SAT Question concerning Newton's Third Law

In summary, the person standing on the floor feels two forces: the downward pull of gravity and the upward supporting force from the floor. According to the book, these two forces have equal magnitudes but do not form an action/reaction pair because they are acting on the same object. However, the push of the person on the floor can be seen as the action, with the reaction being the push of the floor on the person. This is not an action/reaction pair according to Newton's third law, as it refers to forces acting on different objects. It is an example of Newton's first law, where the equilibrium of forces acting on the same object is described. Additionally, the repulsion between the surfaces of the person's feet and
  • #1
yellowenigma
1
0
I didn't understand the explanation, and think perhaps the book is wrong.

A person is standing on a horizontal floor fells two forces: the downward pull of gravity and the upward supporting force from the floor. These two forces are:

I put A) have equal magnitudes and form an action/reaction pair.

according to the book, they B) have equal mags and do not form an action/reaction pair.

The book explains that b/c the two forces are felt by the same object (the person), it cannot be an action/reaction pair, but doesn't the push of the person lead to the reaction of the push on the person from the earth?
 
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  • #2
yellowenigma said:
I didn't understand the explanation, and think perhaps the book is wrong.

A person is standing on a horizontal floor fells two forces: the downward pull of gravity and the upward supporting force from the floor. These two forces are:

I put A) have equal magnitudes and form an action/reaction pair.

according to the book, they B) have equal mags and do not form an action/reaction pair.

The book explains that b/c the two forces are felt by the same object (the person), it cannot be an action/reaction pair, but doesn't the push of the person lead to the reaction of the push on the person from the earth?
Yes, but action-reaction pairs, per Newton 3, refer to forces acting on different objects. What you have described is the equilibrium of forces acting on the same object, which is Newton 1. The upward force of the floor on the person is not the same as the person's weight in the general case of accelerating bodies (that's Newton 2), you know, like if you're in an elevator accelerating up, the floor force on you is greater than than your weight, per Newton 2, but the force of you on the floor is always the same in magnitude as the force of the floor on you, per Newton 3.
 
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  • #3
yellowenigma said:
... but doesn't the push of the person lead to the reaction of the push on the person from the earth?

There is an unfortunate double-use of the term "reaction force." As the gravitational force on the person pulls him down, the floor "gets in the way." As the two surfaces get close to each other, they begin to repel with a mutual electostatic force. The surface of the person's feet repels the surface of the floor and vice versa. These two forces are action/reaction pairs.

Whenever you find a reaction pair, all you need to do is switch the places of the nouns.
action: person pulled by earth
reaction: Earth pulled by person
action: foot repels floor
reaction: floor repels foot
 
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FAQ: SAT Question concerning Newton's Third Law

What is Newton's Third Law?

Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object will exert an equal and opposite force back on the first object.

How does Newton's Third Law apply to SAT questions?

Newton's Third Law can be tested on the SAT by asking questions about how forces interact with each other and how they affect the motion of objects. These questions may involve identifying action and reaction pairs or calculating the net force on an object.

Can you give an example of Newton's Third Law in action?

One example of Newton's Third Law is a person pushing against a wall. The person exerts a force on the wall, but the wall also exerts an equal and opposite force back on the person. This is why the person does not move the wall, but instead feels resistance.

What is the difference between Newton's Third Law and Newton's Second Law?

Newton's Second Law states that the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass. This law deals with the relationship between force, mass, and acceleration. Newton's Third Law, on the other hand, deals with the interaction of forces between two objects.

How can understanding Newton's Third Law be helpful in real life?

Understanding Newton's Third Law can be helpful in many real-life situations, such as understanding how rockets are propelled, how cars move, and how sports involve action and reaction forces. It can also help in designing structures and machines that can withstand and utilize forces effectively.

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