When we open the lid of a bottle by turning it our fingers

Last updated: 6/5/2023

When we open the lid of a bottle by turning it our fingers are applying a couple to the lid Fig 7 21 a Another known example is a compass needle in the earth s magnetic field as shown in the Fig 7 21 b The earth s magnetic field exerts equal forces on the north and south poles The force on the North Pole is towards the north and the force on the South Pole is toward the south Except when the needle points in the north south direction the two forces do not have the same line of action Thus there is a couple acting on the needle due to the earth s magnetic field