Transcript Newton’s Second Law of Motion Force & Acceleration

Newton’s Second Law of Motion
Force & Acceleration
Introduction to Science
A. King
EHS
Acceleration
• Galileo define rate of change of velocity as
acceleration.
• The term acceleration is typically used when the
velocity increases.
• The term deceleration is typically used when the
velocity decreases – another way of saying this is
that “deceleration” is when negative acceleration
is occurring.
• In both cases, “acceleration” is the process that’s
actually occurring.
Calculating Acceleration
ΔV
Acceleration = --------t
Where ΔV is the change
in velocity and t is the
time it takes for the
velocity to change.
ΔV
: :
acc
xt
Units for Acceleration
• Speed and velocity
are easily measured in
distance/time units,
such as miles/hr, m/s,
etc.
• Units of acceleration
are more complicated,
and have to take other
stuff into account.
• Sample problem: What is
the acceleration if we
speed up from 10 km/h to
30 km/h in 10 seconds?
• Acceleration = Δv/t
• (30 km/h – 10 km/h)/ 10
sec =
• 2 km/h/s
Acceleration depends on
the net force.
Acceleration ~ net
force.
Mass
• Mass is a measure of an object’s inertia
and a measure of how much material an
object contains. (It depends on the number
and kind of atoms making the object up.)
• Mass is measured in grams: grams,
kilograms, milligrams…
Weight
• Weight depends on gravity.
• Weight is the force due to gravity that acts
on an object’s mass.
• Although weight and mass are different
from each other, they are directly
proportional to each other.
• 1 kilogram weighs 9.8 newtons.
Volume
• Volume is a measure of space.
Newton’s Second Law states:
• The acceleration produced by a net force
on an object
– is directly to the net force.
– Is in the same direction as the net force.
– Is inversely proportional to the mass of the
object.
– In formula: F = m x a
By using consistent units, such as
newtons (N) for force, kilograms
(kg) for mass, and meters per second
squared (m/s2) for acceleration, we
get the exact equation:
Δv
v f – vi
Acceleration = -------- = ---------t
t
Acceleration Problems
• While drag racing out of our school parking lot, I time
myself at a speed of 40 meters per second seven seconds
after starting. What was my acceleration during this
time?
• A = vf – vi / t
• = (40 m/s – 0 m/s )/7s
• = (40 m/s)/7s
• = 5.7 m/s/s
Friction
• Occurs when one object rubs against
something else.
• Occurs for solids, liquids and gases.
• Always acts in a direction opposite to
motion.
• The amount of friction between two
surfaces depends on the kinds of material
and how much they are pressed together.
Free Fall
• When an object is dropped in a place with gravity
(k.e. not deepspace), gravity causes the object to
fall downward.
• When this occurs, the object is said to be in “free
fall.”
• For problems like this, we’ll imagine that gravity
is the only force that’s acting on the object –
wind resistance and other stuff will be ignored.
How fast is an object moving in free fall?
• The increase in speed
per second is the
• Let’s consider the
acceleration which in
following data table:
this case is 10 m/s/s.
Elapsed time Instantaneous
(sec)
speed (m/sec) • The instantaneous speed
of an object that is
0
0
accelerating is equal to
1
10
the acceleration
2
20
multiplied by the amount
of time the object is
3
30
falling.
t
10t
• In equation form: v = at
Δv
Formulas
: :
acc x
: :
acc x
t
F
vf - vi
Wt
d
v
t
: :
: :
: :
: :
mx a
mx g
½ g x t2
a xt