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AP Physics I.B
Newton’s Laws of Motion
B.1 An interaction between two
bodies resulting in a push or a pull
is a force. Forces are of two types:
contact and field forces
Newton’s First Law – an object at
rest will remain at rest, or an
object in motion at a constant
velocity will continue at a
constant velocity, unless acted
upon by a net force.
The tendency of objects to resist
changes in their state of motion is
called inertia. The First Law is
often called the Law of Inertia.
Why?
An unlikely trio – Mr. Evans, James
Lovell and Sir Isaac Newton
Net force – the sum of all the
forces acting on an object
If the net force is zero . . .
• The object is not moving or . . .
• The object is moving at a constant velocity
therefore . . .
• The object is in equilibrium
B.2 Newton’s Second Law
The acceleration of an object is
directly proportional to the net force
and inversely proportional to its
mass.
Net force is a vector in the same
direction as the acceleration
Note! If an object accelerates
then the net force is NOT zero. If
the net force is zero, then the
object is moving with a constant
velocity or it is at rest.
Use free body diagrams to show
all of the forces acting on an
object. Drawing the correct FBD
is half the battle.
Practice FBD’s
• Crate at rest on the floor.
• Mad dad kicks crate with foot across frictionless
floor.
• Crate slides with constant velocity across
frictionless floor.
• Crate slides across rough area.
• Crate slides up a ramp with friction.
• Crate slides down a ramp with friction.
Ex. Huey pulls on the front legs of a 4.0 kg cat with a force of
8.0 N to the right. Dewey, pulls on the back legs of a cat with a
force of 20.0 N to the left. What is the acceleration of the cat
assuming the legs remain attached?
Newton’s Third Law (highly
misunderstood)
Newton’s Third Law – when one
object exerts a force on a second
object, the second object exerts a
force that is equal in magnitude,
but in the opposite direction to
that of the first
“This third law is confusing!”
Remember – Newton’s Third Law
deals with two forces and two objects,
not two forces on one object.
Problem solving strategies
• Draw a free body diagram.
• Is the object in equilibrium (at rest or
constant velocity)or is it accelerating? If in
equilibrium: the sum of the upward forces =
the sum of the downward forces and the sum
of the forces to the right = the sum of the
forces to the left. If accelerating: write an
expression for the net force and use the
Second Law.
Mass and weight
Ex. What is the mass of a watermelon that has weight of 60.0
N?
The Normal Force
“As opposed to the abnormal force”
The force a surface exerts on an
object, perpendicular to the surface
Some instructive illustrations
Ex. A book with a mass of 2.0 kg rests on a table. What is the
normal force on the book by the table?
The normal force and weight on
an inclined plane
Apparent weight (how much your
mother or father weighs)
Setting up the elevator problem.
Ex. A crate slides across a rough floor with an initial speed
of 2.5 m/s. The coefficient of friction between the floor and
crate is 0.35. What is the distance required for the crate to
come to rest?
Ex. A crate with a mass of 20.0 kg is pulled across a wooden
floor with a force of 90.0 N. The coefficient of kinetic friction
between the crate and the floor is 0.30. What is the
acceleration of the crate?
Tension
“These forces are killing me, give me
an Excedrin.”
Tension is the force a rope, string,
chain, cable, sinewy cat gut, etc.
exerts on an objects. All of these
forces can pull only, and not push.
Therefore, the direction of tension
is always away from the object.
Ex. A can of paint with a mass of 6.0 kg hangs from a rope.
If the can is pulled to a rooftop with a constant velocity of
1.0 m/s, what is the tension in the rope?
Ex. What is the tension in a rope that lifts a 50.0 N object
with an acceleration of 10.0 m/s2?
AP Type Problems
Pulleys and hanging blocks. If you
are not concerned with tension in
the connecting string, treat the
blocks as a single system. If you
need to find tension, you must
consider the blocks separately.
m
M
Ex. Find the acceleration of the blocks if they are released
from rest and m rests on a frictionless surface. What is the
tension between the blocks?
m
6m
3m
Ex. What is the acceleration of the block with mass 3m?
What is the tension in the string above m?
FT
θ θ
FT
M
Ex. What is the FT if each string is at an angle θ with the
vertical?
50º
FT1
25º
FT2
75 kg
Ex. Find FT1 and FT2.
θ
Ex. A block of mass m slides down a frictionless inclined
plane that makes an angle θ with the horizontal. Find the
acceleration of the block in terms of the given variables
and fundamental constants.
θ
Ex. A block slides down an inclined plane that makes
an angle θ with the horizontal. Find the acceleration of
the block if the coefficient of kinetic friction between
the block and plane is μ.