A 400kg horse is connected via a harness to a 498 kg cart. The earth exerts a gravitational force on both the horse and the cart. The gravitational force exerted on the earth by the car is?

The gravitational force exerted on the earth by the cart is the same as the gravitational force exerted by the earth on the cart, by Newton's third law. So the force is m x g = 498 x 9.81 = 39.2 N The gravitational force exerted on the earth by the cart and the horse is (400 + 498) kg x 9.81 m/s^2 = 8809 N