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Newton's First Law says that an object keeps doing what it is already doing unless a net external force acts on it. An object at rest stays at rest, and an object in motion continues in a straight line at constant speed. This idea matters because it explains everyday situations like a soccer ball staying still on grass or a hockey puck gliding across ice. It is the starting point for understanding forces, motion, and why changes in motion require a cause.
Inertia is the property of matter that resists changes in motion, and mass is a measure of inertia. Balanced forces do not change an object's velocity because the net force is zero. Unbalanced forces cause acceleration, which means a change in speed, direction, or both. Low friction surfaces make Newton's First Law easier to observe because moving objects can continue for longer without slowing down.
Key Facts
- Newton's First Law: If Fnet = 0, an object's velocity stays constant.
- An object at rest remains at rest when all forces on it are balanced.
- An object in motion continues in a straight line at constant speed when no net force acts on it.
- Net force is the vector sum of all forces: Fnet = ΣF.
- Balanced forces have equal strength in opposite directions, so Fnet = 0.
- More mass means more inertia, so a larger force is needed to change the motion of a more massive object.
Vocabulary
- Inertia
- Inertia is the tendency of an object to resist a change in its motion.
- Net force
- Net force is the overall force on an object after all individual forces are added as vectors.
- Balanced forces
- Balanced forces are forces that cancel each other so the net force on an object is zero.
- Unbalanced force
- An unbalanced force is a nonzero net force that causes an object to accelerate.
- Velocity
- Velocity is an object's speed in a specific direction.
Common Mistakes to Avoid
- Thinking motion requires a continuous force is wrong because constant velocity can continue when the net force is zero.
- Confusing balanced forces with no forces is wrong because an object can have several forces acting on it that cancel out.
- Ignoring direction when adding forces is wrong because forces are vectors and opposite directions must be subtracted.
- Assuming a heavier object always has a larger net force is wrong because mass affects inertia, while net force depends on the actual forces applied.
Practice Questions
- 1 A box has a 20 N push to the right and a 20 N friction force to the left. What is the net force, and what happens to its velocity if it is already moving?
- 2 A puck moves across nearly frictionless ice at 6 m/s. If the net force on it is 0 N for 4 seconds, what is its speed after 4 seconds and how far does it travel?
- 3 A soccer ball rests on grass with gravity pulling downward and the ground pushing upward. Explain why the ball does not accelerate even though forces are acting on it.