can approach this speed asymptotically, but never reach it. General relativity Unless the state of motion of an object is known, it is impossible to distinguish whether an observed force is due to gravity or to acceleration—gravity and inertial acceleration have identical effects. Albert Einstein called this the equivalence principle, and said that only observers who feel no force at all—including the force of gravity—are justified in concluding that they are not accelerating. Conversions See also Acceleration (differential geometry) Four-vector: making the connection between space and time explicit Gravitational acceleration Inertia Orders of magnitude (acceleration) Shock (mechanics) Shock and vibration data loggermeasuring 3-axis acceleration Space travel using constant acceleration Specific force References External links Acceleration Calculator Simple acceleration unit converter Acceleration Calculator Acceleration Conversion calculator converts units form meter per second square, kilometer per second square, millimeter per second square & more with metric conversion. Dynamics (mechanics) Kinematic properties Temporal rates Vector physical quantities