Newton's first law of motion

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  1. Introduction to Newton's first law
  2. What is Newton's first law?
  3. What is inertia?
  1. Explain physical observations using Newton's first law and inertia

    Explain using Newton's 1st1^{st} Law or inertia:

    1. True or false: a vase sitting still on a desk has no horizontal forces acting on it. Explain your answer.
    2. Why do you press against your seatbelt when you are in a car that stops suddenly?
    3. A shopping cart is pushed at a constant velocity of 1.5 m/s with a force of 40 N. What is the net force acting on the cart?
Topic Notes
In this lesson, we will learn:
  • Newton's first law of motion
  • Inertia
  • Explaining physical phenomena using Newton's first law and inertia


  • Newton's first law of motion can be stated as "if no net force acts on an object, then the velocity of the object does not change."
  • Net force is a vector sum of forces acting on an object.
  • Newton's first law is also called the law of inertia. Inertia is an idea that originated from Galileo, which says that objects have a tendency to resist changes in their motion:
    • Objects in motion will tend to stay in motion by continuing at their velocity in a straight line, and objects at rest tend to stay at rest.
    • Objects with more mass have "more inertia". It is more difficult to get a more massive object change its velocity by speeding up, slowing down, or changing direction.