Download Newton`s laws, part 1(Sep 20)

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What is a force?!
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Forces are important since they are the starting point of
Newtonian mechanics. The idea is to analyze each body
in a problem in isolation to determine the forces that act
on it. If we know the initial position and velocity of the
body, we then solve Newton’s equations of motion, to find
out where the body will be for future times.!
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Forces!
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Fundamental Forces
Gravity
Electromagnetic
Strong
Weak
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Newtonian Dynamics!
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(known as Galileo’s Principle, Law of Inertia, definition of an inertial
reference frame)
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A point-like body on which the net force is zero is said to be
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Often considered to be contained as a special case of
Newton’s second law.
There are no perfect demonstrations of the law, as friction
usually causes a force to act on a moving body
Three examples of noninertial reference frames
(train)!
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Inertial Mass!
A theory developed by Peter Higgs of Edinburgh
University suggests that there is a field (the
Higgs field) throughout all space that causes
fundamental particles to resist motion!
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!!
If this is the case it will show that all inertial mass is
really just bound up energy!
* Inertia is the tendency of an
object (pumpkins) to resist
changes in its velocity: whether !
in motion or, in this case,
motionless!
* If in motion (say a space walker), inertia
is the tendency that he/she “drifts” away
from space station in a linear trajectory
(ignoring the weak gravitational pull he
feels).!
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2nd Law and Mass!
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(SI= kg " m/s2 = N(ewton))
Just like we did for kinematics we can treat the x,y
(and z) components independently for Newton's second
law!
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Fnet"x = ma x
Fnet"y = ma y
Fnet"z = ma z
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Newton II contains Newton I: If the acceleration is 0 (because
the net force is 0), then the velocity is constant.!
ma
IS NOT A SPECIAL TYPE OF FORCE:
Fnet it is immaterial!
Until we get
Newton II refers to EXTERNAL forces – it’s impossible for
a body to affect its own motion in isolation !
Newton II as stated, works only where m=constant AND
generally, works only in inertial reference frames.!
!
Using Newton’s 2nd Law to Solve Problems!
1.!
Identify all forces acting on the object!
-Pushes or Pulls
-Frictional forces
-Tension in a string
-Gravitational Force (or weight = mg where g is 9.8 m/s2)
- “Normal forces” (one object touching another).!
2.!
Draw a “Freebody Diagram”
-draw the object, show all forces acting on that object as vectors pointing in
the correct direction. Show the direction of the acceleration. !
3.!
Chose a coordinate system.!
4.!
Translate the freebody diagram into an algebraic expression based on Newton’s
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second law.!
Using Newton’s 2nd Law to Solve
Problems!
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1.! Identify Forces:
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2.! Draw freebody diagram !0"
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Chose coordinate system: Let up be the +y
direction and down –y. Then :!
4.! Translate the FBD into an algebraic expression.
T-W = m(-a)
so
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T-(100 kg)(9.8 m/s2) = (100 kg)(-2 m/s2)!
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A firework rocket of mass m is launched at an angle of
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is the acceleration horizontally and vertically?!
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To every action there is an
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The normal (or reaction) force is the result of Newton's
third law of motion!
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This is why the book does not fall to the ground!!
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If they did not the book would have a net force and therefore
accelerate!!
*** While I do not condone violence, this !
situation causes confusion: Does the boxer
who threw the punch feel the same amount
of force from the face of the other boxer?
(yes, but face hurts more than knuckles)"
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