Download Chapter 8 concepts

Chapter 7 concepts
 Newton’s 3rd Law
 Action-reaction forces
 Equal forces, not equal effects
 Internal forces
 Ideal strings and pulleys
Physics 151
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Newton’s Third Law
Newton’s Third Law
Forces always come
in pairs, called
action-reaction pairs.
When two objects
interact, they exert equal
and opposite forces on
each other:
“Action = Reaction”
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Third law examples
An action-reaction pair of forces always act on different
objects. They should never both show up in a FBD.
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Problem: A speeding Mack
truck crashes head-on into a
parked Ford Escort.
Compare the sizes of the
forces that the truck and
Escort exert on each other:
PRS Mack truck
1) The truck and the Escort exert equal forces on each other.
2) The force exerted by the truck is larger than the force
exerted by the Escort.
3) The force exerted by the truck is smaller than the force
exerted by the Escort.
4) Insufficient information.
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Problem: A speeding Mack
truck crashes head-on into a
parked Ford Escort.
Compare the sizes of the
forces that the truck and
Escort exert on each other:
Mack truck soln
1) The truck and the Escort exert equal forces on each other.
2) The force exerted by the truck is larger than the force
exerted by the Escort.
3) The force exerted by the truck is smaller than the force
exerted by the Escort.
4) Insufficient information.
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Lessons from Kindergarten…
Physics 151
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What if all the people on earth
gathered together on one continent
and all jumped up at the same time?
Moving the earth
Yes, the Earth would recoil.
… about 310–15 m (3 femtometers)
… roughly the size of an atomic nucleus
Equal forces… but
different masses cause
Different accelerations (F = ma)
Different velocities (v = aDt)
Different heights (Dy = v2/2g)
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Internal Forces
Newton’s 3rd law
explains why we never
need to worry about
internal forces when
calculating net forces.
1st law: In the absence of a
net external force, an object
at rest remains at rest, or an
object in motion remains in
motion with constant
velocity.
T
F red on green = F green on red
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Hand and blocks
Rank the horizontal
forces in descending
order of strength.
F(A on B)
2
F(B on A)
F(H on B)
2
1
F(B on H)
1
F (B on A) = MA a
F (H on B) = (MA+MB) a
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Name Train
If forces on a given letter balance, why does it move?
T
Hint: Which forces are action-reaction pairs?
The forces really look something like this:
Physics 151
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Pulley & hanger
Which rope has the higher tension?
T 1
T
If the masses are not
accelerating, then the tensions
are the same.
2
3
mg
T
mg
mg
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The massless string
If a string is not accelerating,
the tension at each end is the same.
T1
a
0
0
T2 – T1 = mstring a
T2
If a string is massless, it has the same tension at each end
even when it is accelerating.
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Thrust
A rocket engine
pushes gas out the
back; the gas pushes
the rocket forward.
Third law rocket
Newton’s Third Law
When two objects interact, they exert equal and
opposite forces on each other:
“Action = Reaction”
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ATWOOD’S MACHINE
Determine a and T in a
lightweight string
IF the string does not stretch the
acceleration of the two masses
Example Atwood
machine
will be equal in magnitude.
IF friction and the masses
of the string and the pulley
are all negligible, the
magnitude of the tension
force in the string is the
same on both sides.
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ATWOOD’S MACHINE
Atwood machine II
Mass m1
å F- = T - m1g = m1a
Mass m2
å F ¯ = m2 g - T = m2 a
Add both sides:
m2 g - m1g = m1a + m2 a
a
a
æ m2 - m1 ö
Þa=ç
÷ ×g
è m1 + m2 ø
æ 2m1 × m2 ö
T=ç
÷ ×g
è m1 + m2 ø
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a3
a2
are the
Example CoupledWhat
crates
accelerations and
the tensions?
frictionless surface
0.5 kg
a1
How do accelerations
compare?
How do tensions
compare?
a1 = a 2 = a 3
T1 = T2 > T3
Force T2 accelerates 5 kg, T3 accelerates only 3 kg.
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a
a
Coupled crates II
acceleration first…
0.5 kg
a
Physics 151
Treat all the masses
together as one
object.
17
a
a
Coupled
and now the
crates
IIIT…
tension
T
0.5 kg
a
m 1g
T = m1g - m1a
= m1 (g - a)
= 0.5 kg (9.8 - 0.9) m/s
2
SF = m1 g – T = m1 a
= 4.5 N
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a
a
Coupled crates …now
IV for T3
n
0.5 kg
a
T3 = m3 a
3 kg
T3
m3 g
= (3 kg) (0.9 m/s 2 )
= 2.7 N
SF = T3 = m3 a
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Given the masses and assuming
Masses&pulleys
ideal pulleys and strings, what is
the acceleration of the masses?
This is a 1-dimensional problem!
Think of all the masses as one
extended object.
F = ma
-m1g+ m2 g - m3g+ m4 g+ m5 g =
(m1 + m2 + m3 + m4 + m5 )a
Physics 151
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Window
washer
and M
, what
Given Mwasher
platform
is the minimum force a window
washer must supply to hoist
himself up?
T
T
Mtotalg
System is
washer plus
platform.
T(rope on hand)
T = Mtotalg/2
T(hand on rope )
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Tug of war
What’s the tension?
500 N
500 N
500 N
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End
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Physics 151
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Red wagon
Newton’s Third Law
During the time that the rocket is pushing the
gas out the rear, the gas is pushing back on
the rocket, giving it thrust.
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