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Newton’s Second Law
of
Motion
F=ma
Why can a rabbit run faster than a turtle?
It’s really quite
simple. He can apply
more force to his
mass than I can.
Truck A
Truck B
Which of these trucks can accelerate the easiest?
Why?
GET OUTSIDE THE BOX
Prove your answer with realistic numbers.
Remember Force (F) and mass (m)
F = ma
A
F
a= m
10,000 N
a = 8,000 kg
a = 1.26 m/sec2
B
F
a= m
10,000 N
a = 2,000 kg
a=
5 m/sec2
With the same amount of force applied to each, B will
accelerate easier because it has less mass.
How does an ice skater use force?
1. The skater uses force to
get started and to speed
up.
2. The skater uses
force to change
direction.
3. C.
ALL OF THESE ARE
m = 0.5 kg
F=2N
Solve for acceleration.
F
a= m
2N
a=
0.5 kg
a = 4 m/sec2
A car rolls down a ramp
and you measure a force
of 2 newtons pulling the
car down. The mass of the
car is 0.5 kg. What is its
acceleration?
What formula would you
use to solve for Force (F)?
F=mxa
for mass (m)?
_F_
M=
a
BALANCED FORCES
Balanced forces are always equal in size and
opposite in direction.
Balanced forces produce NO MOTION
This man is pushing against a
wall, the wall is pushing back
against the man. Nothing
moves so the forces are in
equilibrium, (balanced)…
NOTHING MOVES !
Notice that the arrows are
the same size and pointing
in opposite directions.
UNBALANCED FORCES
With unbalanced forces the forces will be in
opposite directions but the size of one force will
be larger than the size of the other.
UNBALANCED FORCES ALWAYS PROCDUCE MOTION !
As this man applies an
unbalanced force to the
dolly with the drinks, it
moves because his force is
more than the force of the
object. (notice the arrows)
Analyze this force diagram
What is the actual
“force” applied by
the dirt?(not a number)
The weight of the
dirt
Force applied
by Dozer Rat.
Opposite force
applied by dirt
Direction of motion
We
Need
HELLLP!
Analyze these force diagrams.
Are you pulling ?
The sum of all forces
pulling in this direction
The sum of all
forces pulling in
that direction
Direction of motion
3.2 WEIGHT, GRAVITY, & FRICTION
In order to understand what weight is you must understand what gravity is. So…..
Explain gravity
Gravity is a force that pulls every mass towards
every other mass.
Gravity depends on mass, the greater the mass an
object has, the greater its gravitational pull.
For example, the earth has more mass than our
moon, so the gravitational pull of earth is more
than the moon.
So… how does that relate to the weight of an object?
Let’s look at Mrs. Shelly weighing. How do these scales
“weigh” us?
The top platform of the scale is able to
move and is attached by springs to the
dial which are calibrated to show pounds
of weight.
Gravity pulls down
on the mass of
the person
The floor pushes up
on the scales
Calculating weight
Weight force (N)
Fw = mg
Mass (kg)
Acceleration of
gravity (9.8 m/sec2)
The correct unit for weight force is the newton (N).
What is your metric weight? (reminder 1 kg=~2.2 lb.)
Divide your weight by 2.2 lbs to convert it to mass,
then multiply by 9.8m/sec2. This will give your wt.
in newtons.
FRICTION
Friction is a term used to describe forces that result
from the relative motion between objects. Simply
defined, friction is a force that opposes motion.
Friction is caused by two surfaces rubbing against
one another. The amount of friction depends on the
smoothness or roughness of the surfaces. This is
called texture.
REVIEW
Newton’s 2nd Law of Motion is a mathematical
equation which explains the relationship between
force, mass, and acceleration.
Acceleration (m/sec2)
Force (Newtons, N)
F = ma
Mass (kilogams, kg)