How to do simple physics - Revision history

JoeStrout at 20:46, 4 May 2020

2020-05-04T20:46:26Z

JoeStrout at 16:37, 4 May 2020

2020-05-04T16:37:20Z

JoeStrout at 15:54, 4 May 2020

2020-05-04T15:54:32Z

JoeStrout: Created page with "Games often need simple Newtonian physics to make things move in a believable manner. That basically means, objects have inertia and tend to stay in motion once they're in mo..."

2020-05-04T15:50:13Z

Created page with "Games often need simple Newtonian physics to make things move in a believable manner. That basically means, objects have inertia and tend to stay in motion once they're in mo..."

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Games often need simple Newtonian physics to make things move in a believable manner. That basically means, objects have inertia and tend to stay in motion once they're in motion, affected by forces like gravity. Examples include the ship and rocks in ''Asteroids'', the projectiles in any ballistic game, and even the characters in a platformer.

Fortunately, simple physics are easy to do! The key concept is: in addition to the object's position, you must also keep track of its velocity. This is normally done by adding two new properties, <c>vx</c> and <c>vy</c>, to your object.

* '''vx''' is the object's '''horizontal velocity''': how much its x position changes per second.
* '''vy''' is the object's '''vertical velocity''': how much its y position changes per second.

If we were updating the position of our object once per second, we could just add <c>vx</c> to its <c>x</c> position, and <c>vy</c> to its <c>y</c> position. But usually we're updating the screen much faster than that (typically 60 frames per second). The time step (usually called <c>dt</c>) is much smaller than 1 second, so the object won't move as far on each step. We accomplish this by simply multiplying <c>vx</c> and <c>vy</c> by <c>dt</c>, before adding them to <c>x</c> and <c>y</c>.

== Example ==

The code below loads a sprite in the center of the screen, initially stationary. Then it falls due to gravity, until it hits the bottom of the screen.

<ms>clear
gravity = 100 // acceleration, in pixels/sec^2

obj = new Sprite
obj.image = file.loadImage("/sys/pics/Block.png")
display(4).sprites.push obj
obj.x = 480
obj.y = 320
obj.vx = 0
obj.vy = 0

obj.update = function(dt=0.016)
// update velocity (applying gravity)
self.vy = self.vy - gravity * dt

// update position (applying velocity)
self.x = self.x + self.vx * dt
self.y = self.y + self.vy * dt

// apply any limits, collision checks, etc.
if self.y < 32 then
self.y = 32
self.vy = 0
self.vx = floor(self.vx / 2)
end if
end function

while true // press control-C to exit
obj.update
yield
end while</ms>

'''Things to try''':

# Change the initial values of <c>vx</c> and <c>vy</c> to something other than zero. What happens?
# When <c>self.y < 32</c> (the object hits the ground), instead of setting <c>self.vy</c> to zero, what if you set it to <c>abs(self.vy)</c> (i.e. make it positive)?
# Try adding a thruster you can control, by adding this to the "update velocity" section: <c>if key.pressed("space") then self.vy = self.vy + 500*dt</c>

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	For another example, see the '''flappyBat''' demo in '''/sys/demo/'''.		For another example, see the '''flappyBat''' demo in '''/sys/demo/'''.
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		+	[[Category:Mini Micro]]
		+	[[Category:How To]]

← Older revision		Revision as of 16:37, 4 May 2020
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	# When <c>self.y < 32</c> (the object hits the ground), instead of setting <c>self.vy</c> to zero, what if you set it to <c>abs(self.vy)</c> (i.e. make it positive)?		# When <c>self.y < 32</c> (the object hits the ground), instead of setting <c>self.vy</c> to zero, what if you set it to <c>abs(self.vy)</c> (i.e. make it positive)?
	# Try adding a thruster you can control, by adding this to the "update velocity" section: <c>if key.pressed("space") then self.vy = self.vy + 500*dt</c>		# Try adding a thruster you can control, by adding this to the "update velocity" section: <c>if key.pressed("space") then self.vy = self.vy + 500*dt</c>
		+
		+
		+	For another example, see the '''flappyBat''' demo in '''/sys/demo/'''.

← Older revision		Revision as of 15:54, 4 May 2020
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−	Games often need simple Newtonian physics to make things move in a believable manner. That basically means, objects have inertia and tend to stay in motion once they're in motion, affected by forces like gravity. Examples include the ship and rocks in ''Asteroids'', the projectiles in any ~~ballistic~~ game, and even the characters in a platformer.	+	Games often need simple Newtonian physics to make things move in a believable manner. That basically means, objects have inertia and tend to stay in motion once they're in motion, affected by forces like gravity. Examples include the ship and rocks in ''[https://en.wikipedia.org/wiki/Asteroids_(video_game) Asteroids]'', the projectiles in any [https://en.wikipedia.org/wiki/Artillery_game artillery game], and even the characters in a [https://en.wikipedia.org/wiki/Platform_game platformer].

	Fortunately, simple physics are easy to do! The key concept is: in addition to the object's position, you must also keep track of its velocity. This is normally done by adding two new properties, <c>vx</c> and <c>vy</c>, to your object.		Fortunately, simple physics are easy to do! The key concept is: in addition to the object's position, you must also keep track of its velocity. This is normally done by adding two new properties, <c>vx</c> and <c>vy</c>, to your object.