what is a force?blogs.rsd13ct.org/.../wp-content/uploads/2014/02/7forces-and-motion.pdf · what can...
TRANSCRIPT
3/6/14
1
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
3/6/14
2
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
3/6/14
3
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
3/6/14
4
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
3/6/14
5
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
3/6/14
6
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)
3/6/14
7
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