Physic Labs

Newtonian mechanics

Newton’s third law

When object A exerts a force on B, B simultaneously exerts an equal and opposite force on A along the same line.

When you push a wall, the wall pushes you back. The two forces act simultaneously on different objects, so they do not cancel on one object.

F⃗(A→B)=−F⃗(B→A)F⃗(A→B) = −F⃗(B→A)

Definition: Quantities and model

The interaction forces have equal magnitude and opposite directions, but act on different objects. They therefore cannot be combined as the net force on either object alone.

Use the simulation to observe motion and read quantitative values as time evolves.

Reading the model

When object A exerts a force on B, B simultaneously exerts an equal and opposite force on A along the same line.

Example: Worked example

A person on skates pushes a wall forward. The wall pushes the person and skates backward, making them roll away; the force on the person can change their motion.

Solution

A person on skates pushes a wall forward. The wall pushes the person and skates backward, making them roll away; the force on the person can change their motion.

The two forces in an interaction pair occur together but act on different objects, so they do not cancel on one object’s free-body diagram. If a hand pushes a wall with 40 N, the wall simultaneously pushes the hand with 40 N in the opposite direction. While walking, the foot pushes the ground backward and the ground pushes the person forward; friction between shoe and ground enables this force.

Example: Hand against a wall

A skater pushes a wall with 30 N. What force does the wall exert on the skater?

Solution

30 N in the opposite direction, acting on the skater.

An action–reaction pair does not necessarily keep both objects at rest: objects of different mass have different accelerations under Newton’s second law. Earth pulls on an apple and the apple pulls equally on Earth, but Earth’s acceleration is tiny because its mass is enormous.

Quick check

Which quantity or statement is correct for this motion?

Which statement correctly describes the motion in this topic?

References

  1. Young, Freedman (2019). University Physics
  2. Halliday, Resnick, Walker (2014). Fundamentals of Physics