phys141 principles of physical science chapter 3 force and motion instructor: li ma office: nbc 126...
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Phys141 Principles of Physical Science
Chapter 3 Force and Motion
Instructor: Li Ma
Office: NBC 126Phone: (713) 313-7028Email: [email protected]
Webpage: http://itscience.tsu.edu/ma
Department of Computer Science & PhysicsTexas Southern University, Houston
Sept. 15, 2004
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Topics To Be Discussed
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
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Cause of Motion
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
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Force
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
A force is a quantity that is capable of producing motion or a change in motion
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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:– unbalanced/net force: tug of war
Forces are vector quantities Only net force can cause change in motion
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Newton’s First Law of Motion
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 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
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Newton’s First Law of Motion (cont)
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
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Motion and Inertia
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
Newton’s first law: Law of inertia
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Newton’s Assumptions of Acceleration
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)
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Newton’s Assumptions of Acceleration (cont)
Combining these effects of force and mass on acceleration:
unbalanced forceacceleration ∞
mass
or
a ∞ (F / m)
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Newton’s Second Law of Motion
a = F / m
or
F = m·a F is the net force m is the total mass Unit of the force is newton in metric system:
1 N = 1kg·m/s2
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Example
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)
a = (+8.0N-5.0N)/(1.0kg+1.0kg) = +1.5m/s2
m1
1.0 kg
m2
1.0 kg
aF1 = -5.0N F2 = +8.0N
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Mass and Weight
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
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Newton’s Third Law of Motion 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 reactionF1 = - F2
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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
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)
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Newton’s Law of Gravitation (cont)
F = (G m1m2 / r2)
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?
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Linear Momentum
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
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Linear Momentum (cont)
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
Example of the conservation of linear momentum: man jump out from the boat
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Angular Momentum
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·rr = distance of object from center of motion
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Angular Momentum (cont)
A torque is a twisting effect caused by one or more forces
A torque tends to produce a rotational motion
r
F
T = F·r
v
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Angular Momentum (cont)
Law of conservation of angular momentum:– The angular momentum of an object remains
constant if there is no external unbalanced torque acting on it
Example of the conservation of angular momentum: ice skaters spin on the ice