A good example of a law not working in all cases is Newton's laws of motion. Newton's theories of motion became Newton's laws of motion after several centuries of making accurate predictions; then Einstein came along, and now it turns out that Newton's laws of motion are simply the low-velocity approximations of a more general theory of motion, that being Special Relativity.
E=mc^2 tells us that energy and mass are interchangeable -- they're the same thing. m = m/((1-v^2/c^2)^(1/2)) tells us how mass changes with increased velocity. At the speed of light, that is, v = c, then m = infinity. Infinite mass has infinite inertia; in order to accelerate infinite mass (i.e., accelerate something past c), you would need infinite energy. However, since energy and mass are the same thing, you can't simultaneousl;y have both infinite mass AND infinite energy.
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E=mc^2 tells us that energy and mass are interchangeable -- they're the same thing. m = m/((1-v^2/c^2)^(1/2)) tells us how mass changes with increased velocity. At the speed of light, that is, v = c, then m = infinity. Infinite mass has infinite inertia; in order to accelerate infinite mass (i.e., accelerate something past c), you would need infinite energy. However, since energy and mass are the same thing, you can't simultaneousl;y have both infinite mass AND infinite energy.