Transcript Aim: How do we explain Newton`s 3rd Law?

Aim:
How do we explain the force of
friction?
Visualizing Friction
Friction
When an object is in contact with a surface, the
surface exerts a contact force on the object.
The contact force that acts parallel to the
surface, opposing the motion of an object is
called friction.
Static Friction vs Kinetic Friction
Static friction- the force that must be overcome
to set an object in motion.
Static friction acts on objects at rest
Kinetic Friction- the force of friction that acts on
an object that is moving.
Which is greater, the force of static friction or
kinetic friction?
Equations for Friction
Ff static =< μs FN
Ff kinetic = μk FN
Ff = force of friction
FN= normal force
μs = coefficient of static friction
μk = coefficient of kinetic friction
Sample Friction Problem
1. The coefficients of friction for an old physics book
upon a shelf are: μs = 0.50 and μk = 0.40. Suppose
the book has weight 14.5 N and a horizontal force is
applied to it. (a) Determine the maximum force
that can be applied without moving the book. (b)
Determine the force required to keep the book
moving at a constant speed across the shelf. (c) If
the magnitude of the applied force is increased
gradually, find the initial acceleration of the book
just as it starts to move.
Thought Question 1
You press your physics textbook flat against a
vertical wall with your hand. What is the
direction of the friction force on the book
exerted by the wall?
a) Downward c) Upward
b) Outward from the wall
d) Into the wall
Sample Friction Problems
2. A car with speed vo applies the brakes and
slows to a stop. Derive and simplify an
equation for the stopping distance d in terms
of vo and μ.
(Use Newton’s equations of motion)
Thought Question 2
You are playing with your daughter in the snow.
She is sitting on a sled and asking you to slide
her across a flat, horizontal field. You have a
choice of pushing from behind, by applying a
force downward on her shoulders at 30 degrees
below the horizontal, or attaching a rope to the
front of the sled and pulling with a force at 30
degrees above the horizontal. Which would be
easier for you and why?
Thought Question 2
Sample Friction Problem
3. A traveler pulls a suitcase of mass 8.00 kg across a
level surface by pulling on the handle 20.0 N at an
angle of 50.0° relative to horizontal. Friction against
the suitcase can be modeled by μk = 0.100. (a)
Determine the acceleration of the suitcase. (b) What
amount of force applied at the same angle would be
needed to keep the suitcase moving at constant
velocity?
Sample Friction Problems
4. A horse of mass 509 kg pulls a sleigh of mass
255 kg and both horse and sleigh accelerate at
0.500 m/s2. The coefficient of friction for the
sleigh is 0.15 as it moves over the snow. (a)
Find the force that the horse must exert on
the sleigh. (b) Determine the amount of
horizontal force that the horse’s feet must
exert.
Thought Question 3
A crate is located at the center of a flatbed
truck. The truck accelerates toward the east,
and the crate moves with it, not sliding on the
bed of the truck. What is the direction of the
friction force exerted by the bed of the truck on
the crate?
a) To the west c) To the east
b) There is no friction force because the crate is
not sliding
Problem 5
The driver of an empty speeding truck slams on
the brakes and skids to a stop through a distance
d.
a) If the truck carries a heavy load such that its
mass is doubled, what would be its skidding
distance if it starts from the same initial
speed?
b) If the initial speed of the empty truck is
halved, what would be the skidding distance?
Problem 5
Problem 6
The following is a simple method of measuring
coefficients of friction. Suppose a block is placed
on a rough surface inclined relative to the
horizontal, as shown in the figure. The incline
angle θ is increased until the block starts to
move.
a) How is the coefficient of static friction related
to the critical angle θc at which the block
begins to move?
b) How could we find the coefficient of kinetic
Problem 6
Problem 7
A ball and a cube are connected by a light string
that passes over a frictionless light pulley as
shown. The coefficient of kinetic friction
between the cube and the surface is 0.30. Find
the acceleration of the two objects and tension
in the string.
Problem 7
Problem 8
If the coefficient of kinetic friction were very
large, the ball and cube could move with
constant velocity. What coefficient is required
for this situation to exist?
Problem 9
A warehouse worker places a crate on a sloped
surface inclined at 30 degrees with respect to
the horizontal. If the crate slides down the
incline with an acceleration of magnitude g/3,
determine the coefficient of kinetic friction
between the crate and the surface.
Problem 9
Problem 10
A block moves up a 45 degree incline with
constant speed under the action of a force of 15
N applied parallel to the incline. If the coefficient
of kinetic friction is 0.300, determine
(a) The weight of the block
(b) The minimum force parallel to the incline
required to allow it to move down the incline
at constant speed.