EK Physics Item 21:
Newton's Third Law states that for every action there exists an equal and opposite reaction. What is the force that acts in reaction to the force of Earth's gravity on an object on its surface?
A) The normal force that the object exerts on the Earth
B) The force of gravity that the object exerts on the Earth
C) The normal force that the Earth exerts on the object
D) The force of gravity that the Earth exerts on the object
Answer
I do not understand the rationale for the answer. I had chosen C because the normal force is the reactionary force to the force of gravity.
Thanks!
Newton's Third Law states that for every action there exists an equal and opposite reaction. What is the force that acts in reaction to the force of Earth's gravity on an object on its surface?
A) The normal force that the object exerts on the Earth
B) The force of gravity that the object exerts on the Earth
C) The normal force that the Earth exerts on the object
D) The force of gravity that the Earth exerts on the object
Answer
B is correct. Newton's Third Law refers to the same type of force acting between two objects. In this case the Earth exerts a force of gravity on an object and the object exerts an equal and opposite force on the Earth. Choice A is incorrect because the normal force is not always equal in magnitude to the force of gravity; for example, the normal force on an inclined plane. Choices C and D are incorrect because the reaction force would be exerted by the object on the Earth. Choice D also restates the same force that is acting in the question stem.
I do not understand the rationale for the answer. I had chosen C because the normal force is the reactionary force to the force of gravity.
Thanks!