Download Chapter 3 - Department Of Computer Science

Phys141 Principles of Physical Science
Chapter 3
Force and Motion
Instructor: Li Ma
Office: NBC 126
Phone: (713) 313-7028
Email: [email protected]
Webpage: http://itscience.tsu.edu/ma
Department of Computer Science & Physics
Texas Southern University, Houston
Sept. 15, 2004
Topics To Be Discussed
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Force and Net Force
Newton’s First Law of Motion
Newton’s Second Law of Motion
Newton’s Third Law of Motion
Newton’s Law of Gravitation
Momentum
Cause of Motion
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A push causes something to move
This push is the application of a force
Force and Motion: Cause and Effect
Galileo did experiments on moving objects
Newton formulated the laws of motion and
explained the phenomena of moving objects
on the Earth and the motions of planets
Force
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Easy to describe force
 Define force in terms of what it does:
– A force can produce changes in motion
– A force can produce a change in velocity (speed
and/or direction), or cause a acceleration
– Observed motion is evidence of a force
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A force is a quantity that is capable of
producing motion or a change in motion
Net Force
A force’s capability may be balanced or
canceled by other force(s): the net effect is
then zero
 More than one force acts on an object:
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– unbalanced/net force: tug of war
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Forces are vector quantities
 Only net force can cause change in motion
Newton’s First Law of Motion
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The natural state of motion:
– Aristotle: the natural state of an object is being at
rest – no idea of friction
– Galileo: objects could naturally remain in motion
rather than come to rest
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Newton’s first law of motion
– An object will remain at rest or in uniform
motion in a straight line unless acted on by an
external, unbalanced force
Newton’s First Law of Motion
(cont)
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External force: an applied force
 Internal force: can not change the state of
motion
 Friction and Gravity on the Earth make it
difficult to observe an object in a state of
constant velocity
Motion and Inertia
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Inertia: natural tendency of an object to
remain in a state of rest or in uniform motion
in a straight line - Galileo
 Mass is a measure of inertia – Newton
– The greater the mass of an object, the greater is
its inertia, the greater is its resistance to a change
in motion
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Newton’s first law: Law of inertia
Newton’s Assumptions of
Acceleration
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The acceleration produced by an unbalance
force acting on an object (or mass) is directly
proportional to the magnitude of the force (a
∞ F) and in the direction of the force
 The acceleration of an object being acted on
by an unbalance force is inversely
proportional to the mass of the object (a ∞
1/m)
Newton’s Assumptions of
Acceleration (cont)
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Combining these effects of force and mass on
acceleration:
unbalanced force
acceleration ∞
mass
or
a ∞ (F / m)
Newton’s Second Law of
Motion
a=F/m
or
F = m·a
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F is the net force
 m is the total mass
 Unit of the force is newton in metric system:
1 N = 1kg·m/s2
Example
F1 = -5.0N
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a
m1
m2
1.0 kg
1.0 kg
F2 = +8.0N
Given:
– m1=1.0kg, F1=-5.0N (left, negative direction);
– m2=1.0kg, F2=+8.0N (right, positive direction);
Wanted: a (acceleration)
 Equation: a = (F1+F2)/(m1+m2)
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a = (+8.0N-5.0N)/(1.0kg+1.0kg) = +1.5m/s2
Mass and Weight
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Mass is the amount of matter an object
contains, or a measure of inertia
 Weight is related to the force of gravity
(gravitational force acting on an object)
 They are related:
weight = mass x acceleration due to gravity
w = m·g
Newton’s Third Law of Motion
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The law of action and reaction
 For every action there is an equal and
opposite reaction
 Whenever one object exerts a force on a
second object, the second object exerts an
equal (in magnitude) and opposite (in
direction) force on the first object
action = opposite reaction
F1 = - F2
Comparing Newton’s Second &
Third laws
Newton’s third law relates two equal and
opposite forces acting on two different objects
 Newton’s second law concerns how forces
acting on a single object can cause an
acceleration
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Newton’s Law of Universal
Gravitation
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Gravity: a common fundamental force in
nature
 Every particle in the universe attracts every
other particle with a force that is directly
proportional to the product of their masses
and inversely proportional to the square of
the distance between them
F ∞ (m1m2 / r2)
Newton’s Law of Gravitation
(cont)
F = (G m1m2 / r2)
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G is the universal gravitational constant
G = 6.67 x 10-11 N·m2/kg2
Why objects fall to the ground of the Earth,
Earth doesn’t move?
Why we can’t feel attraction from book?
Astronauts in space shuttle orbiting the Earth
are weightless?
Linear Momentum
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Product of mass and velocity
 Linear momentum is a vector, in direction of
velocity
 If there is no external net force, linear
momentum is conserved
linear momentum = mass x velocity
p = m·v
Linear Momentum (cont)
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Law of conservation of linear momentum:
– The total linear momentum of an isolated system
remains the same if there is no external
unbalanced force acting on the system
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Example of the conservation of linear
momentum: man jump out from the boat
Angular Momentum
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Angular momentum arises when objects go in
the paths around a fixed point
 The angular momentum of a system can be
changed by an external unbalanced torque
L = m·v·r
r = distance of object from center of motion
Angular Momentum (cont)
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A torque is a twisting effect caused by one or
more forces
 A torque tends to produce a rotational motion
F
v
r
T = F·r
Angular Momentum (cont)
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Law of conservation of angular momentum:
– The angular momentum of an object remains
constant if there is no external unbalanced torque
acting on it
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Example of the conservation of angular
momentum: ice skaters spin on the ice