Transcript Normal Force

Honors Physics Semester 1
Review PowerPoint
Distance vs Displacement
0 1 2 3 4 5 6 7 8 9
Distance = magnitude only = 8m
Displacement = magnitude and direction = Δx = x2 – x1
Δx = x2 – x1 = 9m – 1m = +8m
Acceleration
• When you increase your speed in a car, are
you accelerating?
• When you turn in a car, are you accelerating?
• When you slow down in a car, are you
accelerating?
• This is called deceleration or negative
acceleration or acceleration in the direction
opposite of the car’s movement.
• Objectives: define distance and calculate
speed, and explain what is meant by
scalar quantity.
Scalar Quantity:
• Quantity with only a numerical value
• Distance tells us how far but not in what
direction.
• Time
• Mass
• Temperature
Graphical Analysis
Zero acceleration
Positive acceleration
Kinematics
Speed: the rate at which distance is traveled
Constant Speed: speed of object does not
change
Units: meters m
second s
Average speed = total distance/total time
REVIEW:
• Motion: change of position
• Scalar: numbers only
• Vector: direction
• Velocity: change of speed in a given direction
• Acceleration: a change in velocity
• Therefore, a change
in speed or a change
in direction
Graphs
• Position v Time graphs
constant velocity
Δv = acceleration
Speed and Velocity
• There are two types of Velocity:
• 1. Average Velocity V = ∆d/∆t
For example, when a car moved 50 km in 2
hours, the average velocity is 25km/h.
• 2. Instantaneous Velocity
For example, when the speed cameras give
you a ticket, they show the car driving at 90
km/h for that instant.
Position in Meters
Velocity
What is the velocity of this object between 0-4
sec?
What is the initial starting position of the object?
What is the displacement of this object from 3-4
seconds?
What is the object’s final position?
• Assuming
the objects
motion does
not change,
what would
its position
be at t=20s?
Position in Meters
Graphs
Time (seconds)
• What is happening to the velocity of
these two objects?
• What is happening at t=2s?
• At t=4s, which object has greater
speed?
Graphical Analysis
Zero acceleration
Positive acceleration
Newton’s First Law of Motion or Law of Inertia:
in the absence of an unbalanced force, a body
at rest remains at rest, and a body in motion
remains in motion with a constant velocity
(speed and direction).
Inertia and Newton’s 1st Law
• Inertia - tendency of an object to overcome a
change in motion
• Characteristics:
more mass = more inertia
Mass is the quantitative measure of inertia.
Net Forces
Normal Force

When an object is sitting on a level surface
then the normal force is always equal and
opposite of the weight of the object.
Forces Symbols
Fapp – applied force (push or pull)
Fg – force of gravity
(always toward center of earth or down)
Fn – normal force
(always perpendicular to surface)
Ff – force of friction (same as surface)
Balanced forces do not
change the object’s
motion.
FORCES
• Unbalanced forces result in a
change in the object’s motion.
Newton’s First Law of Motion or Law of Inertia:
in the absence of an unbalanced force, a body
at rest remains at rest, and a body in motion
remains in motion with a constant velocity
(speed and direction).
Newton’s 3rd Law of Motion
• For every force (action), there is an equal
and opposite force (reaction).
Free Fall – Force of Gravity ONLY
• Free Fall: A Particular Acceleration
• How fast a falling object moves is
entirely DIFFERENT from how far it
moves.
• We will treat x and y separately
• SI Unit: Newton = kg • m/s2
• Force is a vector
(magnitude/direction)
Like velocity and acceleration, force has a
strength AND a direction
FORCE
Resultant Force: the total of all forces
acting on an object.
• Force 1 pushes upward with 2 N
• Force 2 pushes horizontally with 5 N
Net Forces with angles
Net Forces with angles
What is the net force on this object?
Pull Force of
20 N at 16°
50kg
What is Fs?
What is FN?
Types of Friction
Static Friction:
 Frictional force is sufficient to prevent
motion between surfaces.
Static Friction Formula
 fs

≤ µsN (static conditions = no movement)
µscoefficient of static friction
Friction
(think about ice)
Normal Force
• Normal means perpendicular.
• Force that a surface exerts on an object.
Normal Force
• When an object is sitting on a level surface
then the normal force is always equal and
opposite of the weight of the object.
Force Formula
• Acceleration of an object is directly
proportional to the net force and
inversely proportional to its mass.
Acceleration = Force
Mass
Momentum and Impulse
• The concept of impulse and momentum using
Newton’s 2nd Law:
• F = ma
a = vf – vi = Δv
F = m Δv
t
t
t
• Take t to the other side:
• Impulse-Momentum Theorum = F t = m Δv
•
•
•
•
F(t) is called IMPULSE.
It is defined as a force acting through time.
Impulse is numerically equal to the Δ of momentum.
So a force acting for time on some object gives rise
to a change of the object’s momentum.
Is momentum conserved?
• YES.
• The momentum lost by one object is
gained by the other object.
• The total amount is constant.
Elastic Collisions
Total Kinetic Energy is conserved
• Follows the Law of Conservation of
Momentum
• Kafter = Kbefore
Inelastic Collisions
Kinetic energy is NOT conserved
• Change in original shapes
• Sound and friction – KE lost
Linear Momentum
• Formula: ρ = m v
• ρ = momentum
• m = mass
• v = velocity
SI Units?
kg m/s
Law of Conservation of Linear Momentum:
"the total momentum of an isolated system
of interacting bodies remains constant."
OR
"Total momentum of an isolated system
before collision is always equal to total
momentum after collision.“
Correlates to Newton’s 1st Law of Motion
Radians
 Correct SI unit
for angular
measurements
 radius to arc length = radian (The Rad)
 1 rad = 360˚/2π = 57.3˚
 Calculators: switch to rad when told
Period and Frequency
Frequency: number of cycles
per unit of time.
f = 1/t or s-1
Period: (t) time it takes an
object in circular motion to
complete one revolution or
cycle
t = 1/f
Frequency and period = inverse
relationship
Frequency SI: 1/s = Hertz (Hz)
Heinrich Rudolf Hertz
Uniform Circular Motion
 Needs 3 things
 1.
Centripetal Force
Uniform Circular Motion
 2. Angular Acceleration
 3. Constant Speed
V=
Tangential velocity
wants to go in
a straight line
Gravitational Field Lines
for Two Objects
Kepler’s

rd
3
Law of Planetary
Motion
This Law lets us determine a newly
discovered planet’s distance from the Sun.
Kepler’s Laws of Planetary Motion
Kepler’s 2nd Law: (Law of Areas)
A line from the Sun to a planet sweeps out
equal areas in equal lengths of time.
Escape Velocity

What kind of energy must a man-made
satellite have to escape Earth’s
gravitational pull?

Formula for Escape Velocity:
Escape speed = escape surface of Earth is
about 11km/s or 7mi/s
 Centrifugal is just Inertia – what Law?
 Newton’s 1st: an object in motion wants to stay in that
motion and not change speed or direction unless
acted upon by an outside force.