Skip to content
Grade 9 Intermediate About 15 min Version 1 Awaiting review Premium

Inertia: the sudden stop

Inertia is the tendency of an object to keep doing what it is already doing: a still object stays still, and a moving object keeps moving at the same speed in a straight line, unless a force acts on it. This is Newton's first law of motion. The more mass an object has, the more inertia it has, and the bigger the force needed to start or stop it. That is why passengers lurch forward when a bus brakes suddenly, and why seat belts save lives.

What you will learn

  • State Newton's first law of motion.
  • Explain why a loose object keeps moving when its support stops.
  • Explain why a heavier object needs a bigger force to stop it quickly.

Before you start

  • Know that a force can change an object's motion.

Materials

  • Wooden blocks of 0.25 kg, 0.5 kg and 1 kg
  • Elastic strap

Equipment

  • Trolley and runway
  • Low stopper fixed to the bench

Procedure

  1. Run the trolley slowly into the stopper with the block loose, and record what happens.
  2. Run it fast with the block loose and compare.
  3. Strap the block down and run the trolley fast again.
  4. Use the heavier block and compare the force needed to stop it.

How this simulation is simplified

  • g is taken as 10 N/kg, and friction between the block and the surface is 0.3 of its weight.
  • The trolley stops in 0.1 s.