Consequences of the first law

• If ΣF = 0, then a = 0: the velocity does not change, whether the object is at rest or moving.
• A force is needed to start a stationary object moving or to stop a moving one (that is, to change its velocity).
• No force is needed to keep an object moving at constant velocity (only to cancel friction/air resistance). A car moving steadily on a straight road has an engine force that exactly balances the resistances: ΣF = 0.

Example 1. A car moves at a steady 25 m/s along a straight road. The engine force is 900 N forward. What is the resistance to motion? Since the velocity is constant, ΣF = 0, so the resistance is 900 N backward.

Example 2. A book rests on a table: its weight (downward) is balanced by the table's upward force; ΣF = 0, so it remains at rest.