When a net external force acts on an objectof mass m, the acceleration that results is directly proportional to the net force and hasa magnitude that is inversely proportional tothe mass. The direction of the acceleration isthe same as the direction of the net force.

mF

a

aF

m

Newton’s Second Law

22 smkg

smkg

This combination of units is called a newton (N).

SI Unit for Force

The weight of an object on the earth is the gravitational force that the earth exerts on the object. The weight always acts downwards, toward the center of the earth.

Weight, Fw = mass * acceleration due to gravity = m * g

SI Unit of Weight: newton (N)

Weight: Gravitational Force

Whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body.

Interaction: The Third Law

Oppositely Directed Force

A balloon pressed against a door. FHB and FBH are an interaction pair, as are FDB and FBD. All forces are the result of interactions, and all come in pairs comprising equal and opposite influences.

The normal force is one component of the force that a surfaceexerts on an object with which it is in contact – namely, thecomponent that is perpendicular to the surface.

Definition of the Normal Force

When the two surfaces are not sliding across one another, the friction is called static friction.

Static Friction

The magnitude of the static frictional force can have any value from zero up to a maximum value.

MAXss ff

NsMAXs Ff

10 s is called the coefficient of static friction.

Static Friction

Static Friction Table(Source: http://education-portal.com/academy/lesson/what-is-friction-definition-formula-forces.html#lesson

SurfaceCast iron on cast iron Glass on glass Leather on oak Nonstick coating on steelOak on oakSteel on steelSteel on steel (with castor oil)

Coefficient of Static Friction(µs)

1.10.940.61 0.040.620.780.15