How can you know a physical change has occurred? The substance produces a gas
O The substance starts to smell bad
The substance catches on fire
The substance melted

the focus of lesson 1 is newton's first law of motion - sometimes referred to as the law of inertia. newton's first law of motion is often stated as. an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

explanation:

wavelength divides in half and speed stays the same.