S4.8.3 For the adult
Newton's first law
When I stop pedalling, my bicycle slows down. Does a moving thing need a force to keep moving?
What this chapter teaches
An object keeps its velocity unless a resultant force acts on it. Balanced forces give a resultant of zero. Inertia is the tendency to keep the same velocity, and a seat belt works because the passenger keeps moving when the car stops.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why does a bicycle slow down when you stop pedalling, if no force is needed to keep it moving?
- A box moves at a steady speed under a push of 50 N. How big is the friction, and why?
- Why do passengers move forward when a car stops suddenly, and how does a seat belt help?
After the chapter, the learner can
- Explain why an object keeps moving unless a resultant force acts
- Explain how seat belts protect passengers using inertia
What they copy into their notebook
First law: with no resultant force, an object at rest stays at rest and a moving one keeps a constant velocity. Inertia grows with mass. A seat belt gives the force that slows you with the car.