There are 3 laws of physics: Inertia - a body at rest stays at rest unless acted upon by an outside force; Force = Mass x Acceleration; and for every action there is an equal and opposite reaction. These apply to strength training: a weight remains at rest until acted upon by a force. Muscles generate the force to move a weight. The act of lifting weights can improve muscle strength and appearance. Learn the benefits of strength training and how to add it to your workouts.