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Forces
CHAPTER 3
Force, Mass, & Acceleration
 More force on an object = more acceleration
 The acceleration of an object depends on its
mass as well as the force exerted on it.
 Ex: thrown softball vs baseball
Newton’s 2nd Law of Motion
 The acceleration of an object is in the same
direction as the net force on the object
 Force is Measured in Newtons
 Equation: Acceleration = (net)force/mass or
a = f(net)/m = m/s
 To find force: f = ma
 To find mass: m = f/a
Friction
 A force that opposes the motion of objects
that touch or between 2 objects in contact
 Air resistance: friction acting on objects
moving through the air
 Objects with larger surface areas = greater air
resistance
 Also affected by size, shape, and speed
3 Main Types of Friction
 1.) Static Friction: force that prevents two
surfaces from sliding past each other.(Box
across the floor)
 2.) Sliding: forces that are caused by an
object as it slides over a surface
 3.) Rolling: friction force that acts on rolling
objects
Air Resistance
 Type of fluid friction
 Falling, running, …
 More surface exposed = more resistance
 Ex: flying squirrel, skydiving
High Resist
High Resist
Low Resist
Low Resist
Gravity
 Force that acts between any two masses
 The greater the mass, the larger the
gravitational force
 The closer the objects, the larger the
attraction
 Acceleration due to Gravity = 9.8 m/s2
Gravity Factors
1. Size/Mass


Bigger = more gravity
Ex: Jupiter, Earth, Moon
2. Distance

Farther = weaker
Gravity
 Free fall:
 the motion of an
object when only the
force of gravity is
acting on it
 Projectile Motion:
 The motion of a falling
object after it is given
initial forward velocity
 Follows a curved path
due to initial velocity
and force of gravity
Weight and Mass
 Weight: the force of gravity acting on an
object
 Weight = Mass X Free-fall acceleration or
w = mg (g = 9.8 m/s2)
 Mass: measure of the inertia of an object;
more mass = more inertia and weight
Gravity/Air Resist
 Gravity pulls down
 Air Resist. pushes up
 Terminal Velocity = constant velocity when air
resist is same as gravity
 can’t go any faster
Mass & Weight
Mass
Weight
 Amount of matter
 Force of gravity
 Doesn’t change
 Changes (moon)
 Balance
 Scale
 Unit = grams
 Unit = Newtons (N),
Pounds (lb)
Earth’s Gravitational
Acceleration
 Equation: Force of Gravity =
 Mass x accelaeration

Weight
 Weight = mass . gravity
 W = m .g
 Units: W = N, m = kg, g = 9.8 m/s2
Projectile & Gravity
 2 objects dropped @ same time (no air resist)
– will hit at same time
 Even projectile & dropped object hit same
time
 Gravity only force pulling down
Centripetal Force
 Acceleration toward the center of a curved
path.
 Ex: The swings at a carnival
Swinging a bucket at the end of a
string.
12.3 Newton’s 3rd Law of Motion
 Whenever one object exerts a force on a
second object, the 2nd object exerts an equal
and opposite force on the 1st
 Action – Reaction Force
 The action/reaction force occurs at the same
time; for every action, there is an equal and
opposite reaction
Momentum
 The product of the objects mass and its




velocity
The larger the mass, the more momentum
Equation:
Momentum = mass X velocity
Or: p = mv
Law of Conservation of Momentum
 If no net force acts on a system, then the total
momentum of the system does not change
 In a closed system, the loss of momentum of
one object equals the gain in momentum of
another object