Newton's first Law
Summary
Newton's first law (the law of inertia) says an object keeps its velocity, whether at rest or moving at constant speed in a straight line, unless a nonzero net external force acts on it. Most testable: constant velocity, including zero, means the net force is zero.
Detail
Meaning: Also called the law of inertia. An object at rest stays at rest, and a moving object keeps moving at constant velocity (same speed, same direction), unless a nonzero net external force acts on it. Inertia is the resistance to a change in velocity, measured by mass. The law applies in inertial frames, which are not accelerating.
Equation: ΣF = 0 ⇔ a = 0, where ΣF is the net force (N) and a is acceleration (m/s²); with a = 0 the velocity v (m/s) is constant.
On the MCAT: Passages describe a crate pushed at constant speed, a skydiver at terminal velocity, or a mass hanging from a rope, and ask you to set forces equal: push equals friction, or tension equals weight. If a force is removed, expect constant velocity unless friction or another force acts. The rotational version, zero net torque, appears in static equilibrium of beams.
Common confusion: Students think motion needs a net force, or that constant velocity means no forces act. Constant velocity means the net force is zero; forces can be large but cancel. Objects slow on Earth because friction and drag act.
Sources
- Fundamentals of Physics
- University Physics
- Physics for Scientists and Engineers
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.