Download to the object`s

&
Forces
Isaac Newton
(1642–1727)
proposed that
the tendency of
an object was to
maintain in its
current state of
motion.
INERTIA
the tendency of an object
to resist any change in its motion
Inertia is a property of matter and does not
depend on the position or location of the object.
MASS (kg)
a quantitative measure of inertia
FORCE
“a push or pull”
that may cause an
object to speed up, slow
down or change
directions.
“Law of Inertia”
An object at rest remains at rest,
and an object in motion continues in motion
at a constant speed in a straight line,
unless acted on by a nonzero net force.
* OR *
The velocity of an object remains constant
unless acted on by an unbalanced force.
Identifying a force:
• Source, agent exerting the force
• Type: “contact” or “field”
– “Contact”- the 2 objects HAVE to be touching
• EX. push, normal, pull, thrust, tension, friction, etc.
– “Field”- the 2 objects DO NOT have to be touching
• EX. gravitational, electric, magnetic
The force of friction
• The push things give to each other when they rub
together.
• Friction is the force that acts in the opposite
direction of motion of the object
• In Newton’s world, if you could eliminate friction,
then you would see more things keep on moving
in a straight line.
• Newton’s first law only makes sense when all
forces, including friction, are accounted for.
The net force acting on an object is the
vector sum of all the forces acting on it.
Examples:
8N
8N
4N
7N
12 N
9N
6N
8N
?
5N
4N
If an object is remaining at rest, it
is incorrect to assume that there
are no forces acting on the object.
We can only conclude that the
net force on the object is zero.
8N
3N
7N
Zero Net Force
the condition of an object
that is not accelerating
If an object is at rest
or moving at constant speed,
there will be no net force acting on
the object. All forces will be balanced.
Newton’s
st
1
Law- Inertia
• In the absence of “a net force,” an object moves
with constant velocity.
• In the absence of “the thing that will cause a
“change in velocity,” an object moves with
constant velocity.
• The cause of constant velocity is NOTHING that
will change the velocity.
• The cause of motion is NOTHING!
• Motion Happens!
“Law of Interaction”
Short Version
“For every action there is
an equal and opposite reaction.”
Longer Version
When one object exerts a force on a
second object, the second exerts a force
on the first that is equal in magnitude,
but opposite in direction.
Example
• When you bump into something, the size of
the bruise is determined by Newton's 3rd
Law. When you hit the object it hits you
back with the same force. You only get back
what you put into it.
Newton’s 3rd Law, the Law of Interaction
If (object 1) exerts a (type) force in the
(direction) direction on (object 2), then
(object 2) is exerting a (same type)
force directed (opposite direction) on
(object 1) simultaneously.
Consider the interaction depicted below
between foot A, ball B, and foot C. The three
objects interact simultaneously (at the same
time). Identify the two pairs of action-reaction
forces. Use the notation "foot A", "foot C", and
"ball B" in your statements.
http://www.glenbrook.k12.il.us/gbssci/phys/Cl
ass/newtlaws/u2l4b.html
Identify at least six pairs of action-reaction force
pairs in the following diagram.
http://www.glenbrook.k12.il.us/gbssci/phys/Class/ne
wtlaws/u2l4b.html
Newton’s Third Law deals with two forces
acting on two different objects.
Newton’s Third Law pairs of forces
always,
sometimes,
never
cancel each other out.
The only way for two forces to cancel
each other out is for them to be equal
and opposite and act on the same object!
“Law of Acceleration”
A net force causes an object to accelerate
in the direction of the net force.
The acceleration is
directly proportional to the net force
and inversely proportional
to the object’s mass.
Fnet
a=
m
Fnet = m a
The SI unit of force is the newton, named
in honor of Isaac Newton.
One newton of force
is the amount of force
needed to cause a one
kilogram mass to accelerate
at a rate of 1 m/s2.
WEIGHT
a measure of the gravitational force
that a massive object,
such as a star or planet,
puts on another mass
F = ma
weight = mass x acceleration of gravity
Fg = mag
An object’s weight on planet Earth in newtons
is equal to its mass in kilograms times 9.8 m/s2.
1. As a baseball is being caught it’s velocity goes
from 30.0 m/s to 0.0 m/s in about 0.0050 s. The
mass of the baseball is 0.145kg.
•
•
•
What is the baseball’s acceleration?
a = ΔV / Δt
What is the magnitude and direction of the force
acting on the ball?
What is the magnitude and direction of the force
acting on the player who caught the ball?
a = ? m/s2
m = 65 kg
a = 3.0 m/s2
m = 45 kg
2. A 65 kg boy and a 45 kg girl use an elastic rope
while playing tug – of – war. The two are on an
icy frictionless surface. If the acceleration of the
girl is 3.0 m/s2, then find the magnitude of and
direction of the acceleration of the boy toward the
girl.
• FBD
F b on g = -F g on b
• Newton’s 3rd Law
• F g on b = -F b on g
How do Newton’s Laws of motion
apply to these situations?
•
•
•
•
•
•
•
an object rests in your hand
a ball is tossed upward
a car windshield hits a bug
a person sits on a table
a person jumps up from the floor
a baseball bat hits a baseball
a truck and car hit head-on
• Others?