Download Lesson 3: Newton`s First Law

270
510
1 cm:10 N
1 cm:4 N
1 cm:8 N
1 cm:3 N
Newton’s 1st Law of Motion:
If the ____________________
force, Fnet, acting
net (resultant)
zero
on an object equals ________,
then that object has
0 .
____________________
constant velocity . Its acceleration a = _____
equilibrium
It is said to be in __________________
.
vectors
Because forces are ______________
and have
direction , the x and y _______________
components of the
____________
separately
forces often must be added up _______________
when there are forces along different _________.
axes
Fnet = 0 really means:
net Fx = 0
________
net Fy = 0
________
Ex: A 20-N box on a table
at rest
Free body diagrams:
FN = 20N
20-N
w = FN
"At rest"  __________
w = 20 N
Ex: A 20-N box at rest on a table being pushed
down with a 5 N force.
FN = 25 N
5-N
20-N
w + push = FN
"At rest"  _______________
5N
w = 20 N
Both of these boxes are in ___________________
.
equilibrium
Ex: Same box sliding to the right on a horizontal
frictionless table at a constant velocity 2.0 m/s
v = 2.0 m/s
FN = 20N
20-N
w = FN
"constant velocity"  __________
w = 20N
velocities
Draw only the __________
forces , not the ____________
Notice:
same
1/ The free body diagram is the _________
as the
equilibrium
first example. Both are in ___________________
.
needed to keep box moving.
2/ No force is ___________
forever
The box will continue its motion ________________.
Ex: The forces acting on
a 20-N box are shown
at right:
Ff =
10 N
FN = 20N
pull
= 10N
w = 20 N
equilibrium
This box is in ___________________.
It is either:
at rest
0
1/ _____________
(constant velocity = _____);
moving
2/ or _____________
at a constant velocity.
Unless other information is given, it is
impossible
_______________ to tell if it is at rest or moving.
Ex. The three forces below are the only ones
acting on an 2.0-kg object. What is the
acceleration of the object?
Add them up:
3.0
N
4.0 N
5.0 N
3.0
N
4.0 N
0 so a must = ____
0 .
The net force Fnet = _____
at rest
This object could be _______________
or
constant velocity
moving with ____________________________
.
Restatement of 1st Law:
at rest or moving with
If an object is ____________
constant velocity
in equilibrium
_____________________,
it is_________________.
=0
Then the net force (Fnet ) acting on it _________
.
Ex: A B-2 bomber flying
at constant velocity:
weight ≈ 1.3 x 106 N
thrust ≈ 75 x 103 N/engine
x 4 engines
lift = 1.3 MN
Ff (drag)
= 300 kN
thrust =
300 kN
w= 1.3 MN
The
“air force”
Ex. The two forces shown below act on an
object. What third force is needed to produce
equilibrium?
F1
F2
First way to solve:
• Find resultant force F
• The answer is -F:
F
-F
balances
-F ___________________
the other two forces.
Second way to solve:
1. Add the two forces using head to tail method.
2. Add a force to "bring the total force to zero."
F1
head to tail:
original
F1
F2
F2
-F
Notice:
the same result
• Both ways give ________________________
resultant
• -F is NOT the ________________
of the other
"balances out"
vectors. It is the force that ________________
the other vectors, making Fnet =________.
It is
0
sometimes called the ________________
because it
equilibrant
is what is needed to produce ___________________
.
equilibrium
Ex. Draw the vector that represents the force
that the beam exerts on point A.
beam
force
building
T
A
beam
weight
A
w
equilibrant
The beam force is the ________________
. It must
Fnet = 0
be added to the other forces so that ___________
Ex. Increasing angle between 2 wires supporting a picture:
Case 1: vertical
wires
1
1
2
5
T = ____
in each
wire
2
10 N
w = 10 N
Case 2: greater
angle
2
1
2
1
10 N
7
T = ____
in each
wire
w = 10 N
Case 3: even
greater
angle
1
10 N
2
1
2
w = 10 N
increases
As q increases, the tension _______________
.
10
T = ____
in each
wire
Ex. What has more inertia,
a truck or a baseball?
truck
more __________
mass
inertia
more___________
inertia
not needed to maintain
A net force is ________________
motion. An object will maintain its motion in
absence
the ________________
of a net force. This idea
Galileo
was discovered by _____________.
An object in
stay in motion
motion tends to ____________________
, and an
stay at rest
object at rest tends to _________________
. The
property of matter
_______________________
that “makes” this
true is called its ___________
inertia .
mass
Inertia is the tendency of an object to
resist changes in motion . More inertia (mass)
_________________________
means it is __________________
for an object to…
more difficult
1/ …_____________________
when it is at rest;
start moving
2/ … __________
when it is already moving; and,
stop
move sideways
3/ … ____________________
from a straight path.
inertia
In other words, ______________
is why an object
no net force
needs __________________
equilibrium
to maintain its motion.
This is why Newton's
Law
st
1 Law
of
_____________
is called
Inertia
Law of Inertia .
the ________________
a
F
net
=0
=0
Open your 3-ring binder to the
Worksheet Table of Contents.
Record the title of the worksheet:
Newton’s First Law WS