Real-life examples

10 real-life examples of Newton's first law (inertia)

An object continues at constant velocity unless a net external force acts on it.

  1. Book on a table. Gravity pulls down; the table's normal force pushes up. With net force zero, the book stays at rest.
  2. Passenger lurching forward when a car brakes. The car decelerates due to braking friction; the passenger has no horizontal force on them (until the seat belt acts), so by inertia they continue forward.
  3. Tablecloth pulled quickly. The brief friction impulse on dishes is small compared to their inertia, so they barely move while the cloth is whipped out.
  4. Coin on a card over a glass. Flicking the card sideways gives the coin only a brief friction force; the coin drops nearly straight into the glass.
  5. Astronaut floating inside the ISS. In free-fall the astronaut and capsule fall together; relative to the capsule there is no net force, so the astronaut drifts in a straight line at constant velocity.
  6. Hockey puck on smooth ice. Friction is near zero. After a push, the puck glides almost in a straight line at almost constant speed.
  7. Spacecraft cruising between planets. Far from any star, with engines off, the craft maintains its velocity for years; no net force, no acceleration.
  8. Seatbelts in a sudden stop. The car's chassis stops via the brake force. The passenger has no equivalent force until the belt provides it.
  9. Rolling ball on a level floor that eventually stops. Inertia would keep it rolling forever; in real life rolling friction and air drag slowly remove momentum, so the ball decelerates.
  10. Yo-yo at the end of its string. The string tension provides the centripetal force; remove it (cut the string) and the yo-yo flies off tangentially in a straight line.

Where students go wrong

Newton's first law only holds in inertial frames. In a non-inertial frame (accelerating car, spinning carousel) objects appear to accelerate without a real force — these 'pseudo-forces' are an artefact of the frame.

Where this fits in the library