Changing object mass?

Getting Started Newbie Questions
#1

So, I am trying to create a ball throwing game however I can’t work out how to change the mass.

I want to be able to specify a mass, and input the required density to do so. (lets say it’s a 10kg tennis ball, therefore density is…). To do this, I need the area!

[lua]

local ball = display.newCircle(playerhero.x, playerhero.y, 5)

physics.addBody(ball, “dynamic”, {density = 1, bounce=0.1, friction=1})

ball:setFillColor(0,1,0)

ball.alpha = 1

local ballarea = (math.pi*(5/30)^2) --note pi r ^2    the r is /30 to relate this back to metres due to the scale.

print(“ball area =”…ballarea)

print(“ball mass =”…ball.mass)

[/lua]

I thought the calc for mass was density*area (according to corona docs), but when I calc the area myself I get the following:

ball area =0.087266462599716

ball mass =0.11111111938953

As density is 1, I would expect these to be identical. I can’t manually change mass, so I was planning to calculate required density.  

#2

You would need to remove and re-add the physics body with a new density.

Rob

#3

Thanks Rob, that was my second option.

How is mass calculated then out of curiosity? (How is the body area calculated?)

Secondly, what are the units of mass in Corona :slight_smile: ?I assume as the scale is metres and gravity is m.s^-2 that mass will be kg?

#4

According to our docs:  https://docs.coronalabs.com/daily/guide/physics/physicsBodies/index.html

“density is multiplied by the area of the body’s shape to determine its mass.”

The area of a rectangle would be width * height. Circle are math.PI * radius ^ 2. (If I’m remembering my geometry correctly. For irregular shapes, I’m sure there are other formulas to compute them.

Rob

#5

you don’t specify a radius for your body, so your circle (in the display) is really a rectangle (in physics)

turn on hybrid draw mode and this should be evident if true

so your area is (10/30)^2 = 0.111111  (and thus so is your mass, since density = 1.0)

#6

Thanks Dave that clears it up. I had assumed that as the radius was specified against the display object which the body is derived from it would have used that.

Even though I had got it working, this logic was really bugging me :slight_smile: !

#7

You would need to remove and re-add the physics body with a new density.

Rob

#8

Thanks Rob, that was my second option.

How is mass calculated then out of curiosity? (How is the body area calculated?)

Secondly, what are the units of mass in Corona :slight_smile: ?I assume as the scale is metres and gravity is m.s^-2 that mass will be kg?

#9

According to our docs:  https://docs.coronalabs.com/daily/guide/physics/physicsBodies/index.html

“density is multiplied by the area of the body’s shape to determine its mass.”

The area of a rectangle would be width * height. Circle are math.PI * radius ^ 2. (If I’m remembering my geometry correctly. For irregular shapes, I’m sure there are other formulas to compute them.

Rob

#10

you don’t specify a radius for your body, so your circle (in the display) is really a rectangle (in physics)

turn on hybrid draw mode and this should be evident if true

so your area is (10/30)^2 = 0.111111  (and thus so is your mass, since density = 1.0)

#11

Thanks Dave that clears it up. I had assumed that as the radius was specified against the display object which the body is derived from it would have used that.

Even though I had got it working, this logic was really bugging me :slight_smile: !