Objectives Chapter 6 Define a force and differentiate between
contact forces and long-range forces Recognize the significance of
Newton's second law of motion and use it to solve motion problems
Explain the meaning of Newton's first law and describe an object in
equilibrium
Slide 4
Objectives Chapter 6 Describe how the weight and the mass of an
object are related Differentiate between the gravitational force
weight and what is experienced as apparent weight Define the
friction force and distinguish between static and kinetic friction
Describe simple harmonic motion and explain how the acceleration
due to gravity influences such motion
Slide 5
Objectives Chapter 6 Explain the meaning of interaction pairs
of forces and how they are related by Newton's third law List the
four fundamental forces and illustrate the environment in which
each can be observed Explain the tension in ropes and strings in
terms of Newton's third law
Slide 6
Following Slides Just for Fun May the Force be with you young
Physics Student
Slide 7
Historical Perspective Aristotle: Motion into 2 categories,
Natural and Violent Natural: Objects seek their natural motion.
Either up or down. A ball comes to rest because it is its natural
motion. Violent: Imposed motion, via force. Responsible for things
moving against The earths natural motion: Not moving, no force big
enough to move it.
Slide 8
Galileo and Copernicus Copernicus said that the Earth revolved
around the sun (Contrary to Aristotle who thought the Earth was too
big to move) Galileo countered that it doesnt take a force for an
object to keep moving Changed the notion that it takes a force to
keep something moving
Slide 9
Galileo Galileo: College prof Galileo, not really aware of
politics, first in Venice. One of his Benefactors, a cardinal, good
friend who gave lots of money, become pope. Galileo writes his book
about Aristotle verse Galileo. Different than normal science books,
written like a play. 3 main characters, Simplisico (Greek),
Salvatory (Galileo), and Segacio (Impartial judge). Debate.
Ultimate problem, the pope had a preference to the greek way.
Galileo discovered new things. So into Simplisicos mouth, he doesnt
speek well, argument not well constructed, his name means simple
(Idiot or moron). Idiot says! Sage, the wisemen is won over to
salvatory. Salvatory does a good job with argument. So idio says
what pope thinks, and hero says what Galileo thinks. Galileo gets
in trouble for mocking pope. They were friends, and Galileo makes
fun of pope. Pope asks him not to publish it, Galileo does it
anyway. Gets put under house arrest after inquisition, doesnt
really matter, Galileo is old!
Slide 10
Slide 11
Terminology Force: Any push or Pull Net Force: Combination of
all forces acting on object. If not zero the object will change
speed Friction: Force that acts between objects as they move past
each other. Tension/Compression: Pulling apart of atoms or pushing
together of atoms
Slide 12
Tensionional vs Compressional Tension is going to test the
strength of the atoms to hold onto one another by being pulled
apart Compression is going to test the ability of the atoms to
compress together (form a closer bond) without sliding past one
another (too close and repulsion gets too big leading to
break)
Slide 13
Isaac Newton Three Laws he discovered 1 st Law: Every object
continues in a state of rest, or of motion in a straight line at a
constant speed, unless it is compelled to change that state by
forces exerted upon it. Law of Inertia
Slide 14
1 st Law Unless acted upon by an unbalanced force, this golf
ball would sit on the tee forever.
Slide 15
Newtonss 1 st Law and You Dont let this be you. Wear seat
belts. Because of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stopped by the
brick wall, your body keeps moving at 80 km/hour.
Slide 16
Isaac Newton 2 nd Law: Forces cause acceleration The net force
of an object is equal to the product of its mass and acceleration,
or F=ma. Only forces which act on that object affect the
acceleration of the object.
Slide 17
Newtons 2 nd Law shows that different masses accelerate to the
earth at the same rate, but with different forces. We know that
objects with different masses accelerate to the ground at the same
rate. However, because of the 2 nd Law we know that they dont hit
the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F =
ma 9.8 N = 1 kg x 9.8 m/s/s
Slide 18
If mass remains constant, doubling the acceleration, doubles
the force. If force remains constant, doubling the mass, halves the
acceleration.
Slide 19
Isaac Newton 3 rd Law For every force, there is an equal and
opposite force Only true in certain situations
Slide 20
Examples of Newtons Third Law The baseball forces the bat to
the left (an action); the bat forces the ball to the right (the
reaction).
Slide 21
3 rd Law Consider the motion of a car on the way to school. A
car is equipped with wheels which spin backwards. As the wheels
spin backwards, they grip the road and push the road
backwards.
Slide 22
3 rd Law The reaction of a rocket is an application of the
third law of motion. Various fuels are burned in the engine,
producing hot gases. The hot gases push against the inside tube of
the rocket and escape out the bottom of the tube. As the gases move
downward, the rocket moves in the opposite direction.
Slide 23
3 rd Law Only applicable if objects act on each other If
objects act on an intermediate object, forces do not have to be
balanced Tug of War: Force on rope not balanced on each side
Slide 24
Mass vs Weight Mass is how much matter you have Your mass does
not change when you change location Weight is a measurement of the
forces acting on you Your weight can change when you change
location
Slide 25
Weightlessness Apparent weight is the weight you actually feel
In an elevator that is accelerating, your apparent weight changes
When you accelerate against gravity, you feel heavier When you
accelerate with gravity, you feel lighter On different surfaces,
your weight changes Weigh yourself on carpet vs hardwood vs
cushion
Slide 26
Forces Present Four major Forces to be aware of that act in
this world (from strongest to weakest) 1) The Strong Nuclear Force:
Holds the nucleus of atoms together. Small range (nucleus). Only
effective at ranges less than 1 - 15 meters. Without this force,
the electromagnetic forces between the protons would not allow for
the nucleus to exist at all.
Slide 27
Forces Present 2) Electromagnetic: Causes attraction or
repulsion, depending on charge of objects. This force is roughly 1
40 times stronger than gravity. This force has infinite range
Slide 28
Forces Present 3) The Weak Nuclear Force: Is responsible for
nuclear decay. This force, with a slightly small range of 1 -17
meters, cause emissions to occur from the nucleus.
Slide 29
Forces Present 4) Gravity: Very weak force, but like
electromagnetic, it works over infinite distances. The only
difference is it can only attract, not repel like the
previous.
Slide 30
Long Range Forces Electromagnetic and Gravity are long range
forces, they exert their force over the entire universe. The
objects do not need to be in contact for the force to exist
Example: Objects falling to earth Electrons being repelled by one
another
Slide 31
Contact Forces When objects are in direct contact with one
another (again, this is kind of weird because the objects really
dont touch), that is a contact force Pushing a wheelbarrow
Skidding/sliding on a surface and slowing down due to friction (a
contact force)
Slide 32
Friction A contact force which always opposes the motion of the
object. Friction can be caused by gravity (object is being pulled
to earth and that causes the objects to rub past one another) or by
two objects being forced past one another At the molecular level,
friction is an electromagnetic force
Slide 33
Normal Force This is the force which acts perpendicular to the
weight of the object and supports the weight of the object Normal
Force: Normal means at right angles to.
Slide 34
How Large? On a flat surface, the Normal force is equal to the
weight of the object Weight = mg (mass x gravity) A person who has
a mass of 50 kg has a weight (on earth) of 490 N
Slide 35
Question Check What is the effect of friction on a moving
object? Slows motion, always opposes the direction of motion
Galileo found that a ball rolling down one incline will pick up
enough speed to roll up another. How high up will it roll compared
to the initial incline?
Slide 36
Question Check The law of inertia states that no force is
required to maintain motion. Why then do you have to keep pedaling
a bike to keep it going? Friction What is the weight of 2.0 kg of
yogurt? 19.6 N
Slide 37
Question Check An elephant and a mouse would both have zero
weight in gravity-free space. If they were moving toward you with
the same speed, would they bump into you with the same effect?
Explain No, the elephant has much more inertia
Slide 38
Question Check A massive ball is suspended on a string and
slowly pulled by another string attached to it from below
Slide 39
Question Check Is the string tension greater in the upper or
the lower string? Which string is more likely to break if pulled
slowly from below? What if the string on the bottom is snapped
quickly (pulled quickly)?
Slide 40
Friction in Greater Detail Coefficient of Static Friction: Us
Us How hard is it to slide the object past another (how much force)
starting from rest? It is a % of the total weight of the object
Us(N)=F(static) N is weight of object, Us is some number usually
between 0 and 1, but numbers as high as 5 are seen in drag
racing
Slide 41
Coefficient of Static Friction This value corresponds to the
atoms falling into grooves with one another, and the force required
to pull them past one another A small Us value is associated with
objects that slide easily from rest (object on Ice) to very large
values (examples will vary based off two surfaces in contact)
Slide 42
Question An object with a mass of 15 kg is resting on a table
top. If it takes 22 N to get the object to start moving, what is
the value of Us between the table top and the object? If I place
another 15 kg mass on top of the object, will the force required to
start the object increase? Will the Us value change?
Slide 43
Coefficient of Kinetic Friction When an object is moving
(Relative to the surface that it is in contact with) the friction
is said to be Kinetic. This value is generally much smaller than
the Static Friction value because the molecules jump from mountain
to mountain
Slide 44
Practice Questions An object has a weight of 88 N and has a Us
value of 0.3. What force is required to start moving the object
(from rest) along the surface (pushing horizontally)? That object
has another object placed on top, what happens to the Us value?
Static Friction value
Slide 45
Practice Questions An object at rest has a weight of 800 N and
requires 200 N of horizontal push to overcome static friction. What
is the Us Value between the object and the floor? If the Uk value
is half the value of the Us, what acceleration does the object
undergo when it starts to move?
Slide 46
Uk In the same token, Uk(N) = Kinetic Friction Question: An
object that has a weight of 50 N has a Us value of 0.20 and a Uk
value of 0.10. The object is at rest on a table. AA) How much force
is required to start moving the Object? BB) What is the net force
acting on the object as soon as it starts moving?
Slide 47
Question An elevator with mass of 1200 kg is supported by a
cable. If three people enter the elevator with a total mass of 190
kg, what is the tension in the cable in N?
Slide 48
The Quiz Questions concerning Frictional Forces (Us/Uk)
calculations, thinking about the force of friction Remember, NET
forces cause acceleration, balanced forces cause constant speed
Questions over types of forces Some weight and mass questions Some
questions concerning Newtons 3 Laws