This is a very old physics brain-teaser: A horse is pulling a wooden sled across the ground. According to Newton's third law of motion, the force of the horse acting on the sled pulling it forward must be equal and opposite to the force of the sled pulling backward on the horse. According to Newton's second law of motion, only net forces cause accelerations. Since the two forces must add to zero by the third law, the acceleration of the horse and sled must also be zero. 50 how can the sled ever move, much less accelerate