State Newton's first law, explain inertia as the resistance to a change in motion, and identify the role of balanced and unbalanced (net) forces (MA STE Introductory Physics, Motion and Forces).

What this topic is asking

Newton's first law is the starting point for all of mechanics, and the Massachusetts Introductory Physics MCAS uses it to test a deep idea: that motion does not need a force to continue, only to change. You must state the law, explain inertia as the resistance to a change in motion, and identify when forces are balanced (net force zero) or unbalanced (net force not zero). This connects to the crosscutting concept of cause and effect: an unbalanced force is the cause, a change in motion is the effect.

Newton's first law

The law overturns the everyday intuition that things naturally slow down and stop on their own. They slow because of a force, usually friction or air resistance. Remove that force, as on an ice rink or in space, and an object keeps moving at constant velocity indefinitely. So a force is not needed to keep something moving; it is needed only to change how something moves: to speed it up, slow it down, or change its direction.

Inertia and mass

Inertia explains many MCAS scenarios. When a car stops suddenly, the passengers keep moving forward because of their inertia, until a seatbelt provides the force to stop them. When a car turns, the passengers feel pushed outward because their inertia keeps them moving straight while the car turns under them. In each case nothing is "pushing" the person the way it feels; their inertia is simply keeping their motion unchanged until a force acts.

Balanced and unbalanced forces

The phrase that links the first law to calculations is the net force: the single force equivalent to all the forces acting, found by adding them as vectors.

• When the forces are balanced, they cancel and the net force is zero. By the first law, the object's motion does not change: it stays at rest or keeps moving at constant velocity.
• When the forces are unbalanced, the net force is not zero, and the object accelerates in the direction of the net force.

A book resting on a table feels its weight pulling down and the normal force pushing up. These are equal and opposite, so they balance, the net force is zero, and the book stays still. The first law is really the special case of the second law ($F = ma$) when the net force, and therefore the acceleration, is zero.

Try this

Q1. State what happens to the motion of an object when the net force on it is zero. [1]

• Cue. Its motion does not change: it stays at rest or continues at constant velocity.

Q2. Explain, using inertia, why it is harder to stop a loaded shopping cart than an empty one. [2]

• Cue. The loaded cart has more mass, so more inertia, and therefore resists a change in motion more strongly; a larger force is needed to stop it.