Newton's first law of motion

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Explain physical observations using Newton's first law and inertia

Explain using Newton's $1^{st}$ 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?

In this lesson, we will learn:

Notes:

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.