Modelling free fall of an object

ViewController.h

#import <Cocoa/Cocoa.h>

@interface ViewController : NSView{
    NSBezierPath *path;
    NSImage *image;
    NSPoint downPoint;
    NSPoint currentPoint;
    float angle;
    float velocity;
}

@property (assign) float angle;
@property (assign) float velocity;
@property (assign) IBOutlet NSTextField *angleField;
@property (assign) IBOutlet NSTextField *speedField;

-(IBAction)fly:(id)sender;
-(NSRect)currentRect;
@end

ViewController.m

#import "ViewController.h"
#import "math.h"

@implementation ViewController
@synthesize angle;
@synthesize velocity;



- (id)initWithFrame:(NSRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        path = [[NSBezierPath alloc] init];
        [path setLineWidth:3.0];
    }
    return self;
}
- (void)drawRect:(NSRect)dirtyRect
{
    NSRect bounds = [self bounds];
    [[NSColor greenColor] set];
    [NSBezierPath fillRect:bounds];
    [[NSColor blackColor] set];
    [[self pathWithVelocity:velocity angle:angle] stroke];
}

-(IBAction)fly:(id)sender
{
    angle = [_angleField floatValue];
    velocity = [_speedField floatValue];
    NSLog(@"%f - %f",angle,velocity);
    [self drawRect:[self bounds]];
}

-(NSBezierPath *)pathWithVelocity:(float)v angle:(float)a{
    NSBezierPath* pat = [[NSBezierPath alloc] init];
    NSPoint p;
    p.x = 0;
    p.y = 0;
    [pat moveToPoint:p];
    for (float t = 0.5; t < 100; t += 0.5) {
        p.x = v*cosf(a)*t;
        p.y = -9.81*t*t/2+v*sinf(a)*t;
        NSLog(@"(%f;%f)",p.x,p.y);
        [pat lineToPoint:p];
    }
    [pat closePath];
    return pat;
}

-(NSRect)currentRect{
    float minX = MIN(downPoint.x,currentPoint.x);
    float maxX = MAX(downPoint.x,currentPoint.x);
    float minY = MIN(downPoint.y,currentPoint.y);
    float maxY = MAX(downPoint.x,currentPoint.x);
    return NSMakeRect(minX, minY, maxX-minX,maxY-minY);
}
@end

Don't call -drawRect: directly -setNeedsDisplay:YES is what should be called..