections), the other body has initial velocity, u=v₁+v₂, and constant acceleration in the opposite direction, a=a₁+a₂.

The change in separation between the two bodies, s, at a given time, t, is given by standard SUVAT formulae:
s=ut-at²/2
Since s reaches a maximum when the closing velocity, v=u-at, is 0, we get:
u-at=0
t=u/a
Substituting for t in the equation for s gives:
s=u(u/a) - a(u/a)²/2
=u²/a - u²/2a
=u²/2a
and substituting back for u and a gives the answer above:
L_max=(v₁ + v₂)²/(2(a₁ + a₂))
[/hideb]::

edit: the subscript tags are behaving weirdly, and I can't get them to work properly
edit2: fixed. Thanks towr.

Title: Re: Maximum Separation
Post by towr on Dec 29^th, 2021, 7:46am

The tags are misbehaving because the board inserts spaces in a too-long string without any.