Forces
Definition – Friction – Falling Objects
Projectiles – Newton’s Laws of Motion
Momentum – Universal Forces
Fluid Pressure – Hydraulics – Buoyancy
Definition of Force
• Force = a push or pull that causes a change in the
motion of an object.
–
–
–
–
–
Change may be in speed or direction
Act in a certain direction (vector)
Can be shown using a Free Body Diagram
Forces transfer/transform energy
Unbalanced forces change motion, balanced forces do
not.
• Measured in Newtons, N (1 N = 1 kg·m/s2)
Friction
• A force that opposes the movement of objects that
touch each other.
• 4 main types
– Static friction – resists motion from rest
– Sliding friction – continues to resist motion as an
object is sliding over a surface
– Rolling friction – friction of an object on wheels, ball
bearings, or rollers
– Fluid friction – resists movement of an object through
a fluid (air, water, other liquids)
Comparison of Friction Types
•Static
•Sliding
• Rolling
•
Fluid
Falling Objects
•
•
•
•
Accelerate due to force of gravity.
Acceleration on Earth is at a rate of 9.8 m/s2
Air resistance (friction) increases with speed.
Objects reach a terminal velocity when the
force of gravity is opposed by air resistance.
Projectile Motion
• Projectiles experience a horizontal force and
the force of gravity.
• This leads to a curved path of motion.
NO MORE VEGGIES!
Newton’s 1st Law of Motion
• The Law of Inertia
– The motion of an object remains unchanged
unless acted upon by an unbalanced force.
• An object in motion stays in motion and an
object at rest stays at rest, unless acted upon
by an unbalanced force.
Newton’s 2nd Law of Motion
• The acceleration of an object is equal to the
net force acting on it divided by the object’s
mass.
• F=ma
– Or a = F/m or m = F/a
Weight vs. Mass
• Weight = the force of gravity acting on the
mass of an object
– If F = ma then W = mg
– W is weight in Newtons
– m is mass in kilograms
– g is acceleration due to gravity, m/s2
Newton’s 3rd Law of Motion
• Action-Reaction
• Whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first object.
• List 3 examples in your notes from your
textbook (page 373) or from class discussion.
Momentum
• Momentum = mass x velocity
– The larger the object and the faster it is
moving, the more momentum it has
• Similar to force, but with only velocity instead
of acceleration
Conservation of Momentum
– The total momentum of a closed system remains the
same before and after collisions
– For a collision between objects A and B . . .
(MomentumA)before + (MomentumB)before = (MomentumA)after + (MomentumB)after
(mA x vA)before + (mB x vB)before = (mA x vA)after + (mB x vB)after
– Elastic collisions (objects hit & bounce apart with no
friction or loss of momentum)
– Inelastic collisions (objects stick together)
Electromagnetic Force
• Can Attract or Repel
• Opposite charges attract, Like charges repel
• Moderate strength over medium distance
Nuclear Forces
• Strong and Weak Nuclear Forces
• Forces inside the nucleus of an atom that
hold it together or play a role in its decay
• Very strong, but only over short distances
(like the size of a proton or so)
Gravitational Force
• Newton’s law of Universal Gravitation
– Any two objects in the universe have a force
of gravitational attraction between them
– Affected by the mass of the objects and the
distance between them
– Weakest force but acts over very long
distances
Centripetal Forces
• Describes any force that is center directed
• Acts to change the direction of motion of an
object to keep it in “orbit”
• NOT Centrifical or Centrifugal 
Pressure
• Definition: Force per unit of area
• Equation: P = F / A
• Units: if F is in Newtons and A is in m2, then P is in
Pascals
– Pascals are really small units of pressure, so we often
use kilopascals or kPa
– (1 kPa = 1000 Pa)
– Other units (like psi, atm, torr, mmHg, bar, in. can all
be converted into kPa)
Pressure in Fluids
• Pascal’s Principle – Pressure on a fluid is
transmitted equally and unchanged in all
directions throughout the fluid.
• Application : Hydraulics
– Piston systems are the simplest
Hydraulics
F= 100 N
F=
F=1000
?N N
A= 1 m2
P=
P=
P=100
F/A
? Pa
/1
Pa
A= 10 m2
100
=/FFA/Pa
/ 10
10
100
P=
P= F=
100
x10
x10
Seems like a magical way to
multiply force. What’s the catch?
Bernoulli’s Principle
• As the speed of a fluid increases, the
pressure within the fluid decreases
2 important applications
– Wings and lift
– Atomizers, aspirators, spray bottles
Airplane Wing
Lift & Wings
Atomizer
• Perfume atomizer
Hose-end sprayer
Atomizers
Archimedes’ Principle
• The buoyant force on an object is equal to
the weight of the fluid displaced by the
object
– Pressure increases with depth in a fluid
– Upward force due to pressure on the bottom
of an object is greater than the downward
force due to pressure on the object
Buoyancy
• Buoyant force =
the weight of
the fluid
displaced
Floating and Sinking
• Buoyant force > Weight of the of the object
the object floats
•
< Weight
the object sinks
Buoyant force
of the object,
Floating and Sinking
Density
• Density = mass / volume (D=m/V)
– Units for density depend on the units for mass
and volume.
• kg/m3
• g/cm3
• g/mL
• g/L
Density
• It measures how compact the matter is in an
object
• Density can also be used to predict if an
object will float or sink in a fluid
– If Dobject <
– If
Dfluid the object will float
Dobject > D
fluid
the object will sink
• Average density is important!
Density Benchmarks
• Density of pure water at 4°C (near freezing)
is exactly 1 g/mL
• Density of pure water at 20°C (room temp.)
is 0.9982 g/mL (close enough to 1 for our
purposes)
• Density of air (at sea level, at 20°C) is
approximately 1.2 kg/m3 or 0.0012 g/cm3