3/6/14 WHAT IS A FORCE?
„ Forces are pushes or pulls
„ Force is measured in Newtons(N) – spring scale
„ The total amount of force is called NET FORCE
„ Force has a size (magnitude) and a direction
„  Example 5N è
1 3/6/14 EXAMPLES OF FORCES:
„ NORMAL FORCE
„ GRAVITY
„ FRICTION (KINETIC AND STATIC)
„ CENTRIPETAL FORCE
„ MAGNETIC FORCE
„ ELECTRICAL FORCE
TERMS/VOCABULARY
„ Normal Force-The normal force is the support force
exerted upon an object that is in contact with another
stable object.
„  For example, if a book is resting upon a surface, then the surface is exerting
an upward force upon the book in order to support the weight of the
book.
„ Gravity – a pulling force between 2 objects that depends on
the mass of the 2 objects and the distance between them
„  On Earth, accleration due to gravity = 9.8 m/s/s
„ Friction (kinetic and static) – force that occurs when 2
surfaces are in contact with each other; works against
motion
„ Centripetal Force- a net force that pulls towards the center
2 3/6/14 WHAT CAN FORCES DO?
„ A force can
„  Stop an object’s motion
„  Change an object’s speed
„  Change an object’s direction
„  ALL OF THE ABOVE ARE FORMS OF
ACCELERATION ( an increase in speed, a
decrease in speed or a change in direction)
NET FORCE, BALANCED AND
UNBALANCED FORCES
„ Forces can be in the same direction or different
directions
„ Forces can be balanced ( = 0) or unbalanced
( not = 0)
„ Balanced forces will cause objects to be at rest
or to keep moving the way they are moving
„ Unbalanced forces will cause a change in
motion
3 3/6/14 HOW TO CALCULATE NET FORCE
„ If forces are in the same direction, add them,
movement direction is in the direction of both
forces
HOW TO CALCULATE NET FORCE
„ If forces are in opposite directions, SUBTRACT
them, movement direction is in the same
direction of the larger force
4 3/6/14 HOW TO CALCULATE NET FORCE
„  If forces cancel each other out = balanced
force 0 N, no direction no movement or change
in movement
MORE ON FRICTION FORCES
„ Friction depends on surface type and mass
„ The more “hills and valleys” in the surfaces, the more
friction
„ The more mass, the more friction
„ TYPES OF FRICTION:
„ Kinetic – moving
„ Rolling
„ Ex: wheels
„  Sliding
„  Ex: pushing a box
„  Fluid (liquids or gases)
„  Ex: air resistance
„  Static – not moving
„  In order to set an object in motion, the static friction force needs to be
overcome
5 3/6/14 MORE ON GRAVITATIONAL FORCE
„ Gravity is a pulling force
„ Gravity can change motion by changing speed, direction, or
both
„ As objects fall due to gravity on Earth they accelerate at a
rate of 9.8 meters/second/second
„  This means that there speed increase by 9.8 m/s every second that
passes
„  Air resistance will change this rate
A REMINDER ON MASS AND WEIGHT
„ Mass is the amount of matter in an object
( measured in grams, kilograms on a
balance)
„ Weight is the force of gravity pulling on the
mass of the object (measured in Newtons
on a spring scale)
6 3/6/14 CENTRIPETAL FORCE
„ A net force that acts towards the center of a
moving objects circular path
„ Acceleration is always changing because the object
always changes direction
„ Remember why planets orbit the Sun
„  Net force = 0 = balanced force of gravity of Sun and
planets inertia
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