<form id="form1" name="form1" method="post" action="">
Espaco Inicial
<label>
<input type="text" name="espaco_inicial" id="textfield" />
</label> <br>
Velocidade Inicial
<label>
<input type="text" name="vel_inicial" id="textfield" />
</label><br>
Aceleracao
<label>
<input type="text" name="aceleracao" id="textfield" />
</label><br>
Tempo
<label>
<input type="text" name="tempo" id="textfield" />
</label>
<label>
<input type="submit" name="button" id="button" value="Submit" />
</label>
</form>
<?php
echo "Superficie Final = ".$_POST['espaco_inicial'] + $_POST['vel_inicial'] * $_POST['tempo'] + $_POST['aceleracao'] * $_POST['tempo'] * $_POST['tempo']/2;
// Sf = Si + V0*t + a*t*t/2
?>
