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Forces I
Newtons Laws
Kinematics
• The study of how objects move
Why do objects move?
Dynamics
• The study of why objects move
Newton’s Laws and Forces
• What is force?
• What are they?
Force
•
•
•
•
A push or a pull
Symbol is F
Unit is N (Newton)
One newton is the force necessary to cause
a one kilogram mass to accelerate at the rate
of 1m/s2
• 1N=1 kg m/s2
FORCE
A force is any influence that can change the velocity of
a body. Forces can act either through the physical
contact of two objects (contact forces: push or pull) or
at a distance (field forces: magnetic force,
gravitational force).
Contact Forces
Action-at-a-Distance Forces
Frictional Force
Gravitational Force
Tensional Force
Electrical Force
Normal Force
Magnetic Force
Air Resistance Force
Applied Force
Spring Force
What do we mean by balanced and unbalanced forces?
The forces on the book are unbalanced
Balanced forces are
EQUAL and
OPPOSITE in direction
Unbalanced forces
have a greater force
in one direction
If all the forces are balanced,
we say the object is in EQUILIBRIUM
A net force is the vector sum of the forces
acting on an object
What are the net forces?
FNET = Σ forces ≠ 0 N
These are free-body diagrams
Free Body Diagram Practice
1. A book is at rest on a tabletop. Diagram the forces acting on the book.
2. A girl is suspended motionless from the ceiling by two ropes. Diagram the
forces acting on the girl.
3. An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram
the forces acting on the egg as it is falling.
4. A rightward force is applied to a book in order to move it across a desk
with a rightward acceleration. Consider frictional forces. Neglect air
resistance. Diagram the forces acting on the book.
5. A rightward force is applied to a book in order to move it across a desk at
constant velocity. Consider frictional forces. Neglect air resistance.
Diagram the forces acting on the book.
6. A college student rests a backpack upon his shoulder. The pack is
suspended motionless by one strap from one shoulder. Diagram the
vertical forces acting on the backpack.
7. A skydiver is descending with a constant velocity. Consider air resistance.
Diagram the forces acting upon the skydiver.
8. A force is applied to the right to drag a sled across loosely packed snow
with a rightward acceleration. Neglect air resistance. Diagram the forces
acting upon the sled
Newton Three Laws of Motion
• His laws explain why objects move (or
don’t move) as they do
Newton's First Law of Motion
(Law of Inertia) or [Law of Balanced
Forces]
Newton's First Law of Motion says:
an object at rest stays at rest and an
object in motion stays in motion
unless the object is acted on by an
unbalanced force.
“Things keep doing what they are
doing!”
Constant Velocity
It can be Zero or it can be greater than
zero…..It just can’t change!!!!!!
What is required to change the
constant velocity?
• An unbalanced force
Diagram from the Physics Classroom
http://www.physicsclassroom.com/mmedia/newtlaws/efff.html
Diagram from the Physics Classroom
http://www.physicsclassroom.com/mmedia/newtlaws/efff.html
Other Examples to Consider:
• Blood rushes from your head to your feet
while quickly stopping when riding on a
descending elevator.
• The head of a hammer can be tightened
onto the wooden handle by banging the
bottom of the handle against a hard surface.
Inertia and Mass
• Inertia: the resistance an object has to a
change in its state of motion
• The more mass the more inertia
• As mass increases the more an object resists
change in its motion
• Mass (kg) is the measurement of Inertia
FREE BODY DIAGRAM:Identify the missing
force marked with a (?) in the diagram below.
The car is moving at a constant velocity of 25
m/s eastward.
Air Friction
Force pushing car
Car
20N
?
15N
Ground Friction
Ground
Friction
Newton’s First Law
Remember an object in constant
motion has balanced forces working
on it. This is the first law described
mathmatically
ΣF = 0
• Where Σ (sigma) is sum
• F is force
Hands On
• Using Newton scale:
• drag mass across table at constant velocity.
What was the force?
.
Newton's Second
Law of Motion
(Law of Acceleration)
[Law of Unbalanced
Forces]
SECOND LAW OF MOTION
According to Newton's Second Law of Motion, the net
force acting on a body equals the product of the mass
and the acceleration of the body. The direction of the
force is the same as that of the acceleration. In
equation form:
F = ma
F = force applied to an object
[N = Newtons = kg ∙m/s2]
m = object’s mass [kg]
a = acceleration [m/s2]
What is the relationship between
Force, Mass, and Acceleration?
According to Newton's second law of motion,
the acceleration experienced by an object
increases in a direct variation with?
Force
According to Newton's second law of motion,
the acceleration experienced by an object
increases in an inverse variation with?
Mass
FREE BODY DIAGRAM:Identify the missing
force marked with a (?) in the diagram below.
The car is accelerating eastward with a net
force of 50 N.
Air Friction
Force pushing car
Car
20N
?
15N
Ground Friction
Ground
Friction
nd
2
Law: F = m·a
EX A
• A 2000 N net force acts horizontally to the
right on a 50 kg object. Draw the free body
diagram. Calculate the acceleration.
F  m a
2000 N  50kg  a
a  40m / s
2
Are weight and mass the same
thing?
WEIGHT (of a body or object) aka Force of Weight
The gravitational force with which the Earth attracts the
body. Causes it to accelerated downward with the
acceleration of gravity g.
Vector quantity
Measured in Newtons
Varies with its location near the Earth (or other
astronomical body)
ForceWeight = mass x gravity
FW = m x g
Newton = kg x m/s2
1 N = 1 kgm/s2
Mass
A scalar quantity
Same everywhere in the universe. It
doesn’t change
Measured in kg
Hands On
• Using Newton scale:
• drag mass across table at a changing
velocity. What happens to the force?
• Suspend the mass. What is the force?
What does this represent?
EX B: What is the weight of a person
whose mass at sea level is 72 kg? Draw
the free body diagram.
g = 9.80 m/s2
m = 72 kg
F = mg
Fw = mg
Fw = (72 kg)(9.80 m/s2)
Fw = 706 kg m/s2
Fw = 706 N
Ex C: What is the mass of a box
that weighs 625N? Answer to the
nearest tenth. Draw the free body
diagram.
F = ma
m = F/m
m = 625N/9.8 m/s2
m = 63.8 kg
Constant Force Lab
You will have a quiz over
this
You do not have to convert
grams nor cm
When graphing you must
come up with a reasonable
scale. The y intervals must
be equal, the x intervals
must be equal
What is the third law of motion
• For every action force there is an equal and opposite
reaction force
• Two different objects and two different forces (equal but
opposite)
• Forces always occur in pairs
• Also known as The Law of Force Pairs
• Are the effects of the force the same?
• NO
• They are not always acting on the same amount of mass
• Examples:
• Bug hitting windshield, recoil of shot gun, pushing off
of on a raft
Third Law Mathmatically
F=F
Which can be written as:
ma= ma
Forces occur is pairs…
What forces cause the swimmer to
move forward?
• Her push on the wall?
Or
• The wall’s push on her?
THIRD LAW OF MOTION
According to Newton's third law of motion, when one
body exerts a force on another body, the second body
exerts on the first an equal force in opposite
direction.
The Third Law of Motion
applies to two different forces
on two different objects: "The
action force one object exerts
on the other, and the equal but
opposite reaction force the
second object exerts on the
first."
Ex D
• A 2500 kg car hits a 0.001 kg bug with a
force of 500N. According to the third law
of motion, the bug hits the car with how
many Newton's of force? Why isn’t the car
damaged? Calculate the acceleration of the
bug due to the force. Calculate the
acceleration of the car due to the force.
A 2500 kg car hits a 0.001 kg bug with a force of 500N. According to the
third law of motion, the bug hits the car with how many Newton's of
force? Why isn’t the car damaged? Calculate the acceleration of the bug
due to the force. Calculate the acceleration of the car due to the force.
acceleration of the bug due to impact:
a = F/m
a = 500N/.001 kg = 500,000 m/s2 !
acceleration of the car due to impact :
a = F/m
a = 500N/2500kg = 0.2 m/s2
Third Law of Motion
• The girl pushes the boy with 10N of force.
How much does the boy push back with?
Review F=ma calculations
Net Force (Fnet )
• Fnet in simple situations (only 1 force
implied): Fnet = ma
• Fnet and acceleration are always in the same
direction
• How is net force on an object determined
with multiple?
• It is the sum of all forces
• Fnet = ΣF
• Remember force is a vector, therefore it has
direction (+,-, N,S etc)
Net force (F) = ma
The second law of motion is the key to understanding
the behavior of moving bodies since it links cause
(force) and effect (acceleration) in a definite way.
Example E
A 2000 N net force (horizontally to the right) acts
on a 50 kg object. Draw the free body diagram.
Calculate the acceleration. If the object started
from rest, how far would it move in 5 seconds?
F  m a
2000 N  50kg  a
a  40m / s
2
Example E
A 2000 N net force (horizontally to the right) acts
on a 50 kg object. Draw the free body diagram.
Calculate the acceleration. If the object started
from rest, how far would it move in 5 seconds?
d = vit + .5at2
d = (.5)(40 m/s2)(5sec)2 vf
d = 500m
Could we determine the velocity at 5 seconds? How?
a = (vf – vi)/t
vf = vi + at
vf = 0 m/s + (40 m/s2)(5sec) = 200 m/s
Ex F A 1000 kg car goes from 10 to 20 m/s in 5 s. What force is
acting on it?
m = 1000 kg
vi = 10 m/s
vf = 20 m/s
t=5s
a = (vf – vi)/t
a = (20m/s-10 m/s) / 5s
a = 2 m/s2
F = ma
F = (1000 kg) (2 m/s2) = 2000 N
Ex G A 60-g tennis ball approaches a racket at 15 m/s, is in contact
with the racket for 0.005 s, and then rebounds at 20 m/s. Find the
average force exerted by the racket.
m = 0.06 kg
vi = 15 m/s
t = 0.005 s
vf = - 20 m/s
a = (vf – vi)/t
a = (-20m/s-15 m/s) / 0.005s
a = -7000 m/s2
F = ma
F = (0.06 kg) (-7000 m/s2) = -420 N
Ex H The brakes of a 1000-kg car exert 3000 N.
a. How long will it take the car to come to a stop from a velocity of
30 m/s?
m = 1000 kg
F  3000
F = -3000 N
2
a
=
3
m/s

vi = 30 m/s
m 1000
vf = 0 m/s
v f  vi
0  30
= 10 s

t
3
a
b. How far will the car travel during this time?
d = vit+ .5at2
= 30(10)+ .5 (-3)(10)2
= 150 m
Ex I A net horizontal force of 4000 N is applied to a car at rest
whose weight is 10,000 N. What will the car's speed be after 8 s?
FA = 4000 N
Fw = 10,000 N
t = 8s
Fw 10000
m

ag
9.8 = 1020.4 kg
F
4000
a
2

=
3.92
m/s
m 1020.4
vf = vi + at
= 0 +3.92(8)
= 3.14 m/s
Concept Review questions on
following 5 slides
Which diagram represents a book
sitting on a table?
A
B
C
What vector indicates force due to
gravity?
B
A
plane
D
C
Draw a free body diagram of a car
accelerating to the left with a net
force of 400N
My bag of Halloween candy weighs
3 N. What is it’s mass?
I have a brand new car. It can go
from 0mph to 50mph in 5
seconds!!!!! What is its rate of
acceleration?
• An object is thrown in the air with an initial
velocity of 15m/s. How high above the ground
is it after 2.5 secs?
• d = vit + .5at2
• (-15m/s) (2.5 sec) + [(0.5)(9.8m/s2)(2.5sec)2]
• -6.88 m (the negative indicates height)
• Luke Autbeloe drops an approximately 5.0
kg fat cat (weight = 50.0 N) off the roof of
his house into the swimming pool below.
Upon encountering the pool, the cat
encounters a 50.0 N upward resistance force
(assumed to be constant). Use this
description to answer the following
questions.
Which one of the following dot diagrams best
describes the motion of the falling cat from the time
that they are dropped to the time that they hit the
bottom of the pool? The arrows on the diagram
represent the point at which the cat hits the water.
Support your answer with sound reasoning.
• Tape A is correct.
• The cat first accelerates with a negative
(downward) acceleration until it hits the water.
Upon hitting the water, the cat experience a
balance of forces (50 N downwards due to gravity
and 50 N upwards due to the water). Thus, the cat
will finish its motion moving with a constant
velocity. Diagram A depicts both the initial
downward acceleration and the final constant
velocity.
• Several of Luke's friends were watching the
motion of the falling cat. Being "physics
types", they began discussing the motion
and made the following comments. Indicate
whether each of the comments are correct or
incorrect?
• Once the cat hits the water, the forces are
balanced and the cat will stop.
• Upon hitting the water, the cat will accelerate
upwards because the water applies an upward
force.
• Upon hitting the water, the cat will bounce
upwards due to the upwards force.
Fun with Forces
#1 16 N↑ 4N→
#7 130 lb → if net is 85lb right ; must have
85lb greater than the 120lb left. Therefore need
a total of 205lb right. Already have 75lb right,
so need an additional 130lb right (205lb-75lb)