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 missing explanation is a resistive force. A tyre touching the road can experience friction, and an object moving through air can experience air resistance. When these forces oppose the motion, they slow the object.
• Remove them and the rule changes. An ice-hockey puck on smooth ice keeps sliding. A spacecraft with negligible resultant force keeps moving with constant velocity after its engines switch off.

almost no frictionno continuing push

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.
• At the same speed, a cricket ball has more momentum than a tennis ball because it has more mass. If both are stopped in the same time, the cricket ball needs the larger 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.