Action-Reaction Pairs: What Are They?

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In summary: The net force on m2 is the sum of the forces on m1. The net force on m1 is the sum of the forces on m2.
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OVB
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A concept I don't understand is the force exerted by one block of mass m1 on a block of mass m2 and vice versa when a force F is applied to m1.

What I mean is, say you have to blocks next to each other and a force F is applied to m1. My physics teacher says m1 will accelerate, but m2 will accelerate in the opposite direction. How can the forces be equal if m1 and m2 aren't? My teacher says F - (m2)(a) = (m1)(a). How is this possible? It makes absolutely no sense to me how this can be possible if (m2)(a) and (m1)(a) form an action-reaction pair.
 
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  • #2
hmm...lets say u exert a force on m1 and they both move to the right.draw the FBD of both mass and u will notice that other than the force exerted and frictional force,theres a force on m1 by m2 in the left direction and a force on m2 by m1 to the right.this forms the action reaction pair??
 
  • #3
yeah, but why? Why can you use the acceration derived from the formulae I used for the mass m2 if it is not being accelerated in that direction?
 
  • #4
OVB said:
What I mean is, say you have to blocks next to each other and a force F is applied to m1. My physics teacher says m1 will accelerate, but m2 will accelerate in the opposite direction.
Are you sure that's what he said? If you have two blocks next to each other and you push against the left one with a force acting to the right, both blocks will accelerate together--in the same direction. Perhaps he said that the two blocks will exert equal and opposite forces on each other, which is certainly true.
How can the forces be equal if m1 and m2 aren't?
What's their mass have to do with it? If you push on an elephant with a force F, it will push back on you with a force F. That's Newton's 3rd law.

My teacher says F - (m2)(a) = (m1)(a). How is this possible? It makes absolutely no sense to me how this can be possible if (m2)(a) and (m1)(a) form an action-reaction pair.
Your teacher is correct, but (m2)(a) and (m1)(a) do not form an action-reaction pair. (They aren't even equal!) (m1)(a) will equal the net force on m1; (m2)(a) will equal the net force on m2.

If you consider the two blocks as a single system, the only force acting on them is the applied force F. Apply Newton's 2nd law: F = (m1 + m2)a. That's equivalent to what your teacher said, but it says nothing about the forces between m1 and m2.

To find those forces, consider each block as a separate system. What forces act on m1? What forces act on m2? (Note that two forces act on m1, but only one force acts on m2.)
 

Related to Action-Reaction Pairs: What Are They?

1. What is an action-reaction pair?

An action-reaction pair is a fundamental concept in Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object exerts an equal and opposite force back on the first object.

2. How do action-reaction pairs apply to everyday life?

Action-reaction pairs can be observed in many everyday situations. For example, when you push against a wall, the wall pushes back on you with an equal and opposite force. Similarly, when you swim in a pool, you push against the water and the water pushes back on you, propelling you forward.

3. Can action-reaction pairs cancel each other out?

No, action-reaction pairs cannot cancel each other out. While the forces may be equal and opposite, they act on different objects and therefore do not cancel each other out. Instead, they result in a net force on each object, causing them to accelerate in opposite directions.

4. How are action-reaction pairs related to momentum?

Action-reaction pairs are related to momentum through Newton's second law of motion, which states that the net force on an object is equal to its mass times its acceleration. In action-reaction pairs, the forces on each object have equal magnitudes but opposite directions, resulting in a change in momentum for both objects.

5. Can action-reaction pairs occur between non-contacting objects?

Yes, action-reaction pairs can occur between non-contacting objects. This is known as action-at-a-distance, where the objects exert forces on each other without physically touching. An example of this is the gravitational force between two objects, such as the Earth and the Moon.

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