# Can you justify force calculations using more than one of Newton's laws?

If an object is at rest on a flat surface I remember my school teacher would say, "By Newton's 1st law the normal reaction, R, is mg (mass x g)". Ok, sure this is fine because the object is at constant velocity (velocity=zero) and so there are no net forces and so the two forces must balance. But what's wrong with saying "By Newton's 2nd law R-mg=0. Therefore R=mg." (The net force on the object is zero, therefore the acceleration on the object is zero) But can't you also say, "By Newton's 3rd law R=mg." (The object is pushing on the ground with a force of mg, therefore the ground is pushing on the object with a normal reaction of mg).

Is this correct or have I erred somewhere?

• You're using the third law wrong. The reaction force of mg(for the object) is the Earth's force of mg pushing towards the object. The reaction of the normal force acting from the table to the object is the normal force acting from the object to the table. – resgh Aug 20 '13 at 5:03