Transcript Forces and Newton`s Laws

Forces and Newton’s Laws
Forces
• Forces are ________ (magnitude and direction)
• Contact forces result from ________ ________
• Field forces act ___ __ __________
Fig. 4.2, p. 84
4 Fundamental Forces
•
•
•
•
Strong nuclear force
Electromagnetic force
Weak nuclear force
Gravitational force
• All are ______ forces
• Nuclear are _______
______ (femtometers)
• Electromagnetic and
gravitational range is
_________
Newton’s First Law
• An object moves with a constant velocity,
unless acted on by a ________ net force.
– Velocity is a vector: magnitude and direction
• Net force: _______ _____ of all forces
exerted on object
Inertia vs. Mass
• Inertia: tendency of object to continue its
_______ in the __________ of forces
• Mass: measure of object’s __________ to
________ in _______ due to force
Newton’s Second Law
• Acceleration of an object is
– directly __________ to net force acting on it
– _________ proportional to object’s mass

 F
a
m


F  ma
4.1
This is a vector equation
Fx  max
Fy  ma y
Fz  maz
4.2
Unit of Force, Newton (N)
1 N  1 kg  m s
2
1 N  0.225 lb
US unit of mass
2
1 kg  6.85 10 slug
The Gravitational Force
• Newton’s Universal Law of gravitation:
• Every particle in the Universe attracts every other
particle with a force that is:
– Directly proportional to the product of the particles’
___________
– Inversely proportional to the _______ of the __________
between them
m1m2
Fg  G 2
r
Fig. 4.6, p. 90
4.5
Weight
• Magnitude of Fg acting on object near Earth’s
surface
w  mg
4.6
• g is acceleration of gravity (of Earth)
ME
g G 2
r
4.8
• r is radius of Earth (RE) plus height (y) above
surface of Earth
r  RE  y
Newton’s Third Law
• Forces in nature always exist ___ _______
• A _______ __________ force can’t exist
• The force F12 exerted by object 1 on object 2 is
equal in _________ but opposite in _________ to
the force F21 exerted by object 2 on object 1


F12  F21
Fig. 4.7, p. 92
Force Pairs & the Normal Force
• TV resting on table
attracted by Earth (Fg)
• Pair: ______ attracted
to ______ (Fg’)
• But a = 0, so another
force is present. n is
_________ force of
table on TV
• Pair: normal force of
_____ on _____ (n’)
Fig. 4.8, p. 93


n  Fg
n is ______________ to
contact surface and is
_____________ in origin