Hi there I have been posed a brain teaser: If pushing against a trolley, if there is an equal and opposite opposing force (ie. friction) why may the trolley still acceleerate? The best solution I have is that between the wheels and the ground there is a static friction force. This force on the bottom of the wheels causes a torque by R(cross)F so the wheels begin to rotate, pulling the cart along with it..... . ..... But it still seems a little like a contradiction that something can have a linear acceleration with no net force... . Anyone care to clarify this?