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Forces & Motion
What is a Force?
• Force: push or pull
• Unit: Newton (N)
 Kg x m/s2
• Vector: has both magnitude & direction
Combining Forces
• Use vector addition
• Net Force: the total force acting on an object
• Balanced Forces: when the net force is zero
▫ No motion occurs
• Unbalanced Forces: when the net force is not
zero
▫ Motion occurs (acceleration or change in
direction)
Balanced Forces:
Unbalanced Forces:
Friction
• Friction: a force that opposes (acts opposite
to) motion
• Four types:
▫
▫
▫
▫
Static
Sliding
Rolling
Fluid
Types of Friction
• Static: friction force on objects that are not
moving (greatest friction force)
• Sliding: friction force on an object as it slides
over another
• Rolling: friction force on an object as it rolls
over another (ex. Ball bearings)
• Fluid: friction force on an object in a fluid
(liquid
or gas) (ex. Air resistance)
Gravity
• Gravity: downward force pulling objects
toward the center of Earth
• Acceleration on Earth due to gravity = 9.8 m/s2
• Terminal velocity: constant velocity that a
falling object reaches when
the force of gravity is equal
to the air resistance
Projectile Motion
• An object thrown forward will follow a curved
path due to gravity
• The combination of an initial forward velocity
and the downward vertical force of gravity
causes the object to follow a curved path
Projectile Motion
Scientists who contributed to the ideas
about Force & Motion
• Aristotle: incorrectly proposed that force is
required to keep an object moving at
a constant speed
• Galileo: studied constant acceleration due to
gravity
• Newton: defined force & mass; introduced 3
laws of motion
Newton’s First Law of Motion
• Law of Inertia
• An object at rest will remain at rest and an
object in motion will remain in motion unless
acted upon by an outside force
• If the net force = 0, then the object’s state
motion will remain unchanged
Newton’s Second Law of Motion
• The force acting on an object is equal to the mass
of the object times the object’s acceleration
• Force = mass x acceleration (or F = m x a)
• Units: N = kg x m/s2
Weight & Mass
• Weight: the force of gravity acting on an
object
 W=mxg
 same equation as F = m x a
 Changes depending on the gravitational force
• Mass: a measure of inertia on an object
(how much matter is within an object)
 Constant no matter what the gravity
Newton’s Third Law of Motion
• For every action there is an equal and opposite
reaction
▫ Forces occur in pairs
• Not all forces create motion
Momentum
• The product of an objects mass times its velocity
▫ An object with large momentum is hard to stop
• Momentum = mass x velocity
▫ M=mxv
▫ Units kg*m/s = kg x m/s
Momentum
Law of Conservation of Momentum
• In a closed system, the loss of momentum of one
object equals the gain in momentum of another
object
▫ Momentum is conserved
Universal Forces
• Electromagnetic Forces: forces associated
with charged particles (electric & magnetic)
• Nuclear Forces: forces that occur within the
nucleus of an atom
• Gravitational Forces: an attractive force
between any two masses
Newton’s Law of Universal Gravitation
• Every object in the universe attracts every other
object
▫ Closer distance = greater force
▫ Larger mass = greater force