OpenGL Ball is too slow and getting bugged - c++

Question

I'm trying to do an animation which ball starts dropping with affects of gravity but unfortunately the ball is too slow. I actually found a way for it which is to change
double dt = (cur - last) / 1000.0;
to
double dt = (cur - last) / 100.0;
but that time, the bug is that position of the ball is kind of wrong because the ball starts dying :). If you work the animation you may understand what I'm trying to say!
The main code is there :
#include <GL/glut.h>
#include <vector>
#include <bits/stdc++.h>
using namespace std;
#include <glm/glm.hpp>
#include <glm/gtx/norm.hpp>
using namespace glm;
int seconds_past = 0;
struct Ball{
int radius;
dvec3 pos;
dvec3 vel;
double acc = -9.8;
bool last, made = false;
dvec3 last_vel;
void Integrate(double cur, double dt){
static int count = 0;
count++;
vel += dvec3{vel[0], acc * dt, 0};
pos = pos + vel * dt;
std::cout << vel[1] << " " << count << std::endl;
}
void HandleCollisions(){
if(pos[1] - 10 < -100){
std::cout << "true" << std::endl;
vel = -vel;
}
}
};
void Integrate( vector< Ball >& particles, double cur, double dt)
{
for( size_t i = 0; i < particles.size(); ++i )
{
particles[i].Integrate( cur, dt );
particles[i].HandleCollisions();
}
}
vector< Ball > particles;
void display()
{
// use last frame time to calculate delta-t
static int last = glutGet( GLUT_ELAPSED_TIME );
int cur = glutGet( GLUT_ELAPSED_TIME );
double dt = ( cur - last ) / 1000.0;
last = cur;
Integrate( particles, cur, dt);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
double w = glutGet( GLUT_WINDOW_WIDTH );
double h = glutGet( GLUT_WINDOW_HEIGHT );
double ar = w / h;
glOrtho( -100 * ar, 100 * ar, -100, 100, -1, 1 );
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
// draw particles
glPointSize( 50 );
glEnableClientState( GL_VERTEX_ARRAY );
glVertexPointer( 3, GL_DOUBLE, sizeof( Ball ), &particles[0].pos[0] );
glDrawArrays( GL_POINTS, 0, particles.size() );
glDisableClientState( GL_VERTEX_ARRAY );
glutSwapBuffers();
}
void timer( int value )
{
glutPostRedisplay();
glutTimerFunc( 1000.0/60.0, timer, value );
}
int main(int argc, char **argv)
{
particles.resize(1);
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE );
glutInitWindowSize( 600, 600 );
glutCreateWindow( "GLUT" );
glutDisplayFunc( display );
glutTimerFunc( 0, timer, 0 );
glutMainLoop();
return 0;
}