A projectile has a range R and time of flight T. If the range is doubled by increasing the speed of projection, without changing the angle of projection, the time of flight will become
√2 T
We know that,
R=v20 sin2θ0g
When the range is doubled without changing the angle of projection, the initial velocity becomes √2 times.
Now,
T=2v0 sinθ0g=2×√Rgsin2θ0×sinθ0g
As T∝√R, the time of flight will become √2 times.
Hence, the correct choice is (b).