Transcript Slide 1

nd
2
Newton’s _____ Law:
If a net force Fnet acts on an object of mass m,
the object’s acceleration a will be:
a=
Fnet
cause accelerations
Net forces ___________________
m
resists acceleration
mass ________________________
a
a
Fnet
Compare: a = Δv/t
m
is a _________________ of a
a = Fnet/m tells the______________ of a
nd
2
Newton’s _____ Law:
If a net force Fnet acts on an object of mass m,
the object’s acceleration a will be:
a=
a
Fnet
cause accelerations
Net forces ___________________
m
resists acceleration
mass ________________________
a
direct
inverse
Fnet
m
definition
is a _________________
of a
cause
a = Fnet/m tells the______________
of a
Compare: a = Δv/t
1/ Units:
a =
Fnet
units:
Fnet
m
= ma
[N] = [kg] [m/s2]
[N] = [kg m/s2]
derived fundamental
net force that produces an acceleration
1 N = the _____________
1 m/s2 when applied to a mass of _________
1 kg.
of __________
1 kg
1N
frictionless surface
2/ a is a vector: a = 5.0 m/s2, east
magn.
dir.
a/ The magnitude of a is also called
the________________________.
acceleration
Ex. Names of vectors and their magnitudes.
vector
magnitude
displacement
distance
velocity
speed
acceleration
acceleration
the same as
b/ The direction of a is ________________
the
direction of Fnet.
Fnet.
Ex: In free fall, the weight w equals ________
What is the acceleration at
all five points shown?
g = ___________
9.81 m/s2
magn.: a = __
down because it is
dir.: __________
w
the same direction as ______.
w
w
w
≠ 0
The acceleration is ____
w
w
at the top because Fnet ____
≠ 0
there (and everywhere else).
3/ Compare:
Newton’s 1st Law:
Newton’s 2nd Law:
FN = 20N
FN = 20N
Ff =
10 N
pull
= 10N
w = 20 N
Ff = 2 N
pull
= 10N
w = 20 N
0
8N, right
Fnet = __________
Fnet = __________
constant v
a = ___
0  ___________
not zero
a = __________
0
or v = ______
unbalanced
F's
are
_______________
balanced
F's are _____________
non-equilibrium
____________________
equilibrium
__________________
special case
The First Law is really just a _____________________
of
Fnet = 0
the Second Law that is true when ____________________
.
Ex: A force of 16 N pushes horizontally on a
2.0 kg mass on a frictionless surface. Draw all the
forces acting on the mass. Then find Fnet and a.
FN = 20 N
push
= 16N
2.0
kg
Fnet =
16 N, right
______________
+ 16 N
or ___________
a = Fnet
m
=
2)
(2.0
kg)(9.81
m/s
w = mg = _________________
20 N
≈ _____________
20 kgm/s2 = _____
+16 N
2.0 kg
=
+8 N
kg
=
+8m
s2
(to right)
Ex: Add a friction force of 4.0 N to the previous
example. Again, draw all forces acting on the
mass, find Fnet and find a.
FN = 20 N
2.0
kg
Ff =
4.0 N
w = 20 N
Now, Fnet =
+ 12 N
____________
a = Fnet
m
pull
= 16 N
= +12 N
2.0 kg
=+6N
kg
=+6m
s2
less
less .
a is _________
now b/c Fnet is___________
Which way is the mass in the previous problem moving?
Can’t tell.
the
___________.
Why? Because Fnet is to the right, a is to
_____
right
Dv
_______.
But a only tells the direction of _______,
not v____ .
Case 1:
positive v
vi
Fnet
vf
2.0
kg
2.0
kg
vf ___
> vi so Dv = vf - vi ___
> 0
> 0 and so a = Dv/t __
vi
vf
Case 2:
Fnet 2.0
negative v
2.0
kg
kg
vf ___
> vi so Dv = vf - vi ___
> 0
> 0 and so a = Dv/t __
pos.
pos.
In both cases, Fnet is ________,
so Dv and a are ________
but, in Case 2, v is ________________
.
negative
Ex: A 2.0 kg box slides to the right over a surface
with a constant frictional force of 2 N. Draw all
forces acting on the mass, find Fnet and a.
v
F = 20 N
N
2.0
kg
Ff
= 2N
2 N, left
Fnet = __________
-2 N
= ________
a = Fnet
m
=
-2 N
2.0 kg
w = mg
= -1 N
kg
= 20 N
= -1 m
s2
slowing down
The box is _________________
, because Fnet
opposite directions
and v are in ______________________________
.
Ex: Why do all objects fall with same acceleration
in free fall (no air resistance)?
small mass
m
w
a=
w
m
=
big mass:
Newton's
2nd Law :
Fnet
a=
m
small force
small inertia
= 9.81 m/s2
a=
m
w
w = big force
big inertia
m = 9.81 m/s
2
Which object gets pulled down with a greater force?
big
Which object resists more? big
Which accelerates most?
neither
Ex. A block is pulled along a frictionless table by a
force that makes an angle q to the horizontal.
F
F
FN
q
2.0
kg
w = 20 N
components
a/ Find the x- and y-________________
of F.
b/ Replace F in the free body diagram with them:
F
Fy =
Fsinq
q
Fx = Fcosq
FN
Fy
balanced
w
Fx
unbalanced
y-direction: Because the block is not accelerating
up or down, the y-forces are ________________.
balanced
equilibrium:
w = FN + Fy
Notice: w _____
≠ FN
unbalanced
x-direction: Only 1 force _________________
ax =
Fx/m = Fcosq/m
What will happen to a
if q is increased?
F
Fy
q
Fx
F
Fy
inc q
Fx
Fx ___creases,
de
also
so a ________
decreases
_____________.
Open your 3-ring binder to the
Worksheet Table of Contents.
Record the title of the worksheet:
Newton’s 2nd Law WS