Question

what happens to the speed of a cart when a horse applies more force

Answer 1

when a horse applies more force to the cart the cart starts moving faster than it's earlier speed
for eg: 400N+50N=450N
hence we can say that 450N>400N

Answer 2

when a horse applies more force on the cart the cart moves even faster than earlier because here now the horse is using his full force, energy to speed up the cart . And one more factor behind it is that horse is the most energetic and powerful animal. ( hope that this answer help you)