Identify equal-and-opposite force pairs on different objects and distinguish mass from weight.
Method and worked examples
Newton’s third law states that when object A exerts a force on object B,B simultaneously exerts an equal-magnitude, opposite-direction force on A. The forces act on different objects. A swimmer pushes water backward and the water pushes the swimmer forward. These forces do not cancel in the swimmer’s own force account because only one of the pair acts on the swimmer.
Weight is the gravitational force on an object: W= mg. Mass in kilograms measures matter and inertia, while weight in newtons depends on local gravitational field strength. A book resting on a table experiences weight and an upward support force; these balance on the same book but are not a third-law pair. The partner to Earth’s pull on the book is the book’s gravitational pull on Earth.
Example 1
A hand pushes a wall with 40N. What force does the wall exert on the hand?
The wall exerts 40N in the opposite direction on the hand. The two forces act on different objects.
Example 2
Find the weight of a 6kg object using g=kg10N.
W=mg=6×10=60N downward. Its mass remains 6kg.
Watch out:They act on different objects. Cancellation in a net-force calculation concerns forces acting on the same object.
Newton’s third law and interacting objects | Year 10 Science | Ascent Arcade