Concept Summary Batesville High School Physics

Newton’s Second Law  If there is a net force on an object, the object accelerates.  Its acceleration is directly proportional to the net force  Its acceleration is inversely proportional to the object’s mass  Its acceleration is in the same direction as the net force.

“directly proportional” means:  If the net force doubles, the acceleration doubles.  If the net force triples, the acceleration triples.  If the net force is half as much, the acceleration is half as much.  Etc.

“inversely proportional” means:  If the object’s mass doubles, its acceleration will be half as much.  If the object’s mass triples, its acceleration will be one-third as much.  If the object’s mass is half as much, its acceleration doubles.  Etc.

What is “mass”?  Mass measures the inertia of an object.  All objects made of matter have inertia - that is, they resist accelerations (Newton’s First Law), but some objects resist more than others.  Mass is a scalar quantity.  SI unit of mass is the kilogram (kg).

Newton’s Second Law  In symbols: a = F net m F net = ma F net m a

Preconceptions  There are 2 major preconceptions to address:  Mass is not the same as weight.  Force is not the same as pressure.

Mass is not Weight  Mass is a property of an object that measures how much it resists accelerating.  An object is difficult to accelerate because it has mass.

Weight  Weight is a force - an interaction between 2 objects involving a push or a pull. One of these objects is typically VERY big - the Earth or the Moon, for instance.  Weight is NOT a property of an object.

What does weight depend on?  The weight of an object depends on the object’s mass.  In fact, an object’s weight is directly proportional to the object’s mass.  The weight of an object also depends on the object’s location.  In fact, an object’s weight is directly proportional to its free fall acceleration, g at its current location.

Weight  In symbols: W = mg W m g

Weight of a 1 kg object  Since W = mg, the weight of a 1 kg object is:  W = (9.8 m/s 2 )(1 kg) = 9.8 N on Earth  W = (1.6 m/s 2 )(1 kg) = 1.6 N on the Moon

Mass vs. Weight  We typically think that an object is difficult to accelerate because it is heavy (has weight) - but it is heavy because it has mass.  So, objects are difficult to accelerate because they have mass.

Force is not Pressure  Force determines how much an object will accelerate.  Pressure determines how that acceleration will feel.

What does pressure depend on?  The pressure exerted on an object depends on:  The force exerted on the object.  In fact, pressure is directly proportional to force.  The area over which the force is applied.  In fact, pressure is inversely proportional to area.

Pressure Units  A force of 1 N applied over an area of 1 m 2 exerts a pressure of 1 Pascal.  Another common unit of pressure is pounds per square inch (lb/in 2 ).

Pressure  In symbols: P = F A F AP

The End.