Transcript Newton`s Laws of Motion

Newton’s Laws of Motion
Carl Wozniak
Northern Michigan University
Isaac Newton

Newton published the
first work on theoretical
physics, The Principia,
in 1687.
 In this, he showed us
his principle of universal
gravitation, as well as
the rules guiding bodies
in motion.
Axiomato Sive Leges Motus
Corpus omne perseverari in statu suo quiescendi vel
movendi uniformiter in directum, nisi quatenus illud a viribus
impressis cogitur statum suum mutare.
Mutationem motus proportionalem esse vi motrici impressae,
et fieri secundum lineam rectam qua vis illa imprimitur.
Actioni contratiam semper et aequalem esse reactionem:
sive corporum duorum actiones in se mutuo semper esse
aequades et in partes contraries dirigi.
Units of force
 Units
of force
N = 1 kg * m/s2
 The Newton is defined as the amount of
force required to give a 1-kg mass an
acceleration of 1 m/s/s.
1
First law of motion
 An
object at rest tends to stay at rest
and an object in motion tends to stay in
motion with the same speed and in the
same direction unless acted upon by an
unbalanced force.
First law of motion

Objects tend to keep on doing what they're
been doing.
 These lead to the concepts of:
 Inertia: the resistance an object has to a
change in its state of motion.
 Momentum: mass in motion
P
=m*v
First law of motion

Seeing the first law every day
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blood rushes from your head to your feet while quickly stopping
when riding on a descending elevator.
the head of a hammer can be tightened onto the wooden handle by
banging the bottom of the handle against a hard surface.
to dislodge ketchup from the bottom of a ketchup bottle, it is often
turned upside down and, thrust downward at high speeds and then
abruptly halted.
headrests are placed in cars to prevent whiplash injuries during
rear-end collisions.
while riding a skateboard (or wagon or bicycle), you fly forward
off the board when hitting a curb or rock or other object which
abruptly halts the motion of the skateboard.
Second law of motion

The acceleration of an object as produced
by a net force is directly proportional to the
magnitude of the net force, in the same
direction as the net force, and inversely
proportional to the mass of the object.
 Boiled down, this gives us one of the most
famous equations in physics.
Second law of motion
F = ma
Force = mass X acceleration
Second law of motion
So what is acceleration?

the rate at which an object changes its
velocity.
So what is velocity?

the rate at which an object changes its
position
Um…that sounds like speed.
Second law of motion
And they are very close. Speed,
however, is a scalar quantity, and
velocity is a vector quantity.
Ah…now I don’t understand at all.
 Scalars are quantities which are fully
described by a magnitude alone.
 Vectors are quantities which are fully
described by both a magnitude and a
direction.

Second law of motion
Is it too late to drop this class?
 Here’s an example:
 If
you’re in your car going 60 mph
 That’s
your speed
 If
you’re in your car traveling north at
60 mph
 That’s
your velocity
Second law of motion
Think of speed and velocity as how fast
something is moving
 Acceleration then, is a change in how
fast something is moving over a given
period of time
 Remember though, forces don’t cause
motion, they only cause accelerations

 Force
is not needed to keep an object in
motion
Third law of motion

Newton’s third law of motion is perhaps
the best known:
 For
every action there is an equal but
opposite reaction

This might seem simple, but let’s see if
you understand
Third law of motion

While driving down the road, an
unfortunate bug strikes the
windshield of a bus. Quite
obviously, this is a case of
Newton's third law of motion.
The bug hit the bus and the
windshield hit the bug. Which of
the two forces is greater: the
force on the bug or the force on
the bus?
Third law of motion

While driving down the road, an
unfortunate bug strikes the
windshield of a bus. Quite
obviously, this is a case of
Newton's third law of motion.
The bug hit the bus and the
windshield hit the bus. Which of
the two forces is greater: the
force on the bug or the force on
the bus?
The forces are identical. Why does the
bug splatter? It has less mass.
Third law of motion
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A gun recoils when it is fired. The recoil is
the result of action-reaction force pairs. As
the gases from the gunpowder explosion
expand, the gun pushes the bullet
forwards and the bullet pushes the gun
backwards. The acceleration of the
recoiling gun is ...
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a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the
bullet.
Third law of motion

A gun recoils when it is fired. The recoil is
the result of action-reaction force pairs. As
the gases from the gunpowder explosion
expand, the gun pushes the bullet
forwards and the bullet pushes the gun
backwards. The acceleration of the
recoiling gun is ...
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a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the
bullet.
The force on the gun equals the force on the bullet. Yet,
acceleration depends on both force and mass. The bullet
has a greater acceleration due to the fact that it has a
smaller mass.
Graphing movement

We graph movement with the P-T graph
 Position
vs. time
Car moving at
constant velocity
Graphing movement
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We graph movement with the P-T graph
 Position
vs. time
Car accelerating
Graphing movement
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The shape of the graph tells you what is
going on.
Moving right,
constant
velocity, slow
Moving left,
slow to fast
Moving right,
constant
velocity, fast
Moving left,
constant
velocity, slow
Moving left,
constant
velocity, fast
Moving left,
fast to slow
Graphing movement
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What are these two doing?
Graphing movement

What are these two doing?
•A is moving to the right and accelerating
•B is moving to the left and accelerating