Transcript PowerPoint

Physic² 121:
Fundament°ls of Phy²ics I
September 18, 2006
D. Roberts
University of Maryland
PHYS 121
Outline
• Why do things move?
• Newton’s Laws of Motion
–In Chapter 4
• Skipping “Freely Falling Objects”
and Chapter 3 for now. Will come
back to them later
• Forces
D. Roberts
University of Maryland
PHYS 121
Why do things move?
• We’ve developed a language and some tools to
describe motion
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–
–
–
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Position
Velocity
Acceleration
Graphs
Equations of motion
• Now we want to talk about what makes things
move in the first place
D. Roberts
University of Maryland
PHYS 121
Demo
C4-03
D. Roberts
University of Maryland
PHYS 121
Discussion
• What can you say about what you just
observed?
– Try to use some of the physics language we’ve
developed
– …and maybe even try to use it correctly
• What happens when I tap the ball:
– Along the direction of motion
– Opposed to direction of motion
• What happens when I’m not tapping the ball?
D. Roberts
University of Maryland
PHYS 121
Newton’s First Law
• An object at that is at rest will remain at rest, or
an object this is moving will continue to move in
a straight line with constant velocity, if and only if
the net force acting on the object is zero
• New ideas we need to understand:
– Force
– Net Force
• This Law basically says no cause is needed for
an object to move.
– Uniform (constant) motion is an object’s natural state
Translation from original Latin: “Every body perseveres in its state of
rest, or of uniform motion in a right (straight) line, unless it is
compelled to change that state by forces impressed thereon”
D. Roberts
University of Maryland
PHYS 121
Forces
• Usually think of a force as a push or pull
• Vector quantity
• May be a contact force or a field force
– Contact forces result from physical contact between
two objects
– Field forces act between disconnected objects
• Also called “action at a distance”
• More generally:
– A force is what causes an object to change its
velocity (accelerate)
– Converse: If an object is accelerating, there must
be a non-zero net force acting on it
D. Roberts
University of Maryland
PHYS 121
Contact and Field Forces
D. Roberts
University of Maryland
PHYS 121
A soccer player first kicks a soccer ball and
then a cannon ball using the same kick.
Which ball will leave his foot going faster?
?
1. The soccer ball because
it’s lighter.
2. The cannon ball
because it’s heavier.
3. Both the same since he
used the same kick.
4. The soccer ball, but for
some other reason.
5. The cannon ball, but for
some other reason.
6. Both the same, but for
some other reason.
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Newton’s Second Law
• The acceleration of an object is directly proportional to
the net force acting on it and inversely proportional to its
mass.
– F and a are both vectors
• Can also be applied three-dimensionally
D. Roberts
University of Maryland
PHYS 121
Units of Force
• SI unit of force is a Newton (N)
kg m
1N  1 2
s
• US Customary unit of force is a pound (lb)
– 1 N = 0.225 lb
• See table 4.1 in Textbook
D. Roberts
University of Maryland
PHYS 121