Problema Solution
Take your smooth card and place on the lab bench with its end poking out beyond the edge of the table. Stack a bunch of pennies on it and try to yank the card out from underneath it without disturbing the pennies. Why does the card move, but not the stack?
Answer provided by our tutors
The card should be smoothly pulled out with horizontal force. The pennies exert more vertical force than the horizontal friction force which is overcome when the card is pulled out.