Re: bouncing rectangle

On Mar 19, 12:40 am, b...@xxxxxxxxxxxxxx wrote:
I'm trying to understand computer game physics better, so I'm making a
2d program that features a rectangle bouncing around on the screen and
rotating (no gravity now).

Unfortunately, the rectangle seems to keep gaining energy, and
eventually it goes ridiculously fast. I'm wondering if there is an
error in my basic approach.

Essentially, the edges of the screen are considered walls, so if a
point on the rectangle goes outside an edge, a force and a torque are
applied to the rectangle. The magnitude of the force is essentially
twice the velocity going towards the edge.

This velocity includes the rotational velocity.

For instance, if a point is going at a velocity of 1 to the right
without rotating, and it hits the right edge, the force it gets is 2
in the opposite direction.

If the point is rotating, I try to add w x r (cross product of angular
velocity and radius vector). So, for the aforementioned point, I would
add the x component of w x r, and the force it gets is (2 * (1 + (x
component of w x r))).

(BTW, I'm just assuming mass=1 for the rectangle for simplicity.)

Anyone see an error in this approach?

Yes.
In an elastic collision between a wall and a *non-rotating* object,
the outgoing perpendicular velocity will be the same as, but opposite
to, the incoming perpendicular velocity, which is where I think your
statement that the *change* in velocity (what you call a "force) is
twice the velocity.
However, for an object that *rotates*, this condition no longer
applies, as some of that linear kinetic energy can become rotational
kinetic energy, and so the rebound linear velocity can actually be
much lower than the incoming linear velocity, which you can easily see
with experiments with a superball on a garage floor at home.

PD

.

Relevant Pages

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