For each case, identify the force (including its direction) that causes the acceleration. (a) a sprinter at the very start of the race. the gravitation force by earth on the sprinter the backward direction the normal force of the block on the sprinter the forward direction (b) a hockey puck skidding freely but slowly coming to rest on the ice. the opposite direction to the velocity the frictional force by the ice on the puck the parallel direction to the velocity the gravitation force by earth on the puck (c) a long fly ball at the top of its arc. the gravitation force by earth on the ball the upward direction the downward direction the frictional force by the air on the ball (d) a bungee jumper at the very bottom of her descent. the upward direction the gravitation force by earth on the jumper the force exerted by the bungee cord on the jumper the downward direction