How to add recaptcha to contact form - php

Question

I tried to use the method from this question, but it didn't work for me.
I always get this error after submiting form:
Fatal error: Cannot use object of type JInput as array in contact.php line 76...
Line 76 looks like this:
$res = $dispatcher->trigger('onCheckAnswer',$post['recaptcha_response_field']);
My contact form:
<div id="contact">
<div id="message"></div>
<form method="post" action="<?php
echo JURI::root() ?>modules/module/tmpl/form/contact.php" name="contactform" id="contactform">
<fieldset>
<input name="name" type="text" id="name" size="30" value="" placeholder="<?php
echo $params->get('contactname'); ?>" onfocus="this.placeholder = ''" onblur="this.placeholder = '<?php
echo $params->get('contactname'); ?>'"/>
<br />
<textarea style="width: 275px;" name="comments" cols="40" rows="3" id="comments" style="width: 350px;" placeholder="<?php
echo $params->get('contactcomment'); ?>" onfocus="this.placeholder = ''" onblur="this.placeholder = '<?php
echo $params->get('contactcomment'); ?>'"></textarea>
<br />
<br />
<?php
JPluginHelper::importPlugin('captcha');
$dispatcher = JDispatcher::getInstance();
$dispatcher->trigger('onInit','recaptcha');
?>
<div id="recaptcha"></div>
<input type="submit" class="submit" id="submit" value="<?php
echo $params->get('contactbutton'); ?>" />
</fieldset>
</form>
</div>
I added this to contact.php
$post = JFactory::getApplication()->input->post;
$dispatcher = JEventDispatcher::getInstance();
JPluginHelper::importPlugin('captcha');
$res = $dispatcher->trigger('onCheckAnswer',$post['recaptcha_response_field']);
if(!$res[0]){
die('<div class="error_message">' . $errorcomments . '</div>');
}

Answer 1

Instead of
JFactory::getApplication()->input->post;
Try
JRequest::get('post');