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This is basic course for Physics students. This course introduces every concept we study in detail latter on. Key points in this lecture are: Force and Motion, Force of Gravity, Newton’S 2Nd Law, Newton’S 3Rd Law, Forces, Human Body, Fluid Pressure, Horizontal Tabletop, Change of Momentum, Inertia
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st
nd
nd
rd
What forces govern the motion of the sailboard?
Ans:• Human body: contact force• Wind & water: fluid pressure• Gravity: action-at-a-distance
Net force determines motion.Newton’s 1
st
law of motion:
A body at rest or in uniform motion remains sounless acted on by a nonzero net force.
net
v
net
v
const
On a horizontal tabletop is a curved barrier that exerts a force on a ball,guiding its motion in a circular path.After the ball leaves the barrier, which of the dashed paths shown does it follow?
Loaded truck hasgreater mass,more inertia,less acceleration.
Inertia: resistance to changes in motion.^1
st
law = law of inertia
known
known
f^
m
a
unknown
unknown
m
a
unknown
known
known
unknown
m
a
m
a
Operationaldefinition of mass
1 N (Newton)
force required to give a mass of 1 Kg anacceleration of 1 m / s
1 N = 1 Kg m / s
1 Dyne = 1 g cm / s
A 1200 kg car accelerates constantly in a straight line from rest to 20 m/s in 7.8 s.a)What is the net force on it?b)What is the net force on it if it rounds a bend 85 m in radius at v = 20 m/s?
f^
m a
v
m
t
m
s
kg
s
k N
f^
m a
2 v m
r
2
m
s
kg
m
k N
Examples of forces:• Pushes & pulls.• Car collided with truck & stopped.• Moon circles Earth.• Person sitting on chair.• Climber on rope.
(action-at-a distance)
(contact force)
(tension force)
F
c^
=
F
g
T
=
F
g
The fundamental forces:
large scale phenomena
everyday phenomena
1
10
25
10
36
10
38
These astonauts feel “weightless” becausethey are in freefall.Their weight is about 93% of that on surface.
same
a
Tactics 4.1. Free-Body Diagram1.Identify object of interest & forces on it.2.Object
dot.
3.Draw forces on object as vectors at dot.
How’s
T
compared to
w = m g
if the elevator
a)moves upward starting at rest.b)decelerates to stop while moving upward.c)starts moving downward, accelerating from rest.d)slows to stop while moving downward.e)moves upward with constant speed.
greater greaterequal
lessless
When you stand on a scale, the scale reading shows the force it pushes up to support you.If you stand on a scale at Earth’s equator, is the reading greater or less than your weight?
Ans:You’re in uniform circular motion so that the net force on you is centripetal.
2 v
m a
m
scale F
m a
scale
F
scale
W =
m
g
m
a
Newton’s 3
rd
law: Action = Reaction
Forces of 3
rd
law pair act on different objects.
Hence, they don’t cancel each other out.
Horse-Cart dilemma
2 books lie on frictionless horizontal surface.You push with force
F
on books of mass m
. 1
which in turn on book of mass m
. 2
What force does the 2
nd
book exert on the 1
st
?
1
2
m
m
a
Acceleration of books:
12
2 m
F
a
Net force on 2
nd
book: 21
12
2 1
2
m m
m
Force exerted by 2
nd
book on the 1
st
2 1
2
m m
m