Newton's First Law & Inertia

• Everyday life suggests a wrong rule. Stop pedalling and the bicycle slows down. Stop pushing a box and it stops. It looks like moving needs a constant push.
• The hidden force is friction (and air resistance). These forces act backwards on every moving object near the ground. They are what slows things down.
• Remove them and the rule changes. An ice-hockey puck on smooth ice keeps sliding. A spacecraft far from any planet keeps moving forever with its engines off.

A force is not needed to keep an object moving. A force is needed to change how it moves.

• Newton's first law: an object remains at rest, or continues to move at constant velocity, unless a resultant force acts on it.
• The law has two halves. A still object stays still. A moving object keeps the same speed and the same direction. Both are the same idea, because being at rest means the velocity is zero.
• The resultant force is the vector sum of all the forces on the object. Forces can be large and still give a zero resultant, if they cancel.

• Balanced forces (resultant = 0): velocity does not change. The object is in equilibrium.
• Unbalanced forces (resultant not 0): the velocity changes. The object speeds up, slows down, or turns.

• Objects resist changes to their motion. This resistance is called inertia.
• Mass measures inertia. A large mass is hard to start moving, hard to stop, and hard to turn.
• Catching a cricket ball hurts more than catching a tennis ball at the same speed. The cricket ball has more mass, so stopping it needs a bigger force.

Mass is the property of an object that resists any change in its motion. This sentence is the exam definition of what mass means in dynamics.