When balanced forces act on an object that is at rest, the object remains at rest. This is an application of Newton’s first law of motion, which states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an unbalanced force.
The Law of Inertia
Newton’s first law is also known as the law of inertia. Inertia is the tendency of an object to resist any change in its motion. All objects have inertia, and the more mass an object has, the more inertia it has.
In the absence of any unbalanced force, an object’s inertia keeps it either at rest or moving in a straight line at constant speed. If the forces acting on the object are balanced so that they cancel each other out, there is no net force on the object. Since a net force is required to change an object’s motion, a lack of net force means the object’s motion does not change – it remains at rest or continues moving uniformly in a straight line.
Balanced Forces
Forces are vector quantities that have both magnitude and direction. When two or more forces act on an object and their vectors sum to zero, the forces are said to be balanced. The magnitudes and directions of the forces are such that they completely cancel each other out.
Some examples of balanced forces:
- The gravitational force pulling down on a book resting on a table is balanced by the upward force of the table pushing back.
- For a car moving at constant velocity, the force from the engine is balanced by air resistance and frictional forces.
- A tug-of-war team pulling with equal force in opposite directions results in balanced forces.
In each case, the net force on the object is zero, so the object’s motion does not change.
Object at Rest
Let’s consider a more specific scenario – a hockey puck sitting motionless on ice. The only forces acting on the puck are the downward gravitational force and the equal upward normal force from the ice canceling gravity out.
| Force | Magnitude | Direction |
|---|---|---|
| Gravity | 10 N | Downward |
| Normal force from ice | 10 N | Upward |
| Net force | 0 N | None |
Since the forces are balanced, the net force is zero. Therefore, by Newton’s first law, the puck remains at rest. If another force were applied, such as a hockey stick striking the puck, it would produce an unbalanced force and cause the puck to start moving. But as long as the forces remain balanced, the puck’s inertia keeps it motionless.
Real-World Examples
There are many examples of objects remaining at rest when balanced forces act on them:
- A book lying on a table does not spontaneously slide or fall off, because the downward force of gravity is balanced by the upward force from the table.
- A car remains parked until a force is applied by the engine or by pushing to overcome the frictional forces keeping it still.
- A tug-of-war match remains at a standstill if both teams pull with equal force in opposite directions.
- A ball on the green does not start rolling until struck by a putter with enough unbalanced force to overcome friction.
In all these scenarios, as long as the forces remain precisely balanced, Newton’s first law dictates that the object’s state of rest (or motion) will not change. Even very small objects like atoms and molecules obey this law – they remain stationary unless unbalanced electromagnetic forces act on them.
Conclusion
When forces acting on an object at rest are balanced so that they cancel each other out, Newton’s first law of motion states that the object will remain at rest. With no net force, there is no acceleration, so the object’s inertia keeps it stationary. This principle has applications in everyday objects at rest, from books on tables to parked cars. For an object at rest to begin moving, the forces on it must become unbalanced, with a net force overcoming the object’s inertia. As long as forces remain balanced, even very slight imbalances can be enough to maintain an object’s resting state.