Physics · Grades 8–11
Interaction pairs act on different objects
Identify equal-and-opposite pairs without cancelling the wrong forces.
Learn the idea
Interaction
If A exerts a force on B, B exerts an equal-magnitude opposite-direction force on A.
Different objects
The two forces act on different bodies, so they do not cancel in a force sum for only one body.
Different accelerations
Equal interaction forces can produce different accelerations when the masses differ.
Name both bodies
Describe a pair as force of A on B and force of B on A rather than vaguely calling them two balanced forces.
Worked examples
A hand pushes a wall. What is the matching interaction force?
The partner swaps the interacting bodies.
The wall pushes the hand.
Why do interaction-pair forces not cancel on a diagram of only the hand?
Only forces on the hand belong in that hand's resultant.
They act on different objects.
Check your understanding
A trolley exerts 14 N on a second trolley. Find the magnitude of the second's force on the first.
A small cart and a large cart push apart. Why can their accelerations differ?
Which is the partner of Earth's gravitational force on a book?
- The book's gravitational force on Earth.
- Air's force on the table.
- The table's upward force on the book.
Answers and marking notes
Answers and marking notes
- 14 N
Interaction forces have equal magnitude.
- They experience equal force magnitudes but have different masses.
Use a = F/m; equality of force does not require equality of acceleration.
- The book's gravitational force on Earth.
A force pair belongs to the same interaction with body roles exchanged.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.