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Unformatted text preview: Laws Governing Motion
PS 100 Chapter 2 Physical Science 100
R Skinner Quiz: Which of the following is the smallest?
cell Quiz: Which of the following keeps you from falling Which of the following keeps you from floating “through” your chair?
out of your chair?
c. d. Electromagnetic Force
Weak Force (also a nuclear force with 1/100th the strength of the Strong Force
Newton Using 4 simple ideas, all motion in earth and heaven can be understood. Three laws of motion and one law of gravity
These are the assumptions that the reasoning of motion rests upon. Main Ideas
Main Newton’s First Law (Inertia) natural motion: how things move in the absence of external forces
Newton’s Second Law (Acceleration) how an object responds to an applied force Newton’s Third Law (Action & Reaction)
how objects act on each other
& where forces come from Simplest Case:
Simplest What happens if you leave things alone? “Every object continues in its state of rest, or of uniform motion in a straight line with unchanging speed, unless compelled to do otherwise by the forces acting on it.” This is called The First Law of Motion We must clarify some critical definitions
before we go too far!
before Rest: Uniform Motion: Not Moving Constant Speed, Straight Line Force: Push or Pull
Was that Uniform
Motion? about that?
Yes Another way of changing “rest” or “uniform
motion” is by applying unbalanced forces.
unbalanced What are some examples of unbalanced forces?
aaah, Balanced Forces!
Very Restful! What happens with balanced forces? This leads to Newton’s Second Law, but first
let’s define a few terms:
Acceleration: 1. Any change from uniform motion; includes Acceleration
Centripetal Acceleration Force: 1. “Cause of Acceleration”
Unequal forces or unbalanced forces are the only forces that will cause accelerations! Second Law definitions continued…
Second Net or Resultant Force: The direction of the unbalanced force
If there is only one
force, it is always
unbalanced. Mass The property of objects that determines how much they accelerate in response to a given force, but…
Mass is not weight! Newton’s Second Law Summary:
Newton’s Unbalanced forces cause objects to accelerate. The stronger the net force the greater the acceleration.
Accelerations are less if the mass is larger (and the force is the same).
Acceleration is in the same direction as the net force (forward, backward, sideways, or some combination of these). net force
net force = mass X acceleration f = ma Examples:
Examples: A Styrofoam ball and a steel ball of equal size are each suspended from a cord and subjected to about the same amount of force from a bluntnosed dart fired horizontally from a springloaded gun. Describe what happens Which of Newton's laws applies best? Accelerations are less if the mass is larger (and the force is the same). Allosaurus is the Utah State Dinosaur Named in 1877
Popularized in 1960 16.5 ft. tall
39 ft. long skull over 3 ft. long
4 tons Suppose an elephant and an ant are both moving at the
same speed on a level, frictionless surface: which would
stop first (or neither)? Assume air friction to be unimportant
for both. Straight line constant speed… Are they accelerating? No! Which of Newton’s laws applies here? First Law Now its time for Newton’s Third Law that
tells us where forces come from and how
they “All forces result from interactions between pairs of objects, each object exerting a force on the other.”
“The two resulting forces have the same strength and act in exactly opposite directions.” A large truck moving at high speed collides with an
unsuspecting mosquito that before the collision is hardly
moving at all. What happens to the motion of both the
mosquito and the truck? At the point of impact what forces are exerted on each?
One million times more
acceleration than truck! Equal forces according to the Third Law Unequal accelerations
Acceleration for the truck = 2
4000 Acceleration for the mosquito = 2
0.004 = 0.0005
= 500 Suppose a man jumps forward toward a dock from a small
boat that is not securely anchored. What happens to the
horizontal motion of both the man and the boat if the boat
has less mass than the man? Are the forces the same? Yes, by the Third Law Which will be accelerated the most? The boat What are the forces present when someone walks across the floor? Newton’s Third
Law - examples
Law firing a gun
standing on the floor
bouncing a basketball Sally discovers Newton's Third Law ...
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- Fall '08