You simply have to think in terms of X and Y values for position, the size/radius of your sprites for collision, speed, intertia and gravity.

The rules can all be made up and changed on the spot:
sync on:sync rate 60
gosub MakeSprites
gosub SetValues
do
gosub Collision
gosub PasteSprites
sync
LOOP
PasteSprites:
paste sprite 1,PlayerX#,PlayerY#
paste sprite 2,PlatformX#,PlatformY#
return
MakeSprites:
`make player
red=rgb(255,0,0)
box 0,0,10,10,red,red,red,red
get image 1,0,0,10,10
sprite 1,0,0,1
offset sprite 1,sprite width(1)/2,sprite height(1)
hide sprite 1
`make platform
yellow=rgb(255,255,0)
box 0,0,40,10,yellow,yellow,yellow,yellow
get image 2,0,0,40,10
sprite 2,0,0,2
offset sprite 2,sprite width(2)/2,-sprite height(2)/2
hide sprite 2
return
SetValues:
PlayerX#=screen width()/2
PlayerY#=0
PlatFormX#=screen width()/2
PlatformY#=200
Gravity#=.1
return
Collision:
if PlayerY#<PlatformY# then gosub ApplyGravity else wait 2000:Gravity#=.1:PlayerY#=0
return
ApplyGravity:
inc PlayerY#,Gravity#
Gravity#=Gravity#*1.05
return