Velocity describes speed and direction; acceleration is a change in velocity per unit time. Newton’s first law states that an object remains at rest or moves at constant velocity if its net external force is zero. Balanced forces do not imply no forces exist. An object can move steadily while a forward driving force balances drag and friction. Inertia is resistance to a change in velocity, and mass measures this resistance.
To calculate net force in one dimension, choose a positive direction and combine forces with signs. A 12N rightward force and 5N leftward force give 7N rightward. A non-zero net force causes acceleration, which may speed up, slow down or redirect motion depending on its direction. An object moving right can accelerate left while slowing down.
Example 1
A car moves at constant velocity. Its driving force is 900N forward. What total opposing force acts?
Constant velocity means zero net force. The opposing forces must total 900N backward.
Example 2
A box has forces 18N right and 11N left. Find the resultant.
Taking right as positive, net force =18−11=7N right.
Watch out: With zero net force, an object continues at constant velocity. Unbalanced force changes velocity.