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-rw-r--r--dat.h6
-rw-r--r--main.c38
2 files changed, 31 insertions, 13 deletions
diff --git a/dat.h b/dat.h
index 24799b8..da2038d 100644
--- a/dat.h
+++ b/dat.h
@@ -1,4 +1,6 @@
-#define Eg 9.81
+#define Eg 9.81 /* earth's gravity (m·s⁻²) */
+#define Cd 0.45 /* drag coefficient for a sphere */
+#define ρ 1.293 /* air density (kg·m⁻³) */
#define PIX2M 0.001
#define M2PIX (1.0/PIX2M)
@@ -6,6 +8,7 @@ enum {
Stheta = 0,
Spos,
Svel,
+ Sdrag,
Sdeltax,
Seta,
SLEN,
@@ -17,4 +20,5 @@ struct Projectile
{
Point2 p, v;
double mass;
+ double r;
};
diff --git a/main.c b/main.c
index 1547898..42fbaf8 100644
--- a/main.c
+++ b/main.c
@@ -15,6 +15,7 @@ RFrame worldrf;
Projectile ball;
double t0, Δt;
double v0;
+double A; /* area of the projectile that meets the air */
Point2 target;
char stats[SLEN][64];
@@ -47,11 +48,27 @@ fromscreen(Point p)
}
void
+∫(double Δt)
+{
+ Point2 Fd; /* drag force */
+
+ Fd = mulpt2(mulpt2(ball.v, -vec2len(ball.v)), 0.5 * Cd*ρ*A); /* ½CdρAv² */
+ ball.v = addpt2(ball.v, mulpt2(addpt2(Vec2(0,-Eg), divpt2(Fd, ball.mass)), Δt));
+ ball.p = addpt2(ball.p, mulpt2(ball.v, Δt));
+ snprint(stats[Spos], sizeof(stats[Spos]), "p: %v", ball.p);
+ snprint(stats[Sdrag], sizeof(stats[Sdrag]), "Fd: %v", Fd);
+ if(ball.p.y <= (2+1)*M2PIX){
+ ball.p.y = (2+1)*M2PIX;
+ ball.v = Vec2(0,0);
+ }
+}
+
+void
drawstats(void)
{
int i;
- snprint(stats[Svel], sizeof(stats[Svel]), "v: %gm/s", vec2len(ball.v));
+ snprint(stats[Svel], sizeof(stats[Svel]), "|v|: %gm/s", vec2len(ball.v));
snprint(stats[Sdeltax], sizeof(stats[Sdeltax]), "Δx: %gm", target.x-ball.p.x);
for(i = 0; i < nelem(stats); i++)
stringn(screen, addpt(screen->r.min, Pt(10, font->height*i+1)), statc, ZP, font, stats[i], sizeof(stats[i]));
@@ -86,7 +103,8 @@ mmb(void)
switch(menuhit(2, mc, &menu, nil)){
case SETV0:
enter("v0(m/s):", buf, sizeof(buf), mc, kc, nil);
- v0 = strtod(buf, 0);
+ if(buf[0] != 0)
+ v0 = strtod(buf, nil);
break;
}
}
@@ -124,7 +142,7 @@ mouse(void)
if(ball.p.y <= (2+1)*M2PIX){
p = subpt2(fromscreen(mc->xy), ball.p);
θ = atan2(p.y, p.x);
- snprint(stats[Stheta], sizeof(stats[Stheta]), "θ: %g°", θ*180/PI);
+ snprint(stats[Stheta], sizeof(stats[Stheta]), "θ: %g°", θ/DEG);
dist = v0*v0*sin(2*θ)/Eg;
target = Pt2(ball.p.x+dist, 0, 1);
eta = 2*v0*sin(θ)/Eg;
@@ -204,6 +222,8 @@ threadmain(int argc, char *argv[])
ball.p = Pt2((2+1)*M2PIX,(2+1)*M2PIX,1);
ball.v = Vec2(0, 0);
ball.mass = 106000;
+ ball.r = 0.1; /* 10cm */
+ A = 2*PI*ball.r*ball.r; /* ½(4πr²) */
v0 = 1640; /* Paris Gun's specs */
scrsync = chancreate(1, 0);
@@ -227,14 +247,8 @@ threadmain(int argc, char *argv[])
case 2: key(r); break;
case 3: redraw(); break;
}
- Δt = (nsec()-t0)/1e9;
- ball.v = addpt2(ball.v, mulpt2(Vec2(0,-Eg), Δt));
- ball.p = addpt2(ball.p, mulpt2(ball.v, Δt));
- snprint(stats[Spos], sizeof(stats[Spos]), "p: %v", ball.p);
- if(ball.p.y <= (2+1)*M2PIX){
- ball.p.y = (2+1)*M2PIX;
- ball.v = Vec2(0,0);
- }
- t0 += Δt*1e9;
+ Δt = (nsec()-t0);
+ ∫(Δt/1e9);
+ t0 += Δt;
}
}