Newton's first and third laws

First law (law of inertia)

• A body remains at rest, or continues to move with uniform velocity in a straight line, unless a resultant force acts on it to change that state.
• Converse use: if a body moves with uniform velocity then the resultant force on it is zero — this is how resistance to motion is found from the engine force.
• Inertia is the property by which a body resists any change in its state of motion, and it increases with mass. A train reaches its maximum speed when the engine force equals the total resistance, so a = 0.

Third law

• For every action there is a reaction equal in magnitude and opposite in direction.
• Action and reaction act on two different bodies, which is why they never cancel each other.
• Example: the Earth's reaction to a body resting on it equals the pressure the body exerts on the Earth.