Jack and Annie push a cart to the left with forces of 3 N and 4 N, respectively. If the cart is stationary, what is the force of friction?

A. 1 N to the left

B. 3 N to the right

C. 7 N to the right

D. 7 N in the positive direction

Jack and Annie, working together, are pushing the cart to the left with 7 Newtons of force. Since it's not moving, there must be 7 Newtons of force directed to the right coming from somewhere. That's the friction. (Choice-C.)

If the forces on the cart didn't add up to zero, then the cart would be accelerating.