Download Weight and friction

Draw a free body
diagram for the
skier.
Friction


A block on a
table may not
start to move
when we apply a
small force to it.
This means that
there is no net
force in the
horizontal
direction, and
that the applied
force is balanced
Friction

Based on these
observations we
can conclude :


There are two
different friction
forces: the static
friction force (no
motion) and the
kinetic friction force
(motion).
The static friction
force increases with
the applied force but
has a maximum
value.
Friction and Braking

Consider how you
stop in your car:

The contact force
between the tires
and the road is
the static friction
force (for most
normal drivers).
It is this force
that provides the
acceleration
required to
reduce the speed
Friction and Normal Forces



The maximum static friction force
and the kinetic friction force are
proportional to the normal force.
Changes in the normal force will
thus result in changes in the
friction forces.
NOTE:

The normal force will be
always perpendicular to the
surface.

The friction force will be
always opposite to the
direction of (potential)
motion.
Pushing or Pulling
A Big Difference
More Friction
Bigger Normal
Less Friction
smaller Normal
Sample Problem
A 747 jetliner lands and begins to slow to a stop as it moves along
the runway. If its mass is 3.50 x 105 kg, its speed is 27.0 m/s, the
forward force is 1 x 105 and the braking force is 4.30 x 105 N
a) what is its speed 7.50 s later?
b) How far has it traveled in this time?
Newton’s Third Law


For every action there exists an
equal and opposite reaction.
If A exerts a force F on B, then B
exerts a force of -F on A.
Examples of
Newton’s 3rd
Law
Copyright James Walker, “Physics”, 1st edition
Sample Problem
You rest an empty glass on a table.
a) How many forces act upon the glass?
b) Identify these forces with a free body diagram.
c) Are these forces equal and opposite?
d) Are these forces an action-reaction pair?
Sample Problem
A force of magnitude 7.50 N pushes three boxes with masses m1
= 9.4 kg total. If the friction force is 1.50 N, what is the net
force? What is the acceleration?
Copyright James Walker, “Physics”, 1st edition
Newton’s 2nd Law in 2-D



The situation is more complicated
when forces act in more than one
dimension.
You must still identify all forces and
draw your force diagram.
You then resolve your problem into
an x-problem and a y-problem
(remember projectile motion????).
Forces in 2-D
Copyright James Walker, “Physics”, 1st edition
Forces in 2-D
Copyright James Walker, “Physics”, 1st edition
Forces in 2-D
Copyright James Walker, “Physics”, 1st edition
Forces in 2-D
Copyright James Walker, “Physics”, 1st edition
Forces in 2-D
Copyright James Walker, “Physics”, 1st edition
Sample Problem
A surfer “hangs ten”, and accelerates down the sloping face
of a wave. If the surfer’s acceleration is 3.50 m/s2 and
friction can be ignored, what is the angle at which the face
of the wave is inclined above the horizontal?
Mass and Weight
Sample Problem
How long will it take a 1.0 kg block initially at rest to slide
down a frictionless 20.0 m long ramp that is at a 15o angle
with the horizontal?
Sample Problem
An object acted on by three forces moves with constant
velocity. One force acting on the object is in the positive x
direction and has a magnitude of 6.5 N; a second force has
a magnitude of 4.4 N and points in the negative y
direction. Find the direction and magnitude of the third
force acting on the object.
Mass and Weight




Many people think mass and weight
are the same thing. They are not.
Mass is inertia, or resistance to
acceleration.
Weight can be defined as the force
due to gravitation attraction.
W = mg
Sample Problem
A man weighs 150 pounds on earth at sea level. Calculate
his
a) mass in kg.
b) weight in Newtons.
Newton’s Laws
(inertia: mass vs. weight)
Here is a useful strategy for finding how the motion
of an object depends on the forces exerted on it.
1.Draw a diagram showing
the direction and relative
strength of each force acting
on the system.
2.First, identify all the forces
acting on the object.
3.Then, add the forces to find
the net force.
4.Remember: only outside
forces count (called
EXTERNAL forces). What
happens inside an airplane
doesn’t make it fly. Only the
outside air can do that.
5.Next, use Newton’s second
law to calculate the
acceleration.
REVIEW QUESTIONS
1.
What is the difference between
mass and weight?
2.
Which has more mass, 12 g of
feathers or 12 g of lead?
3.
When an astronaut is living on the
International Space Station, what
happens to his mass?
REVIEW QUESTIONS
4.
What would the mass of a dog be
if it has a weight of 100N?
5.
What would the weight of an apple
be if it has a mass of 1.5 kg?
6.
What is a force?
REVIEW QUESTIONS
7.
What is friction?
8.
What do YOU think causes friction
on a moving object?