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CHAPTER 5: USING NEWTON'S LAWS (FRICTION, CIRCULAR MOTION, DRAG…
CHAPTER 5:
USING NEWTON'S LAWS (FRICTION, CIRCULAR MOTION, DRAG FORCES)
5.1 - APPLICATIONS OF NEWTON'S LAWS INVOLVING FRICTION
Friction is always present when
two solid surfaces
slide along each other.
Sliding friction is called
kinetic friction
.
Approximation of the
frictional force
:
Ffr = μk × Fn
Fn is the
normal force
, and μk is the
coeffiecient of kinetic friction
, which is different for each pair of surfaces
Static friction applies when
two surfaces
are at
rest
with respect to each other (such as a book sitting on a table)
The
static frictional force
is as big as it needs to be to
prevent slipping
, up to a
maximum value
.
Ffr ≤ μs × Fn
In general,
μs > μk
5.4 -HIGHWAY CURVES: BANKED AND UNBANKED
When a car goes around a curve, there must be a net force toward the center of the circle of which the curve is an arc. If the road is flat, that force is supplied by friction.
If the frictional force is insufficient, the car will tend to move more nearly in a straight line.
As long as the tires do not slip, the friction is static. If the tires do start to slip, the friction is kinetic, which is bad in two ways:
The kinetic frictional force is smaller than the static.
The static frictional force can point toward the center of the cirlce, but the kinetic frictional force opposes the direction of motion, making it very difficult to regain control of the car and continue around the curve.
5.3 - DYNAMICS OF UNIFORM CIRCULAR MOTION
For an object to be in uniform circular motion, there must be a net force acting on it.
FR = maR = mv^2 / r
The force must be inward by thinking about a ball on a string. Strings only pull, they never push.
There is no
centrifugal
force pointing outward:
what happens is that the natural tendency of the object to move in a straight line must be overcome.
5.2 - UNIFORM CIRCULAR MOTION (KINEMATICS)
Uniform circular motion
:
motion in a
circle
of constant
radius
at a
consant speed
.
Instantaneous velocity
is always
tangent
to the circle.
This acceleration is called the centripetal, or radial, acceleration, and it points toward the center of the circle
A
centrifuge
works by spinning very fast. This means there must be a v
ery large centripetal force
. The object at A would go in a straight line but for this force, as it is, it winds up at B.
5.6 - VELOCITY (DEPENDENT FORCES : DRAG AND TERMINAL VELOCITY)
When an object moves through a fluid, it experiences a drag force that depends on the velocity of the object.
For small velocities, the force is approximately proportional to the velocity. For higher speeds, the force is approximately proportional to the square of the velocity.
If the drag force on a falling object is proportional to its velocity, the object gradually slows until the drag force and the gravitational force are equal. Then it falls with constant velocity, called the terminal velocity.
Vt = mg / b
5.5 - NON-UNIFORM CIRCULAR MOTION
If an object is moving in a circular path but at varying speeds, it must have a tangential component to its acceleration as well as the the radial one.
This concept can be used for an object moving along any curved path, as any small segment of the path will be approximately circular.
NUR ADIBAH BINTI ROZAINI
