Question about applyLinearImpulse

i want the ball can keep jumping after touch the platform

how come this code does not work?

local stand = display.newRect(0,315,1000,5)

local hero = display.newCircle(40,10,15)

hero.isColidedwithGround = false

local physics = require “physics”

physics.start()

physics.addBody(stand,“static”,{bounce = 0})

physics.addBody(hero,“dynamic”,{bounce = 0})

local function onLocalCollision( self, event )

if event.phase == “began” then

hero.isColidedwithGround = true

–hero:applyForce(0,-2,hero.x,hero.y)

print(hero.isColidedwithGround )

hero:applyLinearImpulse( 0, -0.05, hero.x, hero.y )

–hero:applyForce(0,-2,hero.x,hero.y)

end

print(event.phase)

end

hero.collision = onLocalCollision

print(hero.collision)

hero:addEventListener(“collision”,hero)