Forces
Ch. 12.1
What is a force?
•  Any push or pull on an object = FORCE
•  Forces cause objects to move, accelerate,
and/or change speed or direction
•  Use scales to measure forces
•  The SI unit is a NEWTON
–  1 N=1 kg ● m/s2
•  Use arrows (←, ↑, →, or ↓) to show which
way the force is acting
Combining Forces
•  Put forces together to show an end result
•  If forces are the SAME direction à ADD
•  If forces are DIFFERENT direction à
SUBTRACT
•  NET FORCE à overall force acting on an
object after all forces combine
•  BALANCED FORCE à net force is = and
there’s NO change in motion
–  ex: arm wrestling standoff, tug of war
•  UNBALANCED FORCE à object will
accelerate (move)
–  ex: WINNING arm wrestling or tug of war
–  http://phet.colorado.edu/en/simulation/forcesand-motion-basics
Friction
•  Any force that opposes motion of objects as
they move past each other = FRICTION
Static Friction
•  Acts on an objects that are NOT moving.
Sliding Friction
•  opposes the direction of motion. One object is sliding.
Rolling Friction
a force acting on rolling objects
Fluid Friction
–  opposes motion through a fluid
Gravity
•  Force acting btwn 2 masses is GRAVITY
–  causes objects to accelerate downwards
Projectile Motion
•  CURVED PATH of motion
–  combination of initial forward velocity and
downward vertical force of gravity
–  http://phet.colorado.edu/sims/projectile-motion/
projectile-motion_en.html
Concept Check
•  How is the motion of an object affected
when a force acts on it?
•  How does air resistance affect the
acceleration of a falling object?
•  Describe why a projectile follows a curved
path.