Video narration
You measure a desk in metres, convert it to centimetres, and wonder: are the unit and dimension the same thing? No. A unit is a standard for how much: metre, centimetre, or newton. A dimension says what kind of quantity it is, using the basic quantities mass, length, and time. A desk length can be 1 metre or 100 centimetres. The number and unit change, but the dimension stays length. Now force. From F equals m a, mass contributes M and acceleration contributes L T−2. Therefore, [F] equals [M L T−2]. That's not the unit. The SI unit is the newton, N: 1 N equals 1 kg m s−2. So, metres to centimetres changes the unit. M, L, and T describe the dimension. A unit tells how much; a dimension tells what kind of quantity-and force is [M L T⁻²].