Sin t

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PHYSICS FOR ENGINEERS
Well use the book from P.A. Tipler
... and some more resources from the web
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07 - 2nd week
Dynamics
NEWTONS LAWS
First law. An object at rest stays at rest unless
acted on by an external force.
An object in motion continues to travel with
constant velocity unless acted on by an external
force.
Remark Also called the "law of inertia,"
Newton's first law states that a body at rest
remains at rest and a body in motion continues to
move at a constant velocity unless acted upon by
an external force.
Second law. A net force F acting on a body gives
it an acceleration a which is in the direction of
the force and has magnitude inversely
proportional to the mass m of the body.
Momentum for a single particle of mass m with
velocity v, the momentum is defined as
From Newton's second law, a force F produces a
change in momentum
Which are the SI units of force and momentum?
"SI" stands for "System International" and is the
set of physical units agreed upon by
international convention
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07 - 2nd week
Dynamics
Example. How much horizontal net force is
required to accelerate a 1200 kg car at 2 ms-2
from the rest on a horizontal plane?
To do yourself Example 4-1 (5 min)
To do yourself Example 4-2 (5 min)
To do yourself Example 4-3 (5 min)
For you to read The Force Due to Gravity Weight
(15 min)
Exemple. A box on a lift
The box is inside a lift
Let W0 be the weight of the box
The lift accelerates upwards, let a be its
acceleration
Which is then the apparent weight of the box?
Which is the apparent weight of the box in case
the lift moves up with a constant velocity?
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07 - 2nd week
Dynamics
Also related with Newtons second law impulse.
An impulse is an instantaneous change in momentum
Which S.I. units do we use for impulse?
Can you pose some exemples on impulse?
Also related with Newtons second law friction
forces.
http//www.sc.ehu.es/sbweb/fisica/dinamica/rozamie
nto/general/rozamiento.htm
Third law. Whenever a body exerts a force on
another body, the latter exerts a force of equal
magnitude and opposite direction on the former.
This is known as the law of action and reaction.
A well knowing horse
To do yourself Exemple 4-11 (5 min)
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07 - 2nd week
Dynamics
FRICTION FORCES
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07 - 2nd week
Dynamics
FREE-BODY DIAGRAMS
A diagram that shows schematically all forces
acting on a system, is called free-body diagram.
It is called free-body diagram because the body
is drawn free from (without) its surroundings.
To do yourself Example 4-6 (10 min)
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07 - 2nd week
Dynamics
Find out the acceleration Atwoods Machine
(consider an ideal pulley)
MORE PROBLEMS
What about the tension of the rope?
To do yourself Example 4-9 (10 min)
The block m2 has an inhabitantit has a hollow,
which is the home of a bug.
To do yourself Example 4-10 (10 min)
To do yourself Example 4-12 (10 min)
Let mb be the mass of the bug. What about its
weight when it is at home?
Try it yourself!
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07 - 2nd week
Dynamics
SOME MORE QUESTIONS PROBLEMS
An astronaut on Earth kicks a bowling ball and
hurts his foot. A year later, the same astronaut
kicks the same bowling ball on the moon with the
same force.
Question
(a) more (b) less (c) the same
His foot hurts...
The masses of both the bowling ball and the
astronaut remain the same, so his foot will feel
the same resistance and hurt the same as before.
Question Where (the Earth or the Moon) is easier
for the astronaut to pick up the bowling ball
from the floor?
The weights of the bowling ball and the astronaut
are less on the Moon...
Thus it would be easier for the astronaut to pick
up the bowling ball on the Moon than on the Earth.