Forces I

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Forces I

• Newtons Laws

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Kinematics

• The study of how objects move

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Why do objects move?
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Dynamics

• The study of why objects move

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Newtons Laws and Forces

• What is force?
• What are they?

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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
• 1N1 kg m/s2

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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
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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
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If all the forces are balanced, we say the
object is in EQUILIBRIUM
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A net force is the vector sum of the forces
acting on an object
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What are the net forces? FNET S forces ? 0 N
These are free-body diagrams
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Free Body Diagram Practice A book is at rest on a
tabletop. Diagram the forces acting on the
book. A girl is suspended motionless from the
ceiling by two ropes. Diagram the forces acting
on the girl. 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. A
flying squirrel is gliding (no wing flaps) from a
tree to the ground at constant velocity. Consider
air resistance. Diagram the forces acting on the
squirrel. 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. 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. 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. A skydiver is descending with a
constant velocity. Consider air resistance.
Diagram the forces acting upon the skydiver. 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. A football is
moving upwards towards its peak after having been
booted by the punter. Neglect air resistance.
Diagram the forces acting upon the football as it
rises upward towards its peak. A car is
coasting to the right and slowing down. Neglect
air resistance.

• Free Body Diagram Practice
• A book is at rest on a tabletop. Diagram the
  forces acting on the book.
• A girl is suspended motionless from the ceiling
  by two ropes. Diagram the forces acting on the
  girl.
• 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.
• 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.
• 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.
• 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.
• A skydiver is descending with a constant
  velocity. Consider air resistance. Diagram the
  forces acting upon the skydiver.
• 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

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Newton Three Laws of Motion

• His laws explain why objects move (or dont move)
  as they do

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Newton's 1st Law of Motion
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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.
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Things keep doing what they are
doing! Constant Velocity It can be Zero or it
can be greater than zero..It just cant
change!!!!!!
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What is required to change the constant velocity?

• An unbalanced force

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Diagram from the Physics Classroom http//www.phys
icsclassroom.com/mmedia/newtlaws/efff.html
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Diagram from the Physics Classroom http//www.phys
icsclassroom.com/mmedia/newtlaws/efff.html
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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.

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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

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FREE BODY DIAGRAMIdentify 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
20N
?
15N
Ground Friction
Ground Friction
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Newtons First Law
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Remember an object in constant motion has
balanced forces working on it. This is the first
law described mathmatically

• SF 0
• Where S (sigma) is sum
• F is force

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Hands On

• Using Newton scale
• drag mass across table at constant velocity.
  What was the force?

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Newton's 2nd Law of Motion
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• .
• Newton's Second
• Law of Motion
• (Law of Acceleration)
• Law of Unbalanced
• Forces

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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 objects mass kg a acceleration
m/s2
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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

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FREE BODY DIAGRAMIdentify 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
20N
?
15N
Ground Friction
Ground Friction
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2nd Law F maEX A

• A 2000 N net force acts horizontally to the right
  on a 50 kg object. Draw the free body diagram.
  Calculate the acceleration.

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Are weight and mass the same thing?
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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
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Mass
A scalar quantity Same everywhere in the
universe. It doesnt change Measured in
kg
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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?

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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

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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

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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
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Newton's 3rd Law of Motion
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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

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Third Law Mathmatically

• F F
• Which can be written as
• ma ma

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Forces occur is pairs
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What forces cause the swimmer to move forward?

• Her push on the wall?
• Or
• The walls push on her?

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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."
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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 isnt the car damaged? Calculate the
  acceleration of the bug due to the force.
  Calculate the acceleration of the car due to the
  force.

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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 isnt 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

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Third Law of Motion

• The girl pushes the boy with 10N of force. How
  much does the boy push back with?

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Review Fma calculations
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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 SF
• Remember force is a vector, therefore it has
  direction (,-, N,S etc)

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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.
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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?

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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
• 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

vf
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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 5 s
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
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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
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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 N vi 30 m/s vf 0 m/s
- 3 m/s2
10 s
b. How far will the car travel during this time?
d vit .5at2 30(10) .5 (-3)(10)2 150
m
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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
1020.4 kg
3.92 m/s2
vf vi at 0 3.92(8) 3.14 m/s
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Concept Review questions on following 5 slides
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Which diagram represents a book sitting on a
table?
A
B
C
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What vector indicates force due to gravity?
B
C

• A

plane
D
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Draw a free body diagram of a car accelerating to
the left with a net force of 400N
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My bag of Halloween candy weighs 3 N. What is
its mass?
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I have a brand new car. It can go from 0mph to
50mph in 5 seconds!!!!! What is its rate of
acceleration?
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• 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)

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• 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.

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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.
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• 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.

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• 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?

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• 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.

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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)