One word that sums up the first law is:
Motion
Force
Acceleration
Intertia

Intertia

Explanation:

Inertia can be defined as the tendency of an object or a body to continue in its state of motion or remain at rest unless acted upon by an external force.

In physics, Sir Isaac Newton's first law of motion is known as law of inertia and it states that, an object or a physical body in motion will continue in its state of motion at continuous velocity (the same speed and direction) or, if at rest, will remain at rest unless acted upon by an external force.

For example, the inertia of an object such as a shopping cart is greatly dependent or influenced by its mass; the higher quantity of matter in a shopping cart, the greater will be its tendency to continuously remain at rest.

Hence, one word that sums up the first law is inertia.

substitution is the answer in this case