Projectile motion sand bag drop

[anandzoom]

sand bag is dropped...i can understand that vertical acceleration is 9.8m/s^2...so vertical velocity increases in a.p (9.8, 19.6... m/s)...but this book says horizontal velocity of sandbag is constant and same as that of hot-air balloon (i.e., 12m/s)...I think the horizontal velocity should decrease gradually...but it doesn't...why?

 

[jbriggs444]

sand bag is dropped...i can understand that vertical acceleration is 9.8m/s^2...so vertical velocity increases in a.p (9.8, 19.6... m/s)...but this book says horizontal velocity of sandbag is constant and same as that of hot-air balloon (i.e., 12m/s)...I think the horizontal velocity should decrease gradually...but it doesn't...why?

The book is ignoring air resistance. Can you see that in the absence of air resistance, horizontal velocity must be constant?