Beginner · 10 min
Forces come in pairs
Identify interaction pairs and explain why equal forces can cause different accelerations.
Two objects interact
Newton's third law says that when object A exerts a force on object B, B exerts a force of equal magnitude and opposite direction on A. These forces act on different objects. They do not cancel on a force diagram for just one object. For example, a foot pushes the ground backward while the ground pushes the foot forward.
Worked example
If a hand pushes a wall with 25 N, the wall pushes the hand with 25 N in the opposite direction. Each force acts on a different object.
Equal forces, different effects
Acceleration depends on net force and mass. Two interacting objects can feel equal interaction forces but accelerate by different amounts because their masses differ or because other forces act. Draw a separate force diagram for each object. Balanced forces on one object are not a third-law pair; a third-law pair belongs to one interaction between two objects.
Worked example
Two carts push apart with 6 N on each, with no other horizontal forces. A 2 kg cart accelerates at 3 m/s²; a 3 kg cart accelerates at 2 m/s², in opposite directions.
Draw separate force diagrams
- Draw a person pushing a box. Label the person's force on the box and the box's force on the person on separate diagrams.
- Explain why these equal and opposite forces do not imply that the box's net force must be zero.