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Announcement
• Exam 1 Next Thursday
– Rooms CR 302 and 306
Static Forces Scenario
A block on a frictionless ramp is held by a
rope. As the incline angle q of the ramp
increases,
How does the weight of the block depend
on q?
q
Static Forces Scenario
A block on a frictionless ramp is held by a
rope. As the incline angle q of the ramp
increases,
How does the component of the weight
perpendicular to the ramp depend on q?
q
Static Forces Scenario
A block on a frictionless ramp is held by a
rope. As the incline angle q of the ramp
increases,
How does the component of the weight
parallel to the ramp depend on q?
q
Static Forces Scenario
A block on a frictionless ramp is held by a
rope. As the incline angle q of the ramp
increases,
How does the normal force from the ramp
depend on q?
q
Static Forces Scenario
A block on a frictionless ramp is held by a
rope. As the incline angle q of the ramp
increases,
How does the tension in the rope depend on
q?
q
Forces
That aren’t zero
§ 4.3
What’s the point?
• How do forces affect motion?
Experience Tells Us
The greater the net force on an object, the
more it accelerates.
The greater the mass of an object, the
harder it is to accelerate.
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
• How much is a newton?
F = ma = (1 kg) (1
m/s2)
kg m
=
s2
Board Work
A crate with mass 32.5 kg initially at rest on
a warehouse floor is acted on by a net
horizontal force of 140 N.
a. What acceleration is produced?
b. How far does the crate travel in 10.0 s?
c. What is its speed at the end of 10.0 s?
Whiteboard Problem
A bowl of petunias of mass m accelerates in
free fall at rate g. What is the magnitude of
the net force acting on the bowl?
Weight and Weightlessness
Mass and gravity
§ 4.4
What’s the point?
• What is weight?
Gravitational Force
• Magnitude of gravitational force is
proportional to mass: F = mg.
• g = gravitational field; units N/kg.
• Direction of gravitational force is toward
the center of the earth (down).
• At earth’s surface, g  9.8 N/kg.
Poll Question
Gravity is constantly pulling us downward,
but we are not accelerating downward. This
means that
A. Newton’s second 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.
A Physics Haiku
Elevator breaks
Susie falls and feels no weight
Gravity still there
– Heidi Forbes
Example Problem
A box rests on a frictionless table, and is pushed by two
people. One pushes with a force of F1 = 10 N at an angle
of 30° below horizontal. If she stops pushing, what will be
the acceleration of the box?
F2 = ?
F1 = 10 N
q2 = 60°
q1 = 30°
5 kg