There is a site that explains the old problem of why the horse can pull the cart and move even though Newton's third law says every action has an equal and opposite reaction. It does it well except on this page when it summaries everything. http://www.batesville.k12.in.us/Physics/PhyNet/Mechanics/Newton3/HorseAndCart4.htm I don't quite agree with how it deals with the frictional force. For a wheel, when it spins on normal road, it exerts a force backwards and the ground via friction exerts a force on the wheel forwards. This is what makes the wheel hence cart move forward on a normal road. If there is no friction than the wheel will just spin on the spot. Imagine spinning the wheel in air, it will send air molecules backwards but the force from the air molecules is not directional so do not allow the wheel to travel forwards. The fact that the earth doesn’t move wrt the cart means that for an observer on earth, the cart moves forward. On that website, it shows the force by wagon on ground (which is exerted via the wheel from the diagram) is to the right. And the force by ground on wagon (via wheel) is to the left. This would only be correct if the wheel was locked and can’t turn. But that would be dumb and wasn’t specified on the website. So if the wheels are allowed to turn than the website got their facts wrong. Am I correct? However, I see that it wants to show that by introducing friction, the net force on the cart is reduced. So it might be hard to encoprate what I suggest because on the one hand they want to show the ground reduces the motion of the cart but in reality for a wheel the ground exerts the forward force. Maybe they could show that the net force is exerted by the ground to the cart via the wheel. How would you show everything correctly and systematically?