An acorn falls from a tree and accelerates at 9.8 m/s2. it hits the ground in 0.67 s. what is the speed of the acorn when it hits the ground?

Acceleration means that while it's falling, its speed will increase by 9.8 m/s
every second.  That's the acceleration of everything that falls near the Earth's
surface, no matter how heavy or light the object is.

In 0.67 seconds, then, the speed of the acorn increases from zero to

                 (0.67 x 9.8) = 6.57 m/s .