Transcript PHYSICS 51: Introduction

PHYSICS 50: Lecture 4.1
RICHARD CRAIG
Goals for Today
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To visualize force as a vector
To find the net force acting on a body and
apply Newton’s First Law
Mass, acceleration, and their application
to Newton’s Second Law
To calculate weight and compare/contrast
it with mass
Homework #3
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Read Chapter 3: Sections 4,5 and
Chapter 4 Sections: 1,2,3
Exercises and Problems:3.31, 3.38,
3.40, 4.4 4.12. 4.19
Due Thursday, 2/14
What is a Force?
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A push or a pull
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It has both a magnitude and a direction
(vector)
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It has units of N (Newton) = kgm/s2
There are four common types of
forces
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The normal force—When
an object rests or pushes
on a surface, the surface
pushes back.
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Frictional forces—In
addition to the normal
force, surfaces can resist
motion along the surface.
There are four common
types of forces II
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Tension forces—When a
force is exerted through a
rope or cable, the force is
transmitted through that
rope or cable as a
tension.
Weight—Gravity’s pull on
an object. This force can
act from large distances.
Mass, Weight and Gravity
• Your Mass is a function of the mass of the
molecules that form your body
• Your Weight (a Force) is how that mass
interacts with the Earth’s gravitational field
g, and hence weight, is only
constant on earth, at sea level
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On Earth, g depends
on your altitude.
On other planets,
gravity will likely
have an entirely new
value.
How to denote a force
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Use a vector
arrow to
indicate
magnitude
and direction
of the force.
Use the net (overall) force
• Several forces acting on a point have the same
effect as their vector sum acting on the same point.
Decomposing a force into
components
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Fx and Fy are the parallel and perpendicular
components of a force to a sloping surface.
Use F*Cosθ and F*Sinθ operations to find force
components.
Newton’s First Law
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Simply stated—“objects at
rest tend to stay at rest,
objects in motion stay in
motion.”
More properly, “A body acted
on by no net force moves
with constant (or zero)
velocity and zero
acceleration.”
Examples of Newton’s First Law
Newton’s Second Law
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An unbalanced force (or sum of forces) will cause a mass to accelerate.
Fnet= ma
The relationship of F, m, and a
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Because a depends
linearly on m and F,
an acceleration will
be inversely
proportional to the
object’s mass.
Using the Second Law—
Example
No friction
What is the acceleration of the Box?
How far will it go in 10 seconds?
Many have asked “how lethal is a coin
dropped from atop a tall building”?
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Urban legends have said that a penny
dropped from the top of the Empire State
Building can kill.
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Cable TV has allowed those two science
guys who test such “myths” to debunk this
one.