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Newton’s _____ Law: If a net force F net acts on an object of mass m ,

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Transcript of Newton’s _____ Law: If a net force F net acts on an object of mass m ,
Newton’s _____ Law:
If a net force Fnet acts on an object of mass m,the object’s acceleration a will be:
a = Net forces ___________________
mass ________________________
Fnet
a
m
a
Compare: a = Δv/t is a _________________ of a
a = Fnet/m tells the______________ of a
cause accelerations
resists acceleration
2nd
Fnet
m
Newton’s _____ Law:
If a net force Fnet acts on an object of mass m,the object’s acceleration a will be:
a = Net forces ___________________
mass ________________________
Fnet
a
m
a
Compare: a = Δv/t is a _________________ of a
a = Fnet/m tells the______________ of a
direct inverse
cause accelerations
resists acceleration
2nd
Fnet
m
definition
cause
1/ Units: a =
Fnet =
units: [N] =
[N] =
derived fundamental
1 N = the _____________ that produces an acceleration
of __________ when applied to a mass of _________
1 kg1 N
frictionless surface
[kg] [m/s2]
[kg m/s2]
1 m/s2 1 kg.
Fnet
mm a
net force
2/ a is a vector: a = 5.0 m/s2, east
magn. dir.
a/ The magnitude of a is also called the________________________.
vector magnitude
displacement distance
velocity speed
acceleration acceleration
acceleration
Ex. Names of vectors and their magnitudes.
Fnet.
w
w
w
w
w
9.81 m/s2
b/ The direction of a is ________________ the direction of Fnet.
Ex: In free fall, the weight w equals ________
What is the acceleration at
all five points shown?
magn.: a = __ = ___________
dir.: __________ because it is
the same direction as ______.The acceleration is ____ 0
at the top because Fnet ____ 0
there (and everywhere else).
g
the same as
downw
≠≠
3/ Compare:
a = __________
F's are ___________________________________
Newton’s 1st Law: Newton’s 2nd Law:
FN = 20N
w = 20 N
pull= 10N
Ff = 10 N
a = ___ ___________ v or v = ______
F's are _______________________________
FN = 20N
w = 20 N
pull= 10NFf = 2 N
Fnet = __________ Fnet = __________0 8N, right
0 not zero
balancedequilibrium
unbalancednonequilibrium
constant0
The First Law is really just a _____________________ of
the Second Law that is true when ____________________ .
special caseFnet = 0
w = mg = _________________
≈ _____________ = _____
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.
2.0kg
FN = 19.6 N
(2.0 kg)(9.81 m/s2)
push= 16N
Fnet =
______________
or ___________
+16 N 2.0 kg
+8 N kg
+ 8 m s2
16 N, right+ 16 N
a = Fnet
m
= = =
19.6 kgm/s2 19.6 N
(to right)
Ex: Add a friction force of 4.0 N to the previousexample. Again, draw all forces acting on the mass, find Fnet and find a.
2.0kg
FN = 19.6 N
w = 19.6 N
pull= 16 N
Now, Fnet = ____________
a = Fnet
m
= +12 N 2.0 kg
= + 6 N kg
= + 6 m s2
Ff =4.0 N
a is _________ now b/c Fnet is___________ .
+ 12 N
less less
Which way is the mass in the previous problem moving? ___________. Why? Because Fnet is to the right, a is _____ _______. But a only tells the direction of _______, not ____ .
2.0kg
Case 1: positive v
vi
2.0kg
Fnet
vf ___ vi so v = vf  vi ___ 0 and so a = v/t __ 0
2.0kg
vi
2.0kg
Fnet
Case 2: negative v
In both cases, Fnet is ________, so v and a are ________
but, in Case 2, v is ________________ .
Can’t tell. to the right v v
vf
vf
>
pos. pos.
negative
> >
vf ___ vi so v = vf  vi ___ 0 and so a = v/t __ 0< < <
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.
2.0kg
FN = 19.6 N
w = mg
Ff
= 2N
Fnet = __________
= ________
a = Fnet
m
= 2 N 2.0 kg
= 1 N kg
= 1 m s2
2 N, left
2 N= 19.6 N
The box is _________________ , because Fnet and v are in ______________________________ .
slowing downopposite directions
v
Ex: Why do all objects fall with same acceleration in free fall (no air resistance)?
small mass
big mass:
a =
Newton's2nd Law :
Fnet
Which object gets pulled down with a greater force?
Which object resists more?
Which accelerates most?
w wmm
m
a = a =mw
= =wm
small force
small inertia
= 9.81 m/s2
big force
big inertia
= 9.81 m/s2
bigbig
neither
Ex. A block is pulled along a frictionless table by a force that makes an angle to the horizontal.
2.0kg
F
w = 19.6 N
FN
a/ Find the x and y________________ of F.b/ Replace F in the free body diagram with them:
Fx =
FFy =
FN
Fy
Fx
w
F
balanced
unbalanced
components
Fcos
Fsin
What will happen to a if is increased?
ydirection: Because the block is not acceleratingup or down, the yforces are ________________.
equilibrium: w =
xdirection: Only 1 force _________________
ax =
Fx
F
Fy
Fx ___creases, so a _____________________.
F
Fy
inc
balanced
Notice: w _____ FN
FN + Fy
≠
unbalanced
Fx/m = Fcos/m
Fx
dealso
decreases