Transcript Regents Physics

Regents Physics
 Agenda
 Introduction
to Forces
 Intro to Newton’s three Laws of
Motion
 HW: Read p. 117-125
What’s a Force?
 We’ve learned that acceleration is the
change in an object’s velocity..
And
 Velocity is the change in an objects
position..
 By what causes the change in
acceleration?
A Force!
What’s a Force?
 Forces can be described as a push or a
pull that is applied to an object by
something else..
 Forces are vectors – magnitude and
direction
 The ability to understand how forces
affect us is crucial to success in many
fields

Ex: building of homes and bridges
Kinds of Forces
 Contact force – acts on an object only
by touching it
 Ex:
book on table, friction
 Long-range force – is exerted without
contact
 Ex:
magnetic force, force of gravity
Forces have agents..
 Each force has a specific, identifiable,
immediate cause called the agent
 The agent can be animate – such as a
person
or
 inanimate – such as a desk, floor or
magnet
What’s the agent for the pull of gravity?
Solving Force Problems - intro
 First step is to draw a pictorial model,
called a free body diagram, and identify
the contact and long range forces
 Draw
the vectors
 Example
F desk on book
Book on a table
F book on desk
Solving Force Problems - intro
 Examples
Ball on a rope
F rope on ball
F ball on rope
Skydiver
F air on diver
F gravity on diver
Practice Problems
 Draw a free body diagram for each of
the following:
 Book
held in your hand
 Book pushed across the desk by your hand
 Book pulled across the desk by a string
 Book on a desk with you hand pushing
down
 Ball just after the string that was holding it
broke
The man…Sir Isaac Newton
 300 hundred years ago an apple fell on
his head…and he wondered why?
 Explained the way forces influence
motion
 Summed it up in three famous laws
2nd Law of Motion
 The force exerted on an object is equal
to the objects mass times its
acceleration, or F = ma
 Expressed in newtons = kg x m/s2
 Example: Mr. O
 Mass
= 95 kg
 Acceleration = gravity = 9.80 m/s2
F = ma = (95 kg)(9.80 m/s2) = 931N
2nd law continued
 F = ma can be rearranged to be
a=
F
m
 we say that the force exerted on an
object is proportional to its acceleration
(since the objects mass doesn’t change)
 The larger the force..the greater the
acceleration
 We
have a linear relationship!
2nd law continued
 Multiple forces can combine and act on
a system
 They could act in the same direction or
in different directions
 Because forces are vectors, the total
force on an object is the vector sum of
all forces on the object
 This vector sum is called the net
force
Finding a net force
 Two horizontal forces act, 225 N and
165 N, are exerted in the same direction
on a crate (assume no friction). Find
the net horizontal force on the crate.
 Step 1 – draw a free body diagram
F = +225 N
Fnet = Facting on the crate
Fnet = 225N + 165N = +390N
F = +165 N
worksheet
Regents Physics
 Agenda
 Newton’s
first law of motion - Intro
 Newton’s second law practice probs
 HW Chap Problems:
 RC
#1,3,5,6,8,9
 AC # 10, 15, 16
 Probs # 20, 22, 25, 27, 30, 32, 36, 38, 40
1st Law of Motion
 An object that is at rest will remain at
rest or an object that is moving will
continue to move in a straight line with a
constant speed, if and only if the net
force acting on that object is zero.
 Also
called the Law of Inertial Mass
 What does that mean to us?
1st Law of Motion
 What is inertia?
Inertia is the tendency of an object
to resist change
Anything that has mass has inertia!
We resist what tries to change us!
Examples
Sitting still / moving at a constant velocity
Object in space
Turning in a car
1st Law of Motion
 What is equilibrium?
If an object is at rest or it is moving at
a constant velocity, the net force is zero
in both cases
We are happy just chillin..
Some Types of Forces
 See table 6.2 pg. 123
Common Misconceptions
 When we throw a ball, the force from
our hand stays on it
 A force is needed to keep an object
moving
 Inertia, itself, is a force
 Air does not exert a force
Calculations with Newton’s
2nd Law
 Using F = ma
 Mass and weight..what’s the difference?
 Ex: a bathroom scale
 Draw
a freebody diagram for this and label
the forces
A system of solving..
 Read the problem and visualize!
 Choose a coordinate system
 Write your known and unknowns
 Use f = ma to link acceleration and net
force
 Rearrange, plug in numbers and solve
 Check your answer to see if it’s
reasonable
Practice Problem
 On Earth, a scale shows that you weigh
585 N
 a)
What is your mass?
 B) What would the scale read on the
moon? (g = 1.60 m/s2)
Regents Physics
 Agenda
 More
on Newton’s Second Law
 Review Intro to Forces Worksheet
 Drawing Free-body Diagrams Worksheet
 HW: More Advanced Newton’s Second
Law Problems
Practice Problem - elevator
 You still weight 585 N. Find the reading
on a scale in an elevator when:

a) the elevator moves up at a constant speed
 b) it slows at 2.0 m/s2, while moving upward
 c) It speeds up while moving 2 m/s2 downward
 d) it moves downward at a constant speed
 e) it slows to a stop at a constant magnitude of
acceleration
End
Working with friction...
 Friction is a force that generally
opposes the direction of motion
 Different surfaces have different
amounts of friction that affect an objects
movement
 There are types of friction:
 Kinetic
Friction = uk
 Static Friction = us
Ffr = Fnu
Solving Problems with Friction
 Find the net force on an object that has a
mass of 20.0 kg, acceleration of 2.3 m/s2
on an surface with a friction coefficient of
Known:
0.21
Fn
Ffr
Fn=Fg=mg=(20.0kg)(9.8 m/s2) = 196 N
Fa = ma = (20.0kg)(2.3 m/s2) = 46N
Ffr = Fnuk = (196N)(0.21) = 41.2 N
Fa
Fg
Worksheet
Fnet = Fa - Ffr
Fnet = Fa - Fnuk
Fnet = 46N - 41.2N = 5N
3rd Law of Motion
 Every force has an equal and opposite
force,
or
FA on B = -FB on A
Fyou on wall = Fwall on you