SPH3U: Physics, Grade 11, University Preparation 9. A child is standing on the floor. If the action force is the force of gravity of the Earth pulling down on the child, then the reaction force is: (a) the force of the floor pushing up on the child. (b) the force of gravity of the child pulling up on the Earth. (c) the force of the child pushing down on the floor. (d) a net force causing the child to accelerate. 10.A constant force of 36 N is applied to a 20 kg mass (X) that is in contact with a 4.0 kg mass (Y) on a frictionless surface. F What is the magnitude of the force exerted by mass X on mass Y? (a) 6.0 N (b) 29 N (c) 30 N (d) 36 N