A2L Item 021
- Description: Identify the equal-and-opposite force (from Newton's Third Law) to the gravitation force.
- Goal: Develop the ability to identify 3rd-Law Pairs, the parts of an interaction.
- Source: UMPERG-ctqpe42
- Keywords: Newton's Third Law, Mechanics, Forces
The question for students:
The two blocks shown below are identical. In case A the block sits on a horizontal surface and in case B the block is in free fall.
Which of the following statements are true regarding the reaction force to the gravitational force exerted on each block?
- In case A the reaction force is the Normal force.
- In case B the reaction force is zero.
- The reaction force is larger in A than B.
- All the above are true.
- Only (1) & (2) are true.
- None of the above are true.
- Cannot be determined.
Commentary for teachers:
Answer
(6) The reaction force to the gravitational force exerted (by the earth) on a block is the gravitational force exerted (by the block) on the earth. In both cases the reaction force is non zero and because the blocks are identical the reaction forces for the two cases are equal.
Background
Newton’s third law can be counter intuitive to many students and the concept of reaction force can be very confusing. Students often think that the reaction force to some force exerted on an object is a balancing force on the same object.
(1) Many students will think the normal force is the reaction force because it is equal and opposite to the gravitational force exerted on the block.
(2) Some students may reason that since there is no balancing force there is no reaction force.
Questions to Reveal Student Reasoning
What is a reaction force? What are some examples? Do action-reaction force pairs act on the same, or different bodies? Why is reaction force an important idea?
Suggestions
Reaction force is an abstract concept. It cannot be demonstrated. One needs to make sure students understand its definition. The first step is to make sure students understand the idea of an interaction: When two objects affect each other (i.e., influence each others motion, or shape) then we say that the objects interact. We find that all interactions are two-way: if the motion/shape of one of the interacting objects is affected then the motion/shape of the other object is always affected. We ultimately quantify the effects and refer to the causes of these effects as forces. An interaction involves two forces, one on each object. Action-reaction pair refer to the two parts (forces) of an interaction. Newton’s third law states the relationship between the two parts (forces) of an interaction: the two forces are equal in magnitude and point opposite in direction.