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Here is a no-trig calculation, derived from straight-forward Grade 11 Trig and Physics. It assumes that the origin is the lowest point of the pendulum bob's suspension, that L is the length of the pendulum, and that the normal graphics convention of y increasing down, and x increasing to the right is adopted:

Update: I messed up yAcceleration initially; this is easier.
Update #2: Added explicit time control, and added units of measure.

const float gravity = 9.8;     // units of metres/sec/sec
const float deltaT  = 0.001;   // equals 0.001 sec or 1 millisecond

var xVelocity = 0.010;         // units metres/sec equals 10 cm/sec 
var x = 0.0;                   // units metres
var y = 0.0;                   // units metres

while (true) {
  var xAcceleration = -g * (x/L) * (L-y)/L;
  
  x += (xVelocity + (xAcceleration/2 * deltaT)) * deltaT;
  y  = Math.SQRT(L*L - x*x) - L; 

  xVelocity += xAcceleration * deltaT;
}

Here is a no-trig calculation, derived from straight-forward Grade 11 Trig and Physics. It assumes that the origin is the lowest point of the pendulum bob's suspension, that L is the length of the pendulum, and that the normal graphics convention of y increasing down, and x increasing to the right is adopted:

Update: I messed up yAcceleration initially; this is easier.
Update #2: Added explicit time control, and added units of measure.

const float gravity = 9.8;     // units of metres/sec/sec
const float deltaT  = 0.001;   // equals 0.001 sec or 1 millisecond

var xVelocity = 0.010;         // units metres/sec equals 10 cm/sec 
var x = 0.0;                   // units metres
var y = 0.0;                   // units metres

while (true) {
  var xAcceleration = -gravity * (x/L) * (L-y)/L;
  
  x += (xVelocity + (xAcceleration/2 * deltaT)) * deltaT;
  y  = Math.SQRT(L*L - x*x) - L; 

  xVelocity += xAcceleration * deltaT;
}

Here is a no-trig calculation, derived from straight-forward Grade 11 Trig and Physics. It assumes that the origin is the lowest point of the pendulum bob's suspension, that L is the length of the pendulum, and that the normal graphics convention of y increasing down, and x increasing to the right is adopted:

Update: I messed up yAcceleration initially; this is easier.
Update #2: Added explicit time control, and added units of measure.

const float gravity = 9.8;     // units of metres/sec/sec
const float deltaT  = 0.001;   // equals 0.001 sec or 1 millisecond

var xVelocity = 0.010;         // units metres/sec equals 10 cm/sec 
var x = 0.0;                   // units metres
var y = 0.0;                   // units metres

while (true) {
  var xAcceleration = -g * (x/L) * (L-y)/L;
  
  x += (xVelocity + xAcceleration/2) * deltaT ;
  y  = Math.SQRT(L*L - x*x) - L; 

  xVelocity += xAcceleration * deltaT;
}

Here is a no-trig calculation, derived from straight-forward Grade 11 Trig and Physics. It assumes that the origin is the lowest point of the pendulum bob's suspension, that L is the length of the pendulum, and that the normal graphics convention of y increasing down, and x increasing to the right is adopted:

Update: I messed up yAcceleration initially; this is easier.
Update #2: Added explicit time control, and added units of measure.

const float gravity = 9.8;     // units of metres/sec/sec
const float deltaT  = 0.001;   // equals 0.001 sec or 1 millisecond

var xVelocity = 0.010;         // units metres/sec equals 10 cm/sec 
var x = 0.0;                   // units metres
var y = 0.0;                   // units metres

while (true) {
  var xAcceleration = -g * (x/L) * (L-y)/L;
  
  x += (xVelocity + (xAcceleration/2 * deltaT)) * deltaT;
  y  = Math.SQRT(L*L - x*x) - L; 

  xVelocity += xAcceleration * deltaT;
}

Here is a no-trig calculation, derived from straight-forward Grade 11 Trig and Physics. It assumes that the origin is the lowest point of the pendulum bob's suspension, that L is the length of the pendulum, and that the normal graphics convention of y increasing down, and x increasing to the right is adopted:

Update: I messed up yAcceleration initially; this is easier.

const float gravity = 9.8;

var xVelocity = 2.0;  // adjust this figure to taste; leave other start values as is
var x = 0.0;
var y = 0.0;

while (true) {
  var xAcceleration = -g * (x/L) * (L-y)/L;
  
  x += xVelocity + xAcceleration/2;
  y  = Math.SQRT(L*L - x*x) - L; 

  xVelocity += xAcceleration;
}

Here is a no-trig calculation, derived from straight-forward Grade 11 Trig and Physics. It assumes that the origin is the lowest point of the pendulum bob's suspension, that L is the length of the pendulum, and that the normal graphics convention of y increasing down, and x increasing to the right is adopted:

Update: I messed up yAcceleration initially; this is easier.
Update #2: Added explicit time control, and added units of measure.

const float gravity = 9.8;     // units of metres/sec/sec
const float deltaT  = 0.001;   // equals 0.001 sec or 1 millisecond

var xVelocity = 0.010;         // units metres/sec equals 10 cm/sec 
var x = 0.0;                   // units metres
var y = 0.0;                   // units metres

while (true) {
  var xAcceleration = -g * (x/L) * (L-y)/L;
  
  x += (xVelocity + xAcceleration/2) * deltaT ;
  y  = Math.SQRT(L*L - x*x) - L; 

  xVelocity += xAcceleration * deltaT;
}