Transcript Chap. 5 Newton`s 2nd Law

Newton’s Second law
Force and
Acceleration
How are forces related
to motion?
• What is a force?????
• The unit of force is the
Newton (N).
• 1 kg of mass exerts about 10
N of force.
Forces...
• Can cause a change in an
object’s state of motion…
• A force can increase or
decrease speed OR change
the direction of an object.
In other words…
Force causes acceleration!
TYPES OF FORCES
• Balanced forces…
• Unbalanced forces...
Balanced Forces
• Equal in size • Condition of
equilibrium
• Opposite in
direction
• Net force = 0
• They cancel
• Thus, no
each other out acceleration!
An example...
When you stand
on the floor, your
weight pushes
down while the
floor pushes up...
Another example…
Force the road
Force the tires
applies to the tires apply to the road
One more...
The wall pushes Dave pushes on
on Dave
the wall
Unbalanced Forces...
• Can be
• Net force = 0
unequal
• Causes
• Do not cancel
acceleration!
• No
• Examples...
equilibrium
Here we go...
Force the tires
exert on the road
Force
the road
exerts
on the
tires
One more...
Tires pushing Curve pushing
against the
against the
curve
tires
Newton’s
nd
2
Law
• Acceleration is proportional
to force…
–The more the force…
–The less the force...
Newton’s
nd
2
Law
• Acceleration is proportional
to mass…
–The more the mass…
–The less the mass...
Put it all together...
A = F/m
Newton’s Second Law
An object will accelerate in the direction
you push it…the harder the push, the
greater the acceleration, the bigger the
mass, the less the acceleration will be.
The Equation...
F = ma
Applications...
• Weight : the force due to
gravity
–W = mg : g = 9.8 m/s2
• Air friction…
–Please read Ch. 5!
What are the effects of
air friction while
falling?
In free fall, objects are
being accelerated by
gravity.
• The force due to gravity is
weight.
F = m a
Wt = m g
EXAMPLE
• What is the weight of a 100
kg sky diver?
• m = 100 kg
• g = 10 m/s2
• Wt = ?
EXAMPLE
• Wt = 100 kg x 10 m/s2
• Wt = 1000 N
Air Friction
1. Is also known as Air
Resistance
Air Friction
2. The amount of air
resistance will increase as
a. The speed increases
b. The area of surface increases
Air Friction
3. Opposes gravity
•
Friction always goes against
motion
Air Friction
•
Net Force = weight - air
friction
•
EXAMPLE…...
What is the net force acting on
a 150 kg sky diver with 500 N
of air resistance?
m = 150 kg
2
g = 10 m/s
Wt = 1500 N
1500 N
Wt = 1500 N
Air resistance =
500N
Net Force =
1500N - 500N
500 N
• Net Force = 1000 N
• If the object falls fast
enough, the force of air
resistance will equal the
force of gravity.
1500 N
• Forces are
balanced
– No acceleration
1500 N
• Falling speed
will stop
increasing
The fastest speed when
falling is called terminal
velocity.
Terminal velocity is
greatest when:
• Large weight (more
downward force)
• Small surface area (less
resistance)
PRESSURE
• Amount of force per unit
area.
• P = force / area
•P=F/A
PRESSURE IS
MEASURED IN
PASCALS
• 1 Pa = 1 N/m2
• Very small pressure
• Sometimes use kilopascals
(kPa)
Pressures is greater
when…..
• The force is bigger.
• The area is smaller.
Which exerts the
greatest pressure?
OR
•Why do objects fall at
the same rate (no air
resistance)?
Acceleration is a ratio.
•F / M = F / M
• The ratio of force to mass is
constant and equals gravity.