Transcript Newtons Laws Part 1b - studentx

Acknowledgements
© 2013 Mark Lesmeister/Pearland ISD
 This work is licensed under the Creative Commons Attribution-
ShareAlike 3.0 Unported License. To view a copy of this license,
visit http://creativecommons.org/licenses/by-sa/3.0/ or send a
letter to Creative Commons, 444 Castro Street, Suite 900,
Mountain View, California, 94041, USA.
 Selected graphics and problems from OpenStax College. (2012,
June 12). College Physics. Retrieved from the Connexions Web
site: http://cnx.org/content/col11406/1.7/
 Cartoons from Looney Tunes Movie Collection, © 2005 Warner
Brothers Entertainment. Used under the fair use doctrine for
educational purposes.
 Selected questions from
Pearland High School Physics
FORCE
 Force is a push or pull exerted on some object.
 Forces cause changes in velocity such as:
 Start moving, stop moving or change direction.
 The SI unit for force is the Newton.
 1 Newton = 1 kg m/s2
Types of Forces
 Forces can act through contact or at a distance.
 Contact Force – physical contact between two
objects
 Field Force – does not involve physical contact
between two objects.
 Example include:



electrical forces
magnetic forces
the force of gravity
Part 1
Observation #1
 An object at rest remains at rest, unless something
makes it move.
Is rest the natural state of an object?
 Picture a ball on a table in a moving train car.
Observation #2
 An object in motion continues in motion with
constant velocity, unless something makes it
change its velocity.
 Constant velocity means constant speed in the
same direction.
Combining Observations 1 & 2
 An object left alone will not change it’s velocity.
Something must cause a change in velocity.
 A force is something that causes an acceleration or
change in velocity
 Change in speed
 Change in direction.
Objects and Systems
 An object is something
that has no internal
structure, or that we can
treat as having no
internal structure. Ex:
Electron
 A system is an object or
collection of objects
grouped together for
study. Ex. Atoms
External and Internal Forces
 An object cannot exert a
force on itself.
 Internal forces have no
effect on the motion of a
system as a whole.
 Only external forces are
considered in Newton’s
Laws.
Observation #3
 An object will not change its velocity unless a net
external force acts on it.
Newton’s First Law
 Objects do not change their motion without a
cause.
 Forces are what cause changes in motion.
 It is the net external force acting on an object
that determines whether it will change
motion.
Newton’s First Law
 An object at rest remains at rest, and an object in
motion continues in motion with constant velocity,
unless the object experiences a net external force.
 A net external force is required to change velocity.
Inertia
 Another way to say the First Law is to say that objects
have inertia.
 Inertia is the tendency of objects to resist changes in
motion.
 The amount of inertia an object has is determined by
its mass.
What is happening?
 This is the ball in the train car again. The ball appears
to start moving for no reason.
 Does this violate Newton’s First Law?
Inertial Reference Frame
 An inertial reference frame is one in which Newton’s
First Law holds.
 Accelerating reference frames are not inertial.
Force
 SI unit of force is the Newton (N).
 1 N = 0.225 lb
 1 lb. = 4.448 N
 A force is a vector.
 It has a magnitude, measure in N or lbs.
 It acts in a particular direction.
Forces
 A force is the interaction of two objects.
 There are four fundamental interactions, in other words
four fundamental forces.
P
1 kg
P
Nitrogen
e-
Carbon-14
P
P
4 kg
2 fm
Four fundamental forces
 Gravitational forces
 Strong nuclear forces
 Weak nuclear forces
 Electromagnetic forces
Types of forces
 Contact forces- result from physical contact between
two objects.
 Field forces- force that can exist between two objects
even in the absence of physical contact.
 E.g. gravity, electric forces
 Objects may be in contact, they just don’t have to be.
Common Forces
 The force of gravity (Fg)
pulls straight down.
 The force of friction (Ff )
occurs between two
objects that can slide
against each other.
 It opposes the relative
motion of the surfaces.
Common Forces
 The normal force (FN) is
the support force from a
surface.
 It is called “normal” because
it is always perpendicular to
the surface.
 The tension (FT) is the
force in a rope or string.
 The tension is the same in
every part of a rope.
Newton’s Second Law
 The acceleration experienced
by an object is directly
proportional to and in the
same direction as the net
force that acts on it, and
inversely proportional to the
mass of the object.

FNET  ma
Free-body diagram
 Free-body diagrams consider just one object and
the forces that act on it.
 To draw a free body diagram
 Draw a dot to represent the object.
 Draw and label vector arrows representing all the
forces acting on the object.

All the vectors should be shown as acting at a single point.
Newton’s Second Law in
Component Form
 Force and acceleration are vectors, which can be
broken into components.
 Newton’s Second Law can be applied in each
component direction separately.
FNET  ma
F
x
 max
F
y
 may
Practice problem 1
 Space shuttle astronauts experience accelerations of
about 35 m/s2 during takeoff. What magnitude of
force does a 75 kg astronaut experience during an
acceleration of this magnitude? (answer in the correct significant
figures)
F
NET
 ma
FNET  75 35
 Answer: 2600 N
Practice problem 2
 A 7.5 kg bowling ball initially at rest is dropped from the
top of an 11 m building. It hits the ground 1.5 s later. Find
the net force on the bowling ball while it is in the air,
including direction and magnitude. Down is negative.
FNET  ma
FNET  7.5  (9.81)
 Answer: -74 N
Practice Problems 3
 A 2.0 kg otter starts from rest at the top of a muddy
incline 85 cm long and slides to the bottom in 0.50 s.
1 acts2 on the otter?
What
net
external
force
X  X  v t  at
o
o
1 2
X  at
2
2X
a 2
t
2  0.85
a
0 .5 2
2
a  6.8m / s
2
FNET  ma
FNET  2.0  6.8
FNET  13.6 N
Weight/Mass Relationship
 Weight is the magnitude of the force of the Earth’s
gravity on an object.
 The force of gravity is shown in diagrams as FE-O or
Fg.
 Mass is a measure of the amount of matter in an
object.
 Weight and mass are proportional.
 The constant of proportionality near the surface of
the Earth is g = 9.81 N/kg.
 The weight of an object is often written as mg.
Gravitational Mass vs. Inertial Mass
 The property of an object that determines how
much weight it has at a certain place is called its
gravitational mass.
 The property of an object that determines its resistance
to changes in motion is called its inertial mass.
 Experiments have confirmed that these two
properties are the same.
Inclined Planes
 A box slides down two smooth rails with no friction.
Find the acceleration of the box.
q
FN
θ
Fg = mg
Section 3
Which has more force?
 When the boxer hits the bag, which has more force,
the boxer on the bag or the bag on the boxer?
Newton’s Third Law
 If Object A exerts a force on Object B, then B exerts a
force on Object A that is equal in magnitude but
opposite in direction.
©2012 OpenSTAX College
Newton’s Third Law
 The two forces are called an action-reaction pair.
 The two forces do not balance each other, since they
act on different objects.
©2012 OpenSTAX College
Action-Reaction Pairs
 Identify all the action-reaction pairs involved in a ball
sitting on a table.
Action-Reaction Pairs
 Identify all the action-reaction pairs involved in a ball
sitting on a table.
Acknowledgements
© 2013 Mark Lesmeister/Pearland ISD
 This work is licensed under the Creative Commons Attribution-
ShareAlike 3.0 Unported License. To view a copy of this license,
visit http://creativecommons.org/licenses/by-sa/3.0/ or send a
letter to Creative Commons, 444 Castro Street, Suite 900,
Mountain View, California, 94041, USA.
 Selected graphics and problems from OpenStax College. (2012,
June 12). College Physics. Retrieved from the Connexions Web
site: http://cnx.org/content/col11406/1.7/
 Selected questions from LearnAPPhysics.com, © 2009 Richard
White
 Select problems from Serway and Faughn, Holt Physics, © 2002
Holt Rinehart Winston
Constant Force Model vs.
Equilibrium Model
Equilibrium
Constant Force
 ∑F = 0.
 ∑ F = constant.
 Object will be at rest or move
 Object will accelerate in the
with constant velocity.
 Position vs. time graph-
direction of the net force.
 Position vs. time graph-
x
or
x
t
x
t
t
Problem Solving Tips
 Often the problem will be equilibrium in one direction
and constant acceleration in another.
 You may have to use the constant acceleration model
either before or after finding the acceleration.
Solving Equilibrium Problems
 Givens and Unknowns:
 Sketch and label a diagram of the object and its surroundings.
 Enclose your object in a boundary to help identify outside
forces.
 Draw a free body diagram. If there is motion, choose one axis
in the direction of motion.
 Identify all forces that act on the object, and draw them on
the diagram.
 Model: Equilibrium
 Method
 Apply Newton’s 1st Law in component form.
 Fnet = 0 so ΣFx = 0 and ΣFy=0
Solving Constant Force
(Acceleration) Problems
 Givens and Unknowns:
 Sketch and label a diagram of the object and its surroundings.
 Enclose your object in a boundary to help identify outside
forces.
 Draw a free body diagram. If there is motion, choose one axis
in the direction of motion.
 Identify all forces that act on the object, and draw them on
the diagram.
 Model: Constant Force
 Method
 Apply Newton’s 1st and 2nd Laws in component form.
 Fnet = ma so ΣFx = max and ΣFy= may
Example 1: Equilibrium
 Find the tension in each rope, as a function of the
angle.
30o
5.0 kg
Solution to Example 1
 Givens and Unknowns-
as shown in diagrams to
the right.
 Model:
F1 =?
 Equilibrium
 Method:

𝐹𝑥 = 0 and
Fg=mg
𝐹𝑦 = 0
F2y =F2 sinq
F1y =F1 sinq
F1x =F1 cos q F2x =F2 cos q
Solution to Example 1, Continued
 Implement
F1 =?
 From the horizontal
equation F cos q  F cos q  0
2
1
F1  F2
Fg=mg
 From the vertical
equation
F2y =F2 sinq
F1 sin q  F2 sin q  mg  0
2 F2 sin q  mg
mg
F2 
2 sin q
F2x =F2 cos q
F1y =F1 sinq
F1x =F1 cos q
Example 2: Constant Force
 A truck is pulling a trailer with a force of 2200 N.
Resistive forces opposed to the motion of the trailer
total 1200 N. The trailer has a mass of 500 kg. How
long will the trailer take to reach 20 m/s?
Constant
Force
Example
Givens and Unknown- as in diagram

to the right, and
 m = 500 kg
 v0 = 0 and v = 20 m/s
 t=?
FN
Ffriction-Trailer
= 1200 N
FTruck-Trailer
= 2200 N
 Model:

Equilibrium in y, constant force (so
constant acceleration) in x.
 Method:
F
x
 ma
v  v0  at
Vertical direction not needed, since no accelerati on.
Fg=mg
Example 2 Solution
 Implement
2200 N - 1200 N  ma
1000 N
 a  2.0 m/s 2
500 kg
v  0  at
v
20 m/s
t 
 10 s
2
a
2.0 m/s
FN
Ffriction-Trailer
= 1200 N
FTruck-Trailer
= 2200 N
Fg=mg
Additional Practice, Newton’s
Laws
 From OpenSTAX College Physics:
 Normal, Tension and Other Forces, Exercises 2, 4 and
6.
 Problem Solving Strategies, Exercises 2,4, 10,