# The 'mass' and 'weight' of things?

I’ve been experimenting with mass/physics and I must say I’m a little perplexed by the behaviour.

E.g. in the rolling ball template,

The boxes (TemplateCube_Rounded) appear to have a (default) mass of 442 kg / mass_scale:1.0

The ball (PhysicsBallBP) is set to (default) 676 kg / mass_scale:3.5 in the details panel of the editor but 499 kg / mass_scale:3.5 in the PhysicsBallBP blueprint

Now, for objects that weigh nearly half a ton, they move as if they were empty wooden crates when hit by the ball.

I know can tweak ‘mass_scale’ to reach a desired effect but I’m trying to get a handle on why the weightings don’t seem to match my expectations before I start massaging numbers.

The UE4 documentation didn’t really elucidate my question so if anyone has a concise explanation, (or link to one) then please post.

p.s. I have seen the ‘Physics’ content demo and that didn’t help either.
(you can kick around a 37 kg cube like it was made of cardboard)

Cheers

I’m also curious as to the answer for this.

well the simulation runs in perfect stiff world. so for the simulation only the relative weight to each other matters and not the actual weight.
what Im saying is that there is much more to a collision of that size in the real world than you could ever simulate in real time. the feel of weight you want is simply an illusion you have to create (sound effects, slowing down motion etc.).

I don’t think that’s true. I know almost nothing about unreal but I AM a mechanical engineer.

I think what’s going on is that UE doesn’t factor in deformation, stiffness, elasticity, and density realistically in collisions.

In the real world, when objects collide, the force transfer isn’t instant. The force of the collision causes both objects to deform. The force is gradually distributed through the objects, and each chunk in the object starts moving in the direction of the force as it travels through the object.

Basically, what it sounds like is UE is treating the objects as unreformable. When mass increases, the deformation will substantially increase, the rate at which the forces propagate through the object slows down. And the impact causes more deformation and takes longer. Likely none of this is captured.

I don’t know what variables you can play with, but I would suggest tweaking it with these principles in mind.