Löve2D Jumping Circle controls

local x, y = 400, 100
local vx, vy = 0, 0
local gravity = 200
local jumpspeed = 200
local maxspeed = 300

function love.update( dt )
    x = x + vx*dt
    y = y + vy*dt

    vy = vy + gravity*dt
    vx = vx * 0.9 -- Reibung
    if math.abs( vx ) < 10 then
        vx = 0
    end

    if y >= 500 then
        y = 500 -- nicht in den Boden fallen
        vy = 0
    end

    if love.keyboard.isDown( "left" ) then
        vx = -maxspeed
    elseif love.keyboard.isDown( "right" ) then
        vx = maxspeed
    end

    if love.keyboard.isDown( "up" ) and y >= 500 then
        vy = -jumpspeed
    end
end

function love.draw()
    love.graphics.circle( "fill", x, y, 10 )
end