Newton's third Law - action reaction pair

In summary, the concept of action-reaction pair of forces means that when two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. This applies even if the objects have different masses and are subject to other forces, resulting in different accelerations. In these cases, the equal and opposite forces can help determine the other forces acting on the objects.
  • #1
caligirl
2
0
Hi,

I am a little confused with the concept of action reaction pair of forces. Does this depend on mass?

For example, if a train engine is pulling a buggy with force F, what would be the force applied by the buggy on the engine? The masses of the two are different and there is force of friction on the buggy.


I know that for 2 forces to be action reaction pair, they have to act on different objects. Does this mean that the force the buggy applies on the train engine is also F (in magnitude), but since mass is different, the acceleration would vary? I am not sure if I fully understand the concept.
 
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  • #2
caligirl said:
Hi,

I know that for 2 forces to be action reaction pair, they have to act on different objects. Does this mean that the force the buggy applies on the train engine is also F (in magnitude), but since mass is different, the acceleration would vary?

The force that the buggy applies on the train engine is also F. Since the buggys is attached to the train their accelerations can't be the same. This can happen because the force F isn't the only force on the train or on the buggy.
 
  • #3
Does friction play a role in this situation?
 
  • #4
willem2 said:
Since the buggys is attached to the train their accelerations can't be the same.
I guess you mean "can't be different"?
 
  • #5
caligirl said:
Does friction play a role in this situation?
Both the engine and buggy can have rolling resistance, but to simplify you can neglect that. The engine has also a propelling force which is static friction (traction). For both the engine and buggy the sum of all forces acting on them must produce the same acceleration.
 
  • #6
caligirl said:
I know that for 2 forces to be action reaction pair, they have to act on different objects. Does this mean that the force the buggy applies on the train engine is also F (in magnitude), but since mass is different, the acceleration would vary? I am not sure if I fully understand the concept.
Yes, provided that is the only force acting. Remember, Newton's 2nd law is often mis-stated as f=ma, but it is actually ∑f=ma.

If you have only one force acting on an object and only the reaction force acting on the other object then the accelerations will necessarily be in opposite directions, and the accelerations will be different magnitudes if the masses differ. Consider, for example, the Earth and moon interacting through gravity.

In the train-buggy example, there are other forces acting on each object, so you can have their interaction force obey the third law (equal and opposite) while having them accelerate the same. In many cases you can use that fact to determine the other forces.
 

1. What is Newton's third law of motion?

Newton's third law of motion 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 will exert an equal force in the opposite direction.

2. Can you provide an example of Newton's third law in action?

One example of Newton's third law is when you push against a wall, the wall pushes back on you with an equal force. Another example is when a rocket is launched into space, the exhaust gases push down on the ground and the ground pushes the rocket up with an equal force.

3. Are there any exceptions to Newton's third law?

Newton's third law is a fundamental principle of physics and applies to all types of forces, including gravitational, electrical, and magnetic forces. However, it may seem like there are exceptions in certain situations, such as when an object falls to the ground. This is because the object is not pushing back on the Earth with an equal force, but rather the Earth's mass is so much larger that it is barely affected by the object's force.

4. How does Newton's third law relate to everyday life?

Newton's third law can be seen in many everyday situations, such as when you walk on the ground, the ground pushes back on your feet with an equal force, allowing you to move forward. It also explains why you feel a kickback when shooting a gun or why you feel the wind in your face when riding a bike.

5. How does Newton's third law impact the design of vehicles and structures?

Engineers and designers use Newton's third law to ensure that vehicles and structures can withstand the forces they will encounter. For example, in a car crash, the car's structure is designed to absorb the impact force by exerting an equal and opposite force on the occupants, keeping them safe. Similarly, buildings are designed to withstand strong winds by exerting an equal and opposite force on the wind.

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