ch 4. forces & newton’s laws of motion force = a push or a pull. unit of force = newton net...
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Ch 4. Forces & Newton’s Laws of Motion
force = a push or a pull.
Unit of Force = Newton
net force = vector sum of all forces on object
Individual Forces
Net Force
10 N4 N
6 N
3 N
4 N
5 N 64Individual Net
F
net force =
Newton’s 1st Law:object – stays at rest – or constant speed in a straight line – if no net force.
Inertia = tendency of objectto stay at rest – or constant speed in a straight line – if no net force.
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Mass (m) is the amount of “stuff” in an object
Unit of Mass: kilogram (kg)
Newton’s 2nd Law:Acceleration – proportional to Force – inversely proportional to mass
mF
a
aF
m
22 s
mkg
s
mkg
Unit of Force = Newton
Units
free-body-diagramrepresents object & forces
Net force: 275 N + 395 N – 560 N = +110 Nalong the + x axis
2sm059.0kg 1850
N110
m
Fa
Acceleration: mass of car = 1850kg
3
aFm
xx maF
yy maF Force x component y component
+17 N
+(15 N) cos67
0 N
+(15 N) sin67
+23 N +14 N
P
A
Net force:
forces
2sm 018.0kg 1300
N 23
m
Fa
x
x
2sm 011.0kg 1300
N 14
m
Fa yy
P
A
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Newton’s 3rd Law:Action = ReactionForce from body-A to body-B = Force from body-B to body-A
Magnitude of force is 36 N. Mass of spacecraft is 11,000 kg. Mass of astronaut is 92 kg.
What are the accelerations?
PF
PF
astronaut On the
spacecraft On the
2sm0033.0
kg 11,000
N 36
ss m
Pa
2sm39.0
kg 92
N 36
AA m
Pa
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Newton’s Law of Gravitation
masses m1 and m2 separated by distance r, gravitational force
221
r
mmGF
2211 kgmN10673.6 G
N 104.1
m 1.2
kg 25kg 12kgmN1067.6
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22211
221
r
mmGF
12 kg 25 kg1.2 m
Weight = gravitational force earth attracts object
2r
mMGW E
mgW
2r
MGg E
2
26
242211
2
sm 80.9
m 106.38
kg 1098.5kgmN1067.6
E
E
R
MGg
On earth’s surface
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Normal Force
N 26
0N 15N 11
N
N
F
F
from surface to object
Apparent Weight = normal force on scale by man
mamgFF Ny mamgFN
apparent weight
trueweight
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Frictional Force static friction
MAXss ff NsF
coefficient of static friction.
Nkk Ff coefficient of kinetic friction
After motion begins
kg20sm80.9kg4005.0 2
mgFf kNkk
coefficient of kinetic friction = 0.05 mass = 40kg What is the kinetic frictional force?
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Tension ForceCables and ropes transmit forces through tension.
0xF 0yF
Equilibrium
Reasoning Strategy
• select an object in equilibrium• draw free-body diagram
for object & forces• choose x,y axes • resolve forces into x,y comps• apply equilibrium equations• solve for unknown quantities
035sin35sin 21 TT
035cos35cos 21 FTT
Equilibrium
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Equilibrium Force x component y component
1T
2T
W
0.10sin1T0.80sin2T
0
0.10cos1T0.80cos2T
W
N 3150W
080sin10sin 21 TTFx
080cos10cos 21 WTTFy
21 10sin
80sinTT
080cos10cos10sin
80sin22
WTT
80cos10cos10sin80sin
2
WT
N 5822 T N 1030.3 31 T
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Non-EquilibriumWhen an object is accelerating, it is not in equilibrium.
xx maF yy maF
030sin30sin 21 TTFy
21 TT
x
x
ma
RDTTF
0.30cos0.30cos 21
TTT 21
N 1053.10.30cos2
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DRmaT x
you are given m,a,R,D