Download NEWTON’S SECOND LAW

NEWTON’S
SECOND
LAW
The effect of an
applied force is to
cause the body to
accelerate in the
direction of the force.
The acceleration is in
direct proportion to
the force and in
inverse proportion to
the mass of the body.
F = ma
m, mass in kg
2
a, acceleration in m/s
F, force in Newtons
1N=1
2
kg•m/s
To find the weight of an
object in newtons, use “g”
for the acceleration.
Fw = mg
NEWTON’S
THIRD
LAW
When one body exerts
a force on another, the
second body exerts on
the first a force of equal
magnitude in the
opposite direction.
For every action
there is an equal
and opposite
reaction.
Action and reaction
iinvolves two
different forces
acting on two
different objects.
Mass is a measure
of inertia in that the
larger the mass, the
more an object tends
to resist a change in
motion.
Mass and
weight are not
equivalent.
Equilibrium is the
condition in which
all forces on an
object add up to
zero.
If a person is standing
in a room, his weight
must be equal to the
force with which the
floor is pushing
upward.
This upward
pushing force is
called “ the
normal force”.
If a person is
moving with
constant velocity,
all forces must be
in equilibrium.
Forces are
vectors. Forces
can be added
using vector
addition.
Resolution of forces
procedure for finding
component forces
from an original force
vector.
A lawn mower has a mass of 130
kg. A person tries to push the
mower up a hill that is inclined 15°
to the horizontal. How much force
does the person have to exert
along the handle just to keep the
mower from the rolling down the
hill, assuming that the handle is
parallel to the ground?
FRICTION is a force
that resists motion.
It involves objects
that are in contact
with each other.
CAUSES OF FRICTION
Deformation of a surface.
Interlocking of irregularities
of surfaces.
How can friction be
decreased?
Polishing surfaces
decreases friction.
However, making surfaces
very smooth actually
increases friction due to
the forces of attraction
between the molecules of
substances.
Friction is often desirable.
Without friction you
couldn’t walk across the
room, you couldn’t get your
car to start moving, nails
would spring out of pieces
of wood.
Starting friction Maximum frictional
force between
stationary objects.
(force needed to make
an object start moving)
Sliding Friction frictional force
between objects that
are sliding with
respect to each
other.
1. Friction acts parallel to
the surfaces that are in
contact and in the
direction opposite to the
motion of the object or to
the net force tending to
produce such motion.
2. Friction depends
on the nature of the
materials in contact
and the smoothness
of their surfaces.
3. Sliding
friction is less
than or equal to
starting friction.
4. Friction is
practically
independent of
the area of
contact.
5. Starting or sliding
friction is directly
proportional to the
force pressing the
two surfaces
together.
Coefficient of Sliding
Friction - The ratio of the
force of sliding friction to
the normal (perpendicular)
force pressing the
surfaces together.
µ = Ff / FN
Increasing friction
sand, tire
chains,snow
tires, rosin.
Decreasing friction wax, oil, selflubricating alloys,
rolling friction,
Teflon©.
Decreasing friction wax, oil, self-lubricating
alloys, rolling friction,
Teflon©.
(What makes Teflon©
stick to the pan?)
A box is pushed across a
level floor at a constant
velocity by a force of 100 N.
If the box weighs 200 N,
what is the coefficient of
friction between the box
and the floor?
A box is pushed across a
level floor at a constant
velocity by a force of 100 N.
If the box has a mass of
45 kg, what is the coefficient
of friction between the
box and the floor?
A box with a mass of 175 kg is
pulled along a level floor with a
constant velocity. If the
coefficient of friction between
the box and the floor is 0.34,
what horizontal force is exerted
in pulling the box?
A force of 50 N is directed
30º above horizontal. What
are the vertical and horizontal
components of that force?
A 120 N box is placed on a
25º incline. What are the
perpendicular and parallel
components?
A wooden block weighing 130 N
rests on an inclined plane. The
coefficient of sliding friction
between the block and the
plane is 0.620. Find the angle
of the inclined plane at which
the block will slide down the
plane at constant speed once
it has started moving.
A crate is pulled with a constant
velocity up an inclined floor that
makes an angle of 12° with the
horizontal. The crate weighs
950 N and the pulling force
parallel to the floor is 460 N.
Find the coefficient of
friction between the
crate and the floor.
A box weighing 200 N is pulled along a level floor
at constant speed by a rope that makes an angle
of 40.0° with the floor. If the force on the rope is
100 N, (a) what is the horizontal component (Fh)
of this force? (b) What is the normal force (FN)?
(c) What is the coefficient of sliding friction (µ)?