For this, we will use one of the general kinematic equations that says:

y(t)=y(0)+v(0)*t+(1/2)*a*t^2

where:

y(0)=20 ft (starting height)

v(0)=0 (because it starts from rest)

t=time

a=acceleration (gravity in this case)

y(t)=height of acorn in reference to time (t)

Therefore, we can write:

y(t)=20+(1/2)gt^2

If we use this equation, we must remember that y is currently in ft. That means that we will either need to (1) convert 20ft to meters or (2) use g=-32.15 ft/s^2. We would do this in order to conserve the units.