Even stationary objects are acted on by several forces at the same time. When these forces cancel one another, they are said to be balanced.
Basic Knowledge
Conditions for two forces acting on one object to be balanced:
① The magnitudes are equal
② The directions are opposite
③ The two forces lie on the same straight line
When all three conditions are met, the object remains at rest.
On the other hand, the law of action and reaction is a different concept: “When object A exerts a force on object B, object B exerts a force of equal magnitude in the opposite direction on A.”
The essential difference: balance involves “two forces acting on the same object”, while action-reaction involves “forces acting between two different objects.”
Balanced forces (a state in which multiple forces acting on the same object cancel each other and it does not move)
Law of action and reaction (Newton's third law. Forces between objects occur in pairs)
Resultant force (one force formed by combining multiple forces)
Component force (a force obtained by resolving one force into several directions)
Normal force (the force with which a surface pushes an object perpendicularly)
Inertia (the property of an object that keeps it in its state of motion)
Further Exploration (Background and Significance)
Consider a book placed on a desk. The gravitational force acting on the book and the normal force exerted by the desk on the book are balanced (two forces acting on the same “book” in opposite directions), so the book remains at rest.
On the other hand, the force with which the book pushes the desk downward (equal in magnitude to gravity) and the normal force with which the desk pushes the book back are an action-reaction pair (two forces paired between two objects, “the book and the desk”).
Walking, swimming, and rockets flying──all involve action and reaction. When we kick the ground backward, the ground pushes us forward. When a rocket ejects gas backward, the gas exerts a force that pushes the rocket forward.
In middle school, combining and resolving forces is treated only for forces on the same straight line, but in high school it is extended to two and three dimensions using the parallelogram law.
- Conditions for two balanced forces: ① equal magnitude ② opposite directions ③ same straight line
- Balance = two forces acting on the same object
- Action-reaction = two forces between two different objects
- Walking・swimming・rockets = applications of action and reaction
- Resultant force on the same straight line: sum in the same direction, difference in opposite directions
- Normal force = the force with which a surface pushes an object back
- Inertia = the property by which moving objects tend to keep moving and stationary objects tend to remain at rest
Points to Note (Easily Confused)
① Balance concerns forces acting on one object, while action-reaction concerns forces between two objects. This distinction is most important. ② The gravitational force on a book and the normal force on it are balanced forces, not an action-reaction pair. ③ Although they look the same because they are opposite in direction and equal in magnitude, distinguish them by asking what object each force acts on. ④ Two forces that are not on the same straight line do not balance; a resultant force remains and the object begins to move.
Practice
- State all three conditions for two forces to be balanced.
- What is the decisive difference between balance and action-reaction?
- State the name of the force that balances the gravitational force on a book placed on a desk.