Transcript Newton`s Laws of Motion

Newton’s Laws of Motion
Newton’s First Law
• An object at rest remains at rest and
an object in motion remains in motion
at constant speed and in a straight
line unless acted upon by an
unbalanced force.
– An object at rest means it is not moving
– An unbalanced force is a push or pull.
Inertia
• Newton’s first law is sometimes called
the law of inertia.
• Inertia is the tendency of all objects to
resist any change in motion.
– More mass= more inertia
Newton’s Second Law
• The acceleration of an object depends
on the mass of the object and the
amount of force applied.
• Force= mass x acceleration
F
M
A
Units
•
•
•
•
Force measured in Newtons (N)
Acceleration measured in m/s/s
Mass is measured in kg
A Newton can be described as the
amount of force required to give a 1
kg mass an acceleration of 1 m/s/s
Try it out…
• Find the force needed to accelerate a
800-kg car at a rate of 5m/s/s
– F=MA
– F=(800kg) X (5 m/s/s)
– F= 4000 N
Newton’s Third Law
• If one object exerts a force on another
object, then the second object exerts
a force of an equal strength in the
opposite direction.
“For every action there
is an equal and
opposite reaction.”
“Action…reaction”
• Forces always occur in pairs. Single, isolated
forces never happen!
– However, you cannot always detect the motion.
• You cannot see the Earth’s equal and opposite reaction when the
Earth’s gravity pulls on something.
Ex: When you drop your pencil gravity pulls it downward. At the
same time, the pencil pulls the earth upward. You do not see the
earth accelerate.
• These given names are confusing for two
reasons:
– Either force in an interaction can be the ‘action’
force or the ‘reaction’ force.
– Unfortunately we associate ‘action’ and ‘reaction’
with ‘first an action, then a reaction’.
• This is NOT what occurs in the third law. The action force
and the reaction force exist at the SAME time.
“Equal”
•
Equal means two things:
1. Both forces are exactly the same
size. They are equal in
magnitude.
2. Both forces exist exactly at the
same time. They are equal in
time.
– So why don’t they always
cancel out?
•
They are not always acting on
different objects.
–
Volleyball- setting vs. blocking
“Opposite”
•
Opposite means that the two forces
always act in opposite direction.
•
Exactly 180 degrees apart
Momentum
• The momentum an object has depends on mass
and velocity.
– Formula:
Momentum= Mass x Velocity
– Unit:
kgm/s (“kilogram-meters per second”)
– The more momentum an object has the harder it
is to stop.
– If a car and a Mack truck are
traveling at the same speed the
truck has more mass, therefore
it has more momentum and is
harder to stop.
Try it out….
• What is the momentum of a bird with
the mass of 0.018 kg flying at 15 m/s?
Momentum= M x V
Momentum= 0.018 kg x 15 m/s
Momentum= .27 kgm/s
Try it out…
• Which has more momentum?
A 3 kg sledge hammer swung at 1.5m/s
or a 4kg sledgehammer swung at .9m/s.
Momentum= M x V
Momentum= M x V
Momentum= 3 kg x 1.5 m/s
Momentum= 4 kg x 0.9 m/s
Momentum= 4.5 kgm/s
Momentum= 3.6 kgm/s
The Law of Conservation of Momentum
• The total momentum of any group of
objects remains the same, or is conserved
unless outside forces act on the object.
– The total momentum of objects that interact does not
change, in the absence of outside forces.
– Momentum may be transferred from one to another, but
none is lost.
Collisions With Two Moving Objects
• Some of the momentum is transferred.
• The momentum of one object
decreases while the momentum of the
other increases.
Collisions With One Moving Object
• All of the momentum is transferred
Collision With Connected Objects
• The momentum is transferred so that
both objects are moving at the same
momentum.