Simple answer: gravity is a centripetal force, and can be envisaged clearly as such in Newtonian mechanics.

Centripetal just means a force that is "radially inwards" ("directed towards the centre"). The electric force, for example, is also clearly centripetal. (It's slightly harder to define "centripetal" for the magnetic force.)

Your astronomy teacher is referring to Einstein's theory of general relativity. His description is loosely an overview  of the topology (fabric) of space-time and how it interacts with matter/energy - the manifold is however 4-dimensional, not 3D.

In fact, test particles (particles which do not really disturb the gravitational field) in general relativity follow a geodesic. This is effectively a generalisation of a straight line (shortest route) of normal Euclidian space to the curved space of GR, and may be seen as the source of centripetal force in Newtonian physics.

Planet orbits: what's the difference between gravity and centripetal force?
I have form that sends message me when enter Email and MessageText. I want to use both of jQuery Validation Engine plugin and ajax response to return result. For this I use same jQuery('#messageSendButton').click() function in every case. 

To return response from controller:

&lt;script type="text/javascript"&gt;
                jQuery('#messageSendButton').click(function () {

                    var url = '@Url.Action("SendQuickMessage", "Home")';
                    var email = $('#email').val();
                    var messageText = $('#messageText').val();

                    $.ajax({
                        url: url,
                        type: 'GET',
                        contentType: 'application/json',
                        data: { Email: email, Message: messageText },
                        success: function (result) {
                            if (result.success) {
                                alert("Message sent! Thanks!");
                            }
                            else {
                                alert("Error occured!");
                            }
                        },

                        complete: function () {
                            $(".loading").hide();
                        }
                    });
                });
            &lt;/script&gt;


And, to validate form:

  &lt;script type="text/javascript"&gt;
                jQuery(document).ready(function () {
                    jQuery('#quickMessageForm').validationEngine('init');
                    jQuery('#messageSendButton').click(function () {
                        jQuery('#quickMessageForm .loading').animate({ opacity: 1 }, 250);
                        if (jQuery('#quickMessageForm').validationEngine('validate')) {

                            jQuery.post('/Home/SendQuickMessage', {
                                Email: jQuery('#email').val(),
                                Message: jQuery('#message').val(),
                                formname: 'messageForm',
                                formtype: 'messageF'
                            }, function () {
                                jQuery('#quickMessageForm .loading').hide();
                            });

                            return false;
                        } else {
                         jQuery('#quickMessageForm .loading').animate({ opacity: 0 }, 250);
                            return false;
                        }
                    });
                });
            &lt;/script&gt;


Separately, they both works alone, but together, firstly ajax function works, second validation works. it is not right.. How can I combine those in one? Firstly to validate form, then get ajax response.

Edit:

Here is form:

 &lt;form action="#" method="post" id="quickMessageForm"&gt;
            &lt;div class="form_info cmsms_input"&gt;
                &lt;label&gt;Email&lt;span class="color_3"&gt; *&lt;/span&gt;&lt;/label&gt;
                @Html.TextBoxFor(model =&gt; model.Email, new { name = "email", id = "email", tabidex = 1, @class = "validate[required,custom[email]]" })
            &lt;/div&gt;
            &lt;div class="form_info cmsms_textarea"&gt;
                &lt;label&gt;Message&lt;span class="color_3"&gt; *&lt;/span&gt;&lt;/label&gt;
                @Html.TextAreaFor(model =&gt; model.Message, new { id = "messageText", name = "messageText", tabindex = 2, @class = "validate[required,minSize[10]]" })
            &lt;/div&gt;
            &lt;div class="loading"&gt;&lt;/div&gt;
            &lt;div&gt;
                &lt;input type="submit" name="messageSendButton" id="messageSendButton" value="Send" class="button_small" tabindex="3" /&gt;
            &lt;/div&gt;
        &lt;/form&gt;

