Transcript Newtons laws and Friction spring 2010

Unit 2
Forces & Motion
Forces
Force- Ability to change motion(push or
pull)
Units of lb, N=kg.m/sec2
If forces are balanced then the object won’t
move and it is said to be in equilibrium
Forces cause an object’s velocity to change
& can therefore cause acceleration.
Unbalanced forces=move
Forces always occur in pairs!!!!!
Categories of Forces
Contact forces- force
exerted from an
external source
Applied force
Friction
Support force
Field forces- forces
exerted without
contact
Magnetic
Electrical
Gravitational
Newton’s Laws of Motion
Newton’s First Law:
“AKA” the Law of Inertia
Inertia- property of an object to resist
change in state of motion
an object will stay at rest until
acted on by an unbalanced
force
In other words, things tend to
keep on doing what they were
doing in the first place unless
you apply a force.
Newton’s Laws of Motion
Newton’s Second Law:
force causes an object to
accelerate while the object’s
mass resists acceleration
Fnet=ma
• a=acceleration (m/sec2), F=
force (N), m=mass (kg)
• Fnet= Right – Left
• Fnet= Top - Bottom
EX. Lighter cars go faster than
heavier ones pushed with equal
force. Lighter cars resist the
force of acceleration less
allowing them to move faster
than heavier ones.
Net Force
Net Force - Vector sum of all the forces that act on
an object
In the same direction - add
In opposite directions – subtract
largest vector wins direction
At right angles (a2 + b2 = c2)
When net Force = 0, there is no acceleration
3.3 Newton’s Third Law
Newton’s 3rd Law:
For every action there is a reaction force equal in
strength & opposite in direction
For every action there is an equal & opposite
reaction
Can be + (move to right) or – (move to left)
Examples: rockets, stepping into a boat, throw a
ball on a a skate board
Gravity, Weight & Mass
GravityThe downward pull on you by
the earth
Depends on mass
Mass- amount of matter in an
object compared to a standard
Changes on other planets
On Earth= -9.8m/s2
•Fw=mg
•Fw= weight force (N)
•m=mass (kg)
•g=acceleration due to gravity
(-9.8 m/sec2)
Weight- force created by
gravity depends on mass
It is a force acting on object in
units of lb or N
Fw
m
g
Drag Force
Drag force – the force exerted by a fluid (liquid or
gas) on an object moving thru the fluid.
- We refer to this as air resistance when objects
move thru the air
The faster an object goes the greater the drag force.
- When the drag force equals the force of gravity
there is no acceleration.
- A constant velocity – known as terminal velocity.
- Large surface areas have a lower terminal velocity
- Small surface areas have a higher terminal velocity
Law of Universal Gravitation
Speaking of gravity…
There’s earthly gravity (the earth & objects are attracted to
each other)
There’s universal gravity (attraction between heavenly bodies
like the Sun and moon)
No matter what kind of gravity you speak, two variables
influence the strength of this attractive force:
Object’s mass
Distance between objects
F = G m1 m2
-------r2
Free Fall• Drop straight down in the absence of air
resistance.
• Acceleration due to gravity -9.8 m/sec2
• Object’s will experience uniform acceleration
and uniform increases in velocity when in free
fall!
Terminal VelocityHighest velocity reached by a falling
object
When an object stops accelerating, but
continues to fall terminal velocity is
achieved.
When air resistance balances the
weight of the object pushing down on it.
What is friction?
Friction: A force that…
Reduces acceleration
Works against motion
Occurs when surfaces move against on another
Causes wear on parts
Examples of friction:
Air friction- (air resistance)- aka – “drag”
Sliding friction (rub hands together)
Viscous friction- (oil in car engines & joint fluid)
Rolling friction- (wheel on road, ball bearings)
Static Friction (Ffs) Sliding Friction (Ffk)
The frictional force that must
be overcome to get an object
moving
μs = Ffs μ=coefficient of friction
FN
Fw: Weight Force
= mass(gravity)
FN: Normal Force
Is always = to the Weight Force
(Fw)
μs is always greater than μk
The frictional force that
exists once an object is
in motion.
AKA kinetic friction
μk = Ffk
FN
Rules of Friction
Ff
Fp or Fa
If an object is not moving, then the frictional
force (Ff) is greater than the pulling force
If an object is moving at a constant speed,
then the frictional force (Ff) is equal to the
pulling force
If an object is accelerating, the frictional force
(Ff) is less than the pulling force.
Fp=pulling force = Fa=applied force