For the truss shown below, the member PQ is sort by 3 mm.

For the truss shown below, the member PQ is sort by 3 mm. The magnitude of vertical displacement of joint R (in mm) is ___________.

PQ is short by 3 mm.
Apply unit load at R upwards

U_PR × sin θ = 1/2
U_PQ + U_PR cos θ = 0

U_PQ = - U_PR cos θ = -	l	cos θ
	2sin θ

= -	l	cos θ
	2

=	- 1	×	4
	2		3

= - 2/3
∆R = U_PQ × λ_PQ

=	- 2	(- 3)
	3

= 2 mm upwards