This is usually called "the law of conservation of mass and energy". Kinetic energy is Mass * Speed2. The law of conservation of energy could be linked by explaining how an object looses kinetic energy and gains another form of energy. For instance, when you throw a baseball you transfer kinetic energy from your arm to the baseball. Why doesn't the baseball fly forever - where does the kinetic energy go? One reason is the air friction against the ball; this converts kinetic energy to heat energy. Another reason is the earth's gravity attracts the ball. When it hits the ground the friction converts the kinetic energy to heat energy. These conversions between kinetic and heat energy satisfy the law of conservation of energy.
