Knockback

Lesson 11 of 25 in Coddy's Roblox Game: Battlegrounds course.

A slam should throw what it hits. ApplyImpulse on a root gives its whole body a push. An impulse is a change of speed times mass, so multiplying by AssemblyMass throws light and heavy rigs the same way:

theirRoot:ApplyImpulse((away * 30 + Vector3.new(0, 20, 0)) * theirRoot.AssemblyMass)

away points from you to them, flat on the ground: their position minus yours, with Y zeroed, then .Unit for a length of 1.

Challenge

Medium

Make the slam throw everyone it hits.

In abilities.Slam, after hurt: local away = (theirRoot.Position - root.Position) * Vector3.new(1, 0, 1), then away = away.Unit if its Magnitude is above 0.
Then theirRoot:ApplyImpulse((away * Settings.SlamKnockback + Vector3.new(0, 40, 0)) * theirRoot.AssemblyMass).

Press Play, walk between the two red rigs and press E: they fly apart. Then Check.

Try it yourself

local Players = game:GetService("Players")
local ServerStorage = game:GetService("ServerStorage")
local Settings = require(game.ReplicatedStorage.Settings)

local remotes = game.ReplicatedStorage.Remotes
local lastSwing = {} -- player -> when they last swung

-- Roblox's own sword animations
local slash = Instance.new("Animation")
slash.AnimationId = "rbxassetid://522635514"

print("Swords deal " .. Settings.SwordDamage .. " damage")

-- Every hit in the game goes through here
local function hurt(humanoid, amount)
	if humanoid.Health <= 0 then
		return
	end
	local character = humanoid.Parent
	humanoid:TakeDamage(amount)
end

-- A swing: the animation, then everyone in the box in front of you is hit
local function swing(player)
	local character = player.Character
	local root = character and character:FindFirstChild("HumanoidRootPart")
	if not root or character.Humanoid.Health <= 0 then
		return
	end
	local now = time()
	if lastSwing[player] and now - lastSwing[player] < Settings.SwingCooldown then
		return
	end
	lastSwing[player] = now
	character.Humanoid.Animator:LoadAnimation(slash):Play()
	-- the box in front of you, never your own body
	local params = OverlapParams.new()
	params.FilterDescendantsInstances = { character }
	local box = root.CFrame * CFrame.new(0, 0, -3)
	local hit = {} -- each humanoid once per swing
	for _, part in workspace:GetPartBoundsInBox(box, Settings.HitboxSize, params) do
		local humanoid = part.Parent:FindFirstChildOfClass("Humanoid")
		if humanoid and not hit[humanoid] then
			hit[humanoid] = true
			hurt(humanoid, Settings.SwordDamage)
		end
	end
end

-- The abilities, by name: what each one does
local abilities = {}

-- a player's own device moves their character, so the dash happens there
function abilities.Dash(player)
	remotes.Ability:FireClient(player, "Dash")
end

-- everyone around you is hit
function abilities.Slam(player, character, root)
	local params = OverlapParams.new()
	params.FilterDescendantsInstances = { character }
	local hit = {}
	for _, part in workspace:GetPartBoundsInRadius(root.Position, Settings.SlamRadius, params) do
		local humanoid = part.Parent:FindFirstChildOfClass("Humanoid")
		local theirRoot = part.Parent:FindFirstChild("HumanoidRootPart")
		if humanoid and theirRoot and not hit[humanoid] then
			hit[humanoid] = true
			hurt(humanoid, Settings.SlamDamage)
			-- Lesson 11: Knockback goes here
		end
	end
end

-- The client asks for an ability by name. The server checks it: a real
-- ability, a living player, and its cooldown over. When it is ready again
-- is an attribute of the player, so their screen can show it.
remotes.Ability.OnServerEvent:Connect(function(player, name)
	if typeof(name) ~= "string" or not abilities[name] then
		return
	end
	local character = player.Character
	local root = character and character:FindFirstChild("HumanoidRootPart")
	if not root or character.Humanoid.Health <= 0 then
		return
	end
	local now = workspace:GetServerTimeNow()
	if now < (player:GetAttribute(name .. "ReadyAt") or 0) then
		return
	end
	player:SetAttribute(name .. "ReadyAt", now + Settings.Cooldowns[name])
	abilities[name](player, character, root)
end)

-- Every player gets a sword every life
Players.PlayerAdded:Connect(function(player)
	player.CharacterAdded:Connect(function()
		local sword = ServerStorage.Sword:Clone()
		sword.Parent = player.Backpack
		sword.Activated:Connect(function()
			swing(player)
		end)
	end)
end)

-- The training dummy: its health bar follows its health, and a beaten
-- dummy comes back after a moment, a fresh copy in the same place
local function setupDummy(dummy)
	local humanoid = dummy.Humanoid
	humanoid.HealthChanged:Connect(function(health)
		dummy.HealthBar.Back.Fill.Size = UDim2.fromScale(health / humanoid.MaxHealth, 1)
	end)
	humanoid.Died:Connect(function()
		local spot = dummy:GetPivot()
		task.wait(0.5)
		dummy:Destroy()
		task.wait(Settings.DummyRespawn)
		local fresh = ServerStorage.Dummy:Clone()
		fresh:PivotTo(spot)
		fresh.Parent = workspace
		setupDummy(fresh)
	end)
end
setupDummy(workspace.Dummy)