a shopper pushes a grocery cart 20M at a constant speed a level ground, against a 35N friction force. he pushes in a direction below the horizontal. a) what is the work done on the cart by friction. b) what is the work done on the by the gravitational?., c) what is work done on the cart by the gravitational friction?,d) find the force the shopper exerts using energy consideration ? , c) what is total work done on the cart?​