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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

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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

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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

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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

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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

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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)

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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