My basic implementation of "platformer" physics, based on information from http://freespace.virgin.net/hugo.elias/models/m_main.htm You'll need to supply your own "dude.png" to act as a character.

jump.rb

# Fix require path issues in 1.9.*
$: << "."

begin; require 'rubygems'; rescue; end

require 'gosu'
include Gosu

#require 'texplay'

# Screen size
W = 1024
H = 576

# Full-screen mode
FS = false

# Update interval (Hz)
FPS = 60

# Acceleration due to gravity (not sure what the units are; it doesn't have to be based on the update interval)
GRAVITY = FPS.div(20)

class Point

  attr_accessor :x, :y

  def initialize(x = 0, y = 0)
    @x = x
    @y = y
  end

end # Point class

class Jumpman

  attr_accessor :pos, :speed, :f_ground, :f_air, :tv, :jumps, :maxjumps, :color, :face
  attr_reader :grav

  def initialize (win, x, y, speed = 2, grav = 1.0, f_ground = 0.85, f_air = 0.85, tv = 64)
    @win = win

    @pos = Point.new(x, y)
    @vel = Point.new(0, 0)
    @acc = Point.new(0, 0)
    # @acc = Point.new(0, @grav * GRAVITY)

    @speed = speed
    self.grav = grav
    @f_ground = f_ground
    @f_air = f_air
    @tv = tv

    @color = 0xFFFFFFFF

    @jumps = 0
    @maxjumps = 1

    @face = 1.0

    # TODO: Replace with load_from_tiles for animation
    @player = Gosu::Image.new(@win, "dude.png", true)
    @vel = Point.new(@player.width * -0.5, -@player.height + 5)
  end # Jumpman.new

  def grav=(g)
    @grav = g
    @acc.y = @grav * GRAVITY if @acc
  end

  def draw
    @win.translate(@pos.x, @pos.y) {
      @player.draw_rot(0, 0, 0, 0.0, 0.5, 1.0, @face, @grav < 0 ? -1.0 : 1.0, @color)
    }
  end # Jumpman#draw

  def update
    @pos.x += @vel.x
    @pos.y += @vel.y
    @vel.x += @acc.x
    @vel.y += @acc.y

    # I haven't implemented Hugo's "wind resistance" formula, yet, hence this cludge.
    if @jumps > 0
      @vel.x *= @f_air
    else
      @vel.x *= @f_ground
    end

    if @vel.y.abs > @tv
      @vel.y = @tv * @grav
    end

    # Simple collision to keep player from falling off the screen
    if (@pos.y > H * 0.95)
      @pos.y = H * 0.95
      @vel.y = 0
      @jumps = 0
    elsif (@pos.y < H * 0.05)
      @pos.y = H * 0.05
      @vel.y = 0
      @jumps = 0
    end
  end # Jumpman#update

  def button_down(id)
    case id
    when GpLeft, KbLeft
      @face = -1.0
      @acc.x = -@speed
    when GpRight, KbRight
      @face = 1.0
      @acc.x = @speed
    when GpButton2, KbLeftControl
      # Jump
      if @jumps < @maxjumps
        @jumps += 1
        @vel.y = -@grav * GRAVITY * 12.0
      end
    end
  end # Jumpman#button_down

  def button_up(id)
    case id
    when GpLeft, GpRight, KbLeft, KbRight
      @acc.x = 0.0
    when GpButton2
      # Reserved for fireballs, etc. ;)
    when GpButton0, KbSpace
      # Gravity flip!
      self.grav = -@grav
      @jumps = @maxjumps
    end
  end # Jumpman#button_up

end # class Jumpman

class GfxWindow < Gosu::Window

  def initialize
    # Call the base class' initialize
    super W, H, FS, 1.0 / FPS * 1000
    self.caption = "Jumping Physics!"

    # Sprites
    @actors = []
    # Player
    @actors << Jumpman.new(self, W * 0.5, H * 0.25)

    # Collectables
    @statics = []
    # @statics << Item.new(self, W * 0.85, H * 0.75)

    # @background = Image.new(self, 'blank_room.jpg', false)
    @info = Gosu::Font.new(self, Gosu::default_font_name, 14)

    @last_time = Gosu::milliseconds()
  end

  def button_down(id)
    @actors.each do |a|
      a.button_down(id)
    end

    if id == Gosu::Button::KbEscape then
      close
    end
  end

  def button_up(id)
    @actors.each do |a|
      a.button_up(id)
    end
  end

  def draw
    # @background.draw(0, 0, -1000, W.to_f / @background.width, H.to_f / @background.height)
    @actors.each do |a|
      a.draw
    end
    # @statics.each do |s|
      # s.draw
    # end

    # @info.draw("x: #{@actors[0].x.floor} y: #{@actors[0].y.floor} ms: #{Gosu::milliseconds() - @last_time}", 10, 10, 0)
    @last_time = Gosu::milliseconds()
  end

  def update
    @actors.each do |a|
      a.update
    end
    # @statics.each do |s|
      # s.update
    # end
  end

  def needs_cursor?()
    true
  end
end # class GfxWindow

begin
  $window = GfxWindow.new
  $window.show
rescue Exception => e
  puts e.message, e.backtrace
end