Newton's Laws of Motion - PowerPoint PPT Presentation

Loading...

PPT – Newton's Laws of Motion PowerPoint presentation | free to view - id: 53a3ce-NGYyY



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Newton's Laws of Motion

Description:

Newton's Laws of Motion By Sandrine Colson-Inam, Ph.D References: Conceptual Physics, Paul G. Hewitt, 10th edition, Addison Wesley publisher http://www.glenbrook.k12 ... – PowerPoint PPT presentation

Number of Views:495
Avg rating:3.0/5.0
Slides: 44
Provided by: Centerfor142
Category:
Tags: laws | motion | newton

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Newton's Laws of Motion


1
Newton's Laws of Motion
  • By Sandrine Colson-Inam, Ph.D
  • References
  • Conceptual Physics, Paul G. Hewitt, 10th edition,
    Addison Wesley publisher
  • http//www.glenbrook.k12.il.us/gbssci/Phys/Class/v
    ectors/u3l2a.html

2
Outline
  • Intro
  • Newton's First Law of Motion
  • Forces and Free-Body Diagrams, Net Force
  • Newton's Second Law of Motion
  • Newton's Third Law of Motion
  • Friction

3
INTRODUCTION
  • Newton's Laws of Motion are the most fundamental
    laws that explain the physical world around us.

4
Newton's First Law of Motion
  • An object at rest tends to stay at rest and an
    object in motion tends to stay in motion with the
    same speed and in the same direction unless acted
    upon by an unbalanced force.

5
Inertia and Mass
  • Inertia the resistance an object has to a change
    in its state of motion.
  • The tendency of an object to resist changes in
    its state of motion varies with mass. Mass is
    that quantity which is solely dependent upon the
    inertia of an object.
  • Inertia tendency of an object to resist changes
    in its velocity.
  • Inertia tendency of an object to resist
    accelerations.

6
Check your understanding
  • 1. Imagine a place in the cosmos far from all
    gravitational and frictional influences. Suppose
    that you visit that place (just suppose) and
    throw a rock. The rock will
  • a. gradually stop.
  • b. continue in motion in the same direction at
    constant speed.
  • 2. Ben Tooclose is being chased through the woods
    by a bull moose which he was attempting to
    photograph. The enormous mass of the bull moose
    is extremely intimidating. Yet, if Ben makes a
    zigzag pattern through the woods, he will be able
    to use the large mass of the moose to his own
    advantage. Explain this in terms of inertia and
    Newton's first law of motion.
  • 3. The group of physics teachers are taking some
    time off for a little putt-putt golf. The 15th
    hole at the Hole-In-One Putt-Putt Golf Course has
    a large metal rim which putters must use to guide
    their ball towards the hole. Mr. S guides a golf
    ball around the metal rim When the ball leaves
    the rim, which path (1, 2, or 3) will the golf
    ball follow?

7
Answers
  • 1. According to Newton's first law, the rock will
    continue in motion in the same direction at
    constant speed.
  • 2. The large mass of the bull moose means that
    the bull moose has a large inertia. Thus, Ben can
    more easily change his own state of motion (make
    quick changes in direction) while the moose has
    extreme difficulty changing its state of motion.
    Physics for better living!
  • 3. The answer is 2. Once leaving the rim, the
    ball will follow an "inertial path" (i.e., a
    straight line). At the instant shown in the
    diagram, the ball is moving to the right once
    leaving the rim, there is no more unbalanced
    forces to change its state of motion. Paths 1 and
    3 both show the ball continually changing its
    direction once leaving the rim.

8
About Force
  • A force is a push or pull upon an object
    resulting from the object's interaction with
    another object. Whenever there is an interaction
    between two objects, there is a force upon each
    of the objects. When the interaction ceases, the
    two objects no longer experience the force.
  • A force is a vector quantity.
  • Forces only exist as a result of an interaction.
  • Force is a quantity which is measured using the
    standard metric unit known as the Newton (N).
  • For simplicity sake, all forces (interactions)
    between objects can be placed into two broad
    categories
  • contact forces, and
  • forces resulting from action-at-a-distance

9
Contact Forces
  • Those types of forces which result when the two
    interacting objects are perceived to be
    physically contacting each other. Examples of
    contact forces include frictional forces,
    tensional forces, normal forces, air resistance
    forces, and applied forces.
  • Contact Forces
  • Frictional Force
  • Tension Force
  • Normal Force
  • Air Resistance Force
  • Applied Force
  • Spring Force

10
Actions at Distance
  • Those types of forces which result even when the
    two interacting objects are not in physical
    contact with each other, yet are able to exert a
    push or pull despite their physical separation.
    Examples of action-at-a-distance forces include
    gravitational forces. Electric forces are
    action-at-a-distance forces. magnetic forces are
    action-at-a-distance forces.

11
Balanced Forces
  • The book is said to be at equilibrium.

12
Unbalanced Forces
  • In this case, an unbalanced force acts upon the
    book to change its state of motion.

13
Check your understanding
  • 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? Support your
    answers.
  • a. Once the cat hits the water, the forces are
    balanced and the cat will stop.
  • b. Upon hitting the water, the cat will
    accelerate upwards because the water applies an
    upward force.
  • c. Upon hitting the water, the cat will bounce
    upwards due to the upwards force.

14
Answers
  • a. False.
  • Once the cat hits the water, the forces are
    balanced (50 N down and 50 N up). However, an
    object in motion (such as the cat) will continue
    in motion at the same speed and in the same
    direction. When the cat strikes the water, it
    stops accelerating yet it does not stop moving.
  • b. False.
  • Once the cat hit the water, the forces are
    balanced (50 N down and 50 N up). The upward
    force of the water on the cat is balanced by the
    downward pull of gravity. The cat will continue
    in motion at constant speed.
  • c. False.
  • Once the cat hits the water, the forces are
    balanced (50 N down and 50 N up). The cat would
    only bounce upwards if the water applied an
    upward force greater than 50 N. As stated in the
    problem, the water applies only 50 N of upward
    force. Furthermore, the upward force would first
    contribute to slowing the cat down (an upward
    acceleration) before it could begin to actually
    move it upward.

15
Types of Forces 1/2
16
Types of Forces 2/2
17
Confusion of Mass and Weight
  • Many students of physics confuse weight with
    mass.
  • The force of gravity acting upon an object is
    sometimes referred to as the weight of the
    object.
  • The mass of an object refers to the amount of
    matter that is contained by the object.
  • The weight of an object is the force of gravity
    acting upon that object. Mass is related to how
    much stuff is there and weight is related to the
    pull of the Earth (or any other planet) upon that
    stuff.

18
Weight
  • Weight is the magnitude of the force of gravity
    acting on an object.
  • Weight Fgrav
  • Fgrav mass x gravity

19
Normal Force
  • Normal Force is a force exerted by one object on
    another in a direction perpendicular to the
    surface of contact.
  • The normal force is always perpendicular to the
    surface but is not always opposite the force of
    gravity.

20
Normal Force at an angle
  • On an angle, the normal force is calculated using
    Fn mgcos?.
  • The ? is the angle of the ramp.

21
FRICTION
  • Friction opposes the applied force.
  • The force of friction is proportional to the
    normal force.
  • It is easier to push a chair across the floor at
    a constant speed than to push a heavy desk across
    the floor at the same speed.
  • Because the desk is heavier than the chair, the
    desk experiences a greater normal force and
    therefore greater friction.
  • Two types of friction Static and Kinetic

22
STATIC FRICTION
  • The resistive force that keeps objects from
    moving is called the force of static friction.
  • Static Friction Fs
  • As long as the object doesnt move, the static
    friction is always equal to the opposite in
    direction to the applied force.
  • Fs -Fapplied
  • When the applied force is as great as it can be
    without moving the object, the force of static
    friction reaches its maximum value, called Fsmax

23
KINETIC FRICTION
  • Once an object exceeds Fsmax, it begins to move.
  • The resistive force that opposes the relative
    motion of two contacting surfaces that are moving
    is called the force of kinetic friction (Fk)?

24
Coefficients of Friction
  • Friction depends on the surfaces in contact.
  • The quantity that expresses the dependence on
    frictional forces on the particular surfaces in
    contact is called the coefficient of friction.
  • Coefficient of friction is represented by the
    symbol µ and pronounces mu.

25
Coefficient of kinetic friction
  • µk Fk/Fn
  • Divide the Force of kinetic friction by the
    normal force

26
Coefficient of static friction
  • µs Fsmax / Fn
  • Divide the maximum value of static friction by
    the normal force

27
Finding Friction
  • If the value of µ is known and the normal force
    is known, then the magnitude of the force of
    friction can be calculated.
  • Ff µFn
  • The kinetic friction is always less than or equal
    to the maximum static friction.
  • Think about pushing a car that is sitting still
    or pushing a car that is already moving.
  • The coefficient of kinetic friction is always
    less than or equal to the coefficient of static
    friction.

28
Materials µs µk
29
Air Resistance
  • Whenever an object moves through a fluid medium,
    like air or water, the fluid provides a
    resistance to the motion.
  • When an object falls through the air, its
    velocity increases until the air resistance
    balances the downward force of gravity.
  • The object falls with a constant speed, called
    terminal speed.

30
Check Your Understanding
  • A 24 Kg crate initially at rest on a horizontal
    floor requires a 75 N horizontal force to set it
    into motion. Find the coefficient of static
    friction.
  • Knowns?
  • m 24 kg, Fsmax 75 N
  • Unknown?
  • Coefficient of static friction ?
  • Equations?
  • µs Fsmax / Fn and Fn mg

31
ANSWER
  • Fn 24(9.8) 235.2 N µs 75/235.2 .32

32
More Practice
  • A student moves a box down the hall with a rope
    by pulling with a force of 185 N at an angle of
    25 degrees to the horizontal. The box has a mass
    of 35 kg and a coefficient of kinetic friction
    of .27. What is the acceleration?
  • On Board
  • Draw it first and find the resultant force.
  • Write knowns, unknowns, equations, and work it.

33
ANSWER
  • 2.7 m/s2 in the positive x direction

34
Drawing Free-Body Diagrams
  • Free-body diagrams are diagrams used to show the
    relative magnitude and direction of all forces
    acting upon an object in a given situation. A
    free-body diagram is a special example of the
    vector diagrams.

35
Check your understanding
  • A book is at rest on a table top. Diagram the
    forces acting on the book.
  • A girl is suspended motionless from the ceiling
    by two ropes. Diagram the forces acting on the
    combination of girl and bar.
  • 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.

36
Answers
3
1
2
5
4

37
Determining the Net Force (or Unbalanced Force)?
  • The unbalanced force refers to that force which
    does not become completely balanced (or canceled)
    by the other individual forces. If either all the
    vertical forces (up and down) do not cancel each
    other and/or all horizontal forces do not cancel
    each other, then an unbalanced force exists.
  • The existence of an unbalanced force for a given
    situation can be quickly realized by looking at
    the free-body diagram for that situation.
  • Free-body diagrams for three situations are shown
    below. Note that the actual magnitude of the
    individual forces are indicated on the diagram.
    What is the net force?

38
Answer

39
Summary for Net Force/Newton's 1st Law
  • The net external force is the vector sum of all
    the forces acting on an object.
  • A simple problem occurs when all forces act
    directly along the x and y axis. You would just
    add and subtract and use Pythagorean theorem.
  • However, most of the time, we must use vector
    component method.
  • Newtons 1st law states one condition must be
    true for equilibrium the net external force
    acting on a body in equilibrium must be equal to
    zero.

40
Newtons 2nd Law of Motion
  • Force is proportional to mass and acceleration.
  • Force Mass x Acceleration (Fma)?
  • a (Vf Vi)/ (tf ti)?

41
Newtons 3rd Law of Motion
  • Forces always exist in pairs.
  • Every action has an equal and opposite reaction.

42
Check your understanding
  • The net external force on the propeller of a 3.2
    kg model airplane is 7 N forward. What is the
    acceleration of the plane?
  • Knowns?
  • m 3.2 kg, F 7 N
  • Unknown?
  • a ?
  • Equation?
  • Fma
  • Answer?
  • 7 3.2a, a 2.2 m/s2 Forward (vector need
    direction)?

43
THE END!
About PowerShow.com