Concept Summary

Batesville High School Physics

Newton’s Second LawIf there is a net force on an object,

the object accelerates.Its acceleration is directly

proportional to the net forceIts acceleration is inversely

proportional to the object’s massIts 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 =

Fnet

m

Fnet = ma

Fnet

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 WeightMass is a property of an object that

measures how much it resists accelerating.

An object is difficult to accelerate

because it has mass.

WeightWeight 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 Wm g

Weight of a 1 kg object

Since W = mg, the weight of a 1 kg object is: W = (9.8 m/s2)(1 kg) = 9.8 N on Earth W = (1.6 m/s2)(1 kg) = 1.6 N on the

Moon

Mass vs. WeightWe 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 m2 exerts a pressure of 1 Pascal.

Another common unit of pressure is pounds per square inch (lb/in2).

Pressure

In symbols:

P = FA

FAP

The End.