Consider the the front wheels of a roller coaster cart. Presumably, when the roller coaster in in a straight section the left and right wheels are moving at the same speed. But when the roller coaster enters a turn, the wheels on the outside of the turn must move faster than the wheels on the inside, since the outer wheels need to travel a longer distance (larger radius) than the inside wheels.
What force causes the outer wheels to speed up (or the inner wheels to slow down)?
See the picture below. At (A) the wheels are turning the same speed in the straight section of track. At (B) the cart is just about to enter a turn, still going straight. At (C) the cart is in a turn, and the outer (left) wheel must be spinning faster than the inner (right) wheel. What force was applied between (B) and (C) to make this happen?