title: "Property 'pos' must be a vector" Error when I run VPython Code
body: <p>When I run this code I get an error "Error: Property 'pos' must be a vector." Do I have to write another vector somewhere? Because I wrote vector at </p>
  
  <pre><code>grav_force = vector(0,-object.mass*grav_field,0)
  </code></pre>
  
  <p>This is my whole code</p>
  
  <pre><code>GlowScript 2.7 VPython
  
  from visual import *
  
  display(width = 1300, height = 1000)
  
  projectile = sphere(pos = (-5,0,0),
                      radius = 0.1,
                      color = color.red,
                      make_trail = True)
  
  projectile.speed = 3.2 # Initial speed.
  projectile.angle = 75*3.141459/180 # Initial angle, from the +x-axis.
  
  projectile.velocity = vector(projectile.speed*cos(projectile.angle),
                               projectile.speed*sin(projectile.angle),
                               0)
  
  projectile.mass = 1.0
  grav_field = 1.0
  
  dt = 0.01
  time = 0
  
  while (projectile.pos.y &gt;=0):
      rate(100)
  
      # Calculate the force.
      grav_force = vector(0,-projectile.mass*grav_field,0)
  
      force = grav_force
  
      # Update velocity.
      projectile.velocity = projectile.velocity + force/projectile.mass * dt
  
      # Update position.
      projectile.pos = projectile.pos + projectile.velocity * dt
  
      # Update time.
      time = time + dt
  </code></pre>