Transcript Lesson 8 - WordPress.com

Physics
Newton’s Laws
Newton’s First Law:
The Law of Inertia. A body at rest and a body in
motion continues to move at constant velocity
unless acted upon by an unbalanced external
force.
Newton’s Laws
Newton’s Second Law:
The Law of Acceleration. A net or unbalanced
force acting on an object equals the mass of
the object times the acceleration of that
object.
Fnet  ma
Fnet  force
m  mass
a  acceleration
Newton’s Laws
For Centripetal acceleration:
2
Fnet
v
m
R
1newton  1kg  m / sec
2
1 pound  1slug  ft / sec
2
Newton’s Laws
Sample Problem:
Find the acceleration of a 3 slug object acted
upon by a net force of 1.5 lbs.
Fnet  ma
F
a
m
1.5lbs

3slugs
1.5slug  ft / sec 2

3slugs
 0.5 ft / sec 2
Newton’s Laws
Sample Problem:
A 50-kg block rests at the top of the plane
whose length is 2.00 m and whose height is
0.50 m. How long will it take for the block to
slide to the bottom of the plane when
released?
Newton’s Laws
Solution:
Draw the vector diagram first.
Newton’s Laws
Next establish the relationship:
F
sin  
W
F  W sin 
0.50m
sin  
2.00m
 0.25
Newton’s Laws
m  50kg
W  mg
F  W sin 
F  mg sin 
 (50kg)(9.81m / sec )(0.25)
2
 1.2 10 2 N
Newton’s Laws
F
a
m
1.2 10 2 kg  m / sec 2

50kg
 2.4m / sec 2
1 2
s  vi t  at
2
1
s  0  at 2
2
2s
2(2.00m)
t

a
2.4m / sec 2
 1.3 sec
Newton’s Laws
Newton’s Third Law:
The Law of Interaction. No matter what the
circumstance, when one object exerts a force,
the second must exert an exactly equal and
oppositely directed force on the first.
Newton’s Laws
For propeller problems, it is given by the
formula:
T M
v f  vi
t
where:
T  the force the air mass exerts back in the propeller
M  mass of the air
v f  vi  change in velocity
t  time ellapsed
Newton’s Laws
Sample Problem:
Each second a propeller accelerates an air mass
of 12.2 slugs from rest to a velocity of 137
ft/sec. How much thrust is provided?
Newton’s Laws
Solution:
T M
v f  vi
t
 (12.2slugs )(
 1670lbs
137 ft / sec  0
)
1sec
Exercises
1. Find the net force on a 5 slug object which is
accelerating at 3 ft/sec?
2. At an altitude of 10,665 m and a cruising speed of
252m/sec, the Pratt & Whitney JT 9D-7F in the
Boeing 747 has a mass flow of 879 kg/sec and an
exhaust velocity of 306 m/sec. Find the thrust.
3. A 60 lb block rests on a smooth plane inclined at an
angle of 20˚ with the horizontal. The block is pulled
up the plane with a force of 30 lb parallel to the
plane. What is its acceleration?