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Announcements
• Exam 1/2 today
– On-line on Moodle
– Available 11 AM–midnight
– Contact me if you need accommodations
Influencing Motion
Philosophy, definition, and reality
Objectives
• Relate uniform motion and zero net force.
• Relate force, acceleration, and mass.
• Compute the direction and magnitude of
the forces of gravity, support, and friction.
What’s the point?
• What governs an object’s motion?
Newton’s First Law
• An object’s state of motion does not change
unless an outside net force acts upon it.
• If at rest, it
remains at rest.
• If moving, it
continues straight
at constant speed.
Representation of Newton by William Blake, 1795
Does it Make Sense?
• Do you feel a force while cruising in a
plane? A train? An automobile?
• If you were moving in a perfectly straight
line at a constant velocity inside a closed
box, could you tell?
How Can We Use It?
• Equilibrium Rule: An object moving at a
constant velocity experiences zero net
force.
• That means that the forces acting on it add
to zero.
• A body with zero net force is in mechanical
equilibrium.
What does Newton’s
first law mean?
• Being at rest is nothing special; it is just
another value (zero) of constant velocity.
rest
constant velocities
Newton’s Second Law
F
a=
m
equivalently,
F = ma
a = acceleration F = net force m = mass
Quantify Force
• Unit of force: 1 newton (N) = force needed
to accelerate 1 kg at 1 m/s2
• Unit named for a person
– Unit name lowercase
– Abbreviation capitalized
Group Work
F = ma.
If m = 10 kg and a = 1 m/s2, what is F?
Group Work
A net force of 100 N acts on a 10-kg
steel block. What is its acceleration?
Working with CommonlyEncountered Forces
Gravity
Support (Normal force)
Friction
Gravity
• Magnitude of gravitational force is
proportional to mass: F = mg.
• (g is gravitational field strength.)
• Direction of gravitational force is toward
the center of the earth.
• At earth’s surface, g  9.8 N/kg.
Group Work
A. What is the gravitational force, in N,
acting on a 2-kg chicken at the surface
of the earth?
B. If gravity is the only force acting on it,
what will the chicken’s acceleration be?
(Acceleration is a vector, so specify both
magnitude—including units— and
direction.)
Poll Question
Gravity is constantly pulling us downward,
but we are not accelerating downward. This
means that
A. Newton’s first law does not apply here.
B. Gravity does not apply a physical force.
C. Some other force exactly opposes the
force of gravity.
D. Gravity stops at the earth’s surface.
Support Force
• Exerted by a surface perpendicular
(normal) outward
• Magnitude as needed to cancel inward
forces
support
weight
weight
zero
net
force
Friction
• Commonplace but complicated
• Surface friction acts parallel to a surface
• Friction f opposes applied sliding force F
N
F
perpendicular (normal) force
f
Friction
N
F
perpendicular (normal) force
f
• f  mN (directions of f and N are different)
• m = coefficient of friction
• Coefficient m depends only on surface
materials, not mass, weight, area of
contact, sliding speed (one exception).
Friction
• Static friction: when the two surfaces are
not moving past each other
• Kinetic friction: the two surfaces slide
along each other
• Static friction > kinetic friction
Group Question
Rank the magnitude of frictional force on
a car in these different situations.
I. Skidding on dry pavement.
II. Skidding on ice.
III. Braking just short of a skid on dry pavement.
Static vs. Kinetic Friction
•
•
•
•
Start pushing an object
Anti-lock brakes
Learning to drive
Stick-slip motion
Reading for Next Time
• Projectiles: falling in gravity
• Big ideas:
– Vertical acceleration = g downward
– Horizontal acceleration = 0
– Horizontal and vertical motion (x, vx, ax; y, vy,
ay) are independent