EVERY-DAY FORCES Force of gravity Normal force Force of
friction Universal force of gravity
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FORCE OF GRAVITY
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Newton s Law of Universal Gravitation Every particle in the
universe exerts an attractive force on every other particle. A
particle is a piece of matter, small enough in size to be regarded
as a mathematical point. The force that each exerts on the other is
directed along the line joining the particles.
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For two particles that have masses m 1 and m 2 and are
separated by a distance r, the force has a magnitude given by
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INVERSE PROPORTION
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WEIGHT OF AN OBJECT The weight of an object on or above earth
is the gravitational force that the earth exerts on the object. On
or above another astronomical body, the weight is the gravitational
force exerted on the object by that body. Weight = mg, where m-
mass of the body, g acceleration due to gravity (-9.8 m/s 2 )
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MASS VS. WEIGHT
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Mass is a measure of the amount of stuff contained in an
object. The weight of an object on or above the earth is the
gravitational force that the earth exerts on the object. The weight
always acts downwards, toward the center of the earth. On or above
another astronomical body, the weight is the gravitational force
exerted on the object by that body. SI Unit of Weight: newton
(N)
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Relation Between Mass and Weight
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4.7 THE GRAVITATIONAL FORCE On the earth s surface :
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Definition of the Normal Force The normal force is one
component of the force that a surface exerts on an object with
which it is in contact namely, the component that is perpendicular
to the surface.
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4.8 THE NORMAL FORCE
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Apparent Weight The apparent weight of an object is the reading
of the scale. It is equal to the normal force the man exerts on the
scale.
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EXAMPLE A box with a mass of 10.0 kg is being pulled by a
string with a force of 40.0N A) horizontally B) at an angle of 30
degrees to the horizontal with a Neglecting friction find: the
acceleration of the box the magnitude of the normal force
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EXAMPLE A 10.0-kg box is resting on a 20.0 o incline. Determine
the normal force acting on the incline. Find the force of friction
acting on the incline. If the incline is frictionless, find the
acceleration of the box.
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FORCE OF FRICTION When an object is in contact with a surface
there is a force acting on that object. The component of this force
that is parallel to the surface is called the frictional
force.
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STATIC FRICTION When the two surfaces are not sliding across
one another the friction is called static friction.
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Note that the magnitude of the frictional force does not depend
on the contact area of the surfaces.
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Static friction opposes the impending relative motion between
two objects. Kinetic friction opposes the relative sliding motion
motions that actually does occur. is called the coefficient of
kinetic friction.
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The sled comes to a halt because the kinetic frictional force
opposes its motion and causes the sled to slow down.
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Suppose the coefficient of kinetic friction is 0.05 and the
total mass is 40kg. What is the kinetic frictional force?