a safety apparatus for a tool driven by a motor removes motor power under certain circumstances . the tool includes a housing moveable over a ground surface for mounting the tool , and a ground - engaging member moveable relative to the housing and disposed to follow the ground surface as the housing moves over same . the apparatus further includes a sensor for measuring the relative position of the housing relative to the ground engaging member . motor control apparatus selectively stops the motor upon the ground - engaging member and the housing reaching a predetermined relative position .