Transcript Notes on Forces Part 2 - The Quinlan Corner of Physics

Quinlan Physics
 While
most people know what Newton's laws
say, many people do not know what they
mean (or simply do not believe what they
mean).
 Born
1642 December, 25
 Calculus
 Gravity
 3 Laws of Motion
 Alchemy
 Theology
 1st
Law – An object at rest will stay at rest,
and an object in motion will stay in motion
at constant velocity, unless acted upon by an
unbalanced force. (Inertia: Resists change)
 2nd
Law – Force equals mass times
acceleration. (F= m x a)
 3rd
Law – For every action there is an equal
and opposite reaction. (Action/Reaction)
Newton’s 1st Law – class demo
 Inertia:
the ability of an object to resist
change
 Net Force: the total force of a system
 Mass: the amount of matter in an object
 Force: something that is capable of
changing an object’s state of motion
 If
it has mass it has inertia…
 An
object at rest will stay at rest, and an
object in motion will stay in motion at
constant velocity, unless acted upon by an
unbalanced force.
 Inertia
is the tendency of an object to
resist changes in its velocity: whether
in motion or motionless.
 These
apples will not move unless acted on
by an unbalanced force.
 Once
airborne, unless acted on by an
unbalanced force (gravity and air – fluid
friction), it would never stop!
 Unless
acted upon by an unbalanced force,
this football would sit on the tee forever.
 Why
then, do we observe every day objects
in motion slowing down and becoming
motionless seemingly without an outside
force?
 It’s
a force we sometimes cannot see –
friction.
 Objects
on earth, unlike the frictionless
space the moon travels through, are under
the influence of friction.
 Cleats give us extra friction on the grass!
 Forces
acting on a body
 Examples of Cone
 Balanced
forces occur when Fnet= 0 N
 (It’s not moving!)
 Unbalanced
forces occur when Fnet = 0 N
 (It is moving!)
 Weight:
the force due to the acceleration of
gravity
 Fw= m(g)
 Alexis
is driving a Moving Truck which is
towing a trailer with a constant force of
440N. If the total mass of the trailer and its
contents are 375kg, what is the trailer’s
acceleration and direction?
 Newton
identified force as the cause of
motion (Hect)
F
=mxa
 Action-Reaction means all forces occur in
pairs
 Forces
are VECTORS!
 Molly
is playing in the yard with the wagon.
She puts our puppy in the wagon. The mass
of the wagon and the puppy is 39 kg. The
wagon accelerates at 0.95 m/s2. What force
does Molly pull the wagon with?
 Flash,
the Super Hero, has a mass of 68 kg
exerts a force of 390 N. What is the
acceleration of the Flash?
 Push/Pull
 Electrical
 Magnetic
 Nuclear
 Gravity

As weight
 Friction
 We
will deal with
primarily contact
forces here!
 These are “action
at a distance”
forces- move
mysterious
 Push
o’ war with the chair
 Clearly, the stronger force wins
 Concurrent Forces- Forces acting on the
same point at the same time
 Net Force= the sum of Concurrent Forces=
the Resultant Force
 Free
120N
Fs=120N
Body Diagram
(FBD)
 Simplifies Picture
 Centralizes mass
100N
Fp=100N
Fnet=20N
A drag racer screams
along at 190 mph, its
engine is delivering a
force of 40,000N and
a friction of 770N.
The parachute
accidentally deploys,
adding 2975N of drag.
Find the net force.
Use FBD. Is the car
still speeding up?
 Fnet=36,255N

 Read
section 4.5, including the “Harrier”,
sample problems and solutions.
 Then skip to example 4.8 pg. 121-122
 Watch
http://kevandang.textamerica.com/?r=34750
57
 Fn

is called the normal force
It means it is perpendicular to the surface being
used
Θ
will usually be less than Fn*Fw
 Fn= Fw*cos(Θ)
 Fp= Fw*sin(Θ)

P means it is parallel to surface
 Equilibrium-
state of motion where
acceleration = 0
 Translational Equilibrium- a=0, all forces in
balance. (Can be rotating at constant speed.)
 Equilibrant Force- The force that operates
opposite to the resultant force of a system,
causing it to slow down.
 Moving



and groovin’ with a spring scale
Lifting
Pulling
Angled
 Statics-
The study of equilibrium, using
reaction forces through supports to cancel
forces.

1st true engineering class!!!!
 The
table weighs
200N. It has 162N of
boxes centered
perfectly on the table.
Find the weight
supported by each leg.
Use a FBD.
Let’s combo force and F=m x a concept.
 The Mine excavator with the coal…

 Calculate
the acceleration, and FT
 Try
it on your own
 In your lab book write forces in 2D
 Select: -car
-pulley
-string
-a
mass
 Calculate the acceleration going off the side
of the table and solve the FBD for your
system.
 Weighed
down on two sides
 In
an Atwood's
machine experiment,
the larger mass is 1.8
kg and the smaller
mass is 1.2 kg.
 Ignoring friction, what
is the acceleration of
the masses?
 What is the tension in
the string?









The net force on each mass is given in the two
equations
1.8a = 1.8g -T
1.2a = T - 1.2g
Since a and T are the same for both masses, add the
two equations:
3.0a = .6g
a = .2g = 1.96 m/s^2
Substituting for a in the first equation:
1.8(1.96) = 1.8(9.8) - T
T = 14.1 N
 An
elevator has a mass of 1150kg and the
elevator has a counterweight with the mass
of 1000kg. Find the acceleration and Tension
Force.
 Jack
and Jill ran down the hill, carrying a
10kg pail of water. The they held created a
60 degree angle. Calculate the tension in the
rope.
A
20.0 kg mass is pulled by along a surface by a
horizontal force of 100 N. Friction is 20.0 N.
What is the acceleration of the mass?
 Sum of forces (vertical forces cancel as
evidenced by lack of acceleration in the vertical
dimension)

Fnet = T + Ff
Fnet = (100 N) + (-20.0 N) = 80 N
A
49-N block is pulled by a horizontal force of
50.0 N along a rough horizontal surface at a
constant acceleration of 6 m/s^2. What is the
coefficient of friction?
 In
the vertical axes forces are equal and
opposite as there is no vertical acceleration.
 FN = -Fg
 FN = mg = 49 N
 m = 49/9.81 = 4.99 kg
 In
the horizontal axis
 Fnet = T + Ff
 Fnet = (50 N) + Ff
Newton's Second Law:
Fnet = ma
 Fnet = (4.99 kg)(6) = 30.0 N
 Ff=20 N
 Obviously,
if forces are vectors, they can be
resolved into components as well as added.
 While the resolution of forces will not focus
as much on the concepts of vertical and
horizontal, the goal will be to create right
triangles and ease the trigonometry.
 An
18 kg box is secured on an incline plane at
an angle of 35 degrees by string that can
support 97 N of force.


A. Show mathematically whether or not the
string will hold.
B. If the string breaks and the block accelerates
down the ramp, and if there is no friction, what
is the acceleration of the box?
 Friction-
a force that operates in the
direction opposite to motion due to contact
between surfaces.
 Draw ridged surfaces
 1.
Lubrication-wax on snowboard
 2. Rollers
 3. Smooth Surfaces- air hockey table
 4. Reduce Weight-car, skis
I
push a lawn mower with 450N of force. The
handle forms a 35 degree angle with the
ground.


What force do I push forward with?
What force do I push down with?
 Find
the normal force if the mover is 10kg,
find a is optional
 The
1500 kg truck is on a 40 degree incline.
Find the acceleration down hill of the truck
if the emergency brake is on and reset it.
 Studies
have shown that even the smoothest
surface can develop friction- mysterious, but
most likely “sticky” electrical interactions
between the molecules.
 Jack
and Jill ran down the hill, carrying a
10kg pail of water. The pail they held
created a 60 degree angle between their
arms. Calculate the tension in the rope.
 Ben
is walking his 93kg pot belly pig with a
and pulling with a force of 120N at 37 degree
above the horizontal. What is the
acceleration of the pot belly pig? What is the
Normal Force?
A
baby elephant pulls a small tree with the
mass of 30 kg with a force of 200N at an
angle of 30 degrees. Ignore friction, what’s
the acceleration and Normal Force?