Newtons Laws of Motion

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Newtons Laws of Motion
For teachers of grades 5 - 8
Goals
To enhance your understanding of the concepts of
Newtons Laws of motion. Time permitting I will
also include a session on the physics of heat
mixtures.
How
Examine how to apply each law both conceptually
and numerically.
Presentation will consists of applications with
demonstrations in the lab.
This is for you, so please feel free to ask
questions or make comments.
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Laws of Motion
Historical Development
Aristotle (384 322 BC)
The natural state of an object was to be at
rest, force was required to maintain motion. To
maintain greater velocity, greater force was
required.
Galileo (1564 1642)
Just as it is natural for object to be in motion
at constant velocity as at rest.
Isaac Newton (1643 1727)
Developed the correct mathematical formulation of
three laws of motion of objects
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Newtons Three Laws of Motion

• Inertia The tendency for objects to maintain
  uniform motion in a straight line.

Inertia is the resistance to change
A ball hanging from a rear view mirror seems to
move backward as the car accelerates forward.
implication
acceleration
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An object moving in a circle will immediately
begin to move in a straight line once the force
causing it to move in a circle is removed.
implication
Example 1
A car going around curve encounters ice
A car moving around a curve will continue in the
curve as long as the force between the tire and
road is large enough to force the car to go in a
circular path. If there is ice on the road, then
this force disappears and the car goes straight.
The perception of a passenger
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Demo 1
A sphere follows a circle as long as it is forced
to by the hose. When hose no longer forces ball
to move in circle, its path is straight.
The same principle can be used to explain why a
the path of a discus is straight after the
thrower spins in a circle before release, after
which it travels in a straight line.
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What does the 1st Law say about motion in a
straight line?
An object in motion in a straight line will
continue in the same direction at the same
velocity unless an external force acts on it.
A car moving 30 mph north, and the shift is put
in neutral to allow it to coast. The first law
does not imply that its motion will be
unchanged, since there is an external force
(friction), which brings the car to a stop.
Because friction is practically everywhere, it is
difficult to demonstrate the 1st law. However in
outer space there is little or no friction, so
the 1st law could be demonstrated there. A rock
thrown by an astronaut would continue
indefinitely at the same velocity and direction,
unless there was a collision with another object.
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Newtons 2nd Law
2)
F
Force if more than one, add them algebraically.
Mass of the object being accelerated
Use the concept of mass as a measure of the
inertia of an object
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• Concept of mass
• Mass is one of four fundamental quantity of
  physics.
• Mass
• Length
• Time
• Force
• Mass to weight (force) relationship
• Since all objects fall with the same acceleration
  (32 ft/s2 9.8 m/s2), and F m x a , then

Metric system
English system
Example Using the metric system. If a person
stepped on a scale and showed their mass to be 80
kg, then their weight is
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• Confusion between weight mass due to metric
  English systems

English system usually specify weight ( force
lbs) Metric system usually specify mass (kg)
Europe
USA
80 kg
176 lbs
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A large object has more mass than a small object
so it is more difficult to slow down or speed
up.
implication
Demo 2
2nd Law
The sphere rolls down the plane into the tray of
water, then moves toward the opposite end of the
tray. We measure the distance to the end of the
tray.
The next step is to replace the sphere with a
heavier one and repeat the first part. The
distance d will be smaller since its harder to
stop the heavier sphere.
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Demo 3
spring
2nd Law
ruler
The cart is attached to a spring and allowed to
roll down the incline. The ruler is used to mark
the max distance reached by the cart. This is
repeated with more mass added to the cart.
Because the larger mass is requires more force to
stop, the cart goes farther down the plane
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Demo 4
102 g
200 g
( 0.2 kg)
(The total force causing the cart to move is F
1.96 N )
The following shows that the heavy car has
smaller acceleration
If the mass of the cart 102 g and 50 g is
added,, then the total mass that moves is
If 50 more g is added to the cart then the total
mass that moves is
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Heat Mixtures
Background needed

• If two objects at different temperatures are
  mixed, the heat lost by the hot object the heat
  gain by the cold object.
• The Specific Heat of a material is the amount of
  heat Q(calories) needed to raise one gram of the
  material by 1 degree Centigrade.

This is a table of the specific heats of a few
materials
Water 1 calorie/g deg C Iron 0.11 Aluminum
0.217
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Given the Specific Heat of aluminum 0.217
calories/g deg. C, this means that 1 calorie of
heat is needed to raise the temperature of 1 gram
of aluminum by 1 deg. C.
To raise the temperature of 5 g by 1 degree
requires
To raise the temperature of 5 g by 7 deg. C
requires
How many calories are needed to raise the
temperature of 12 g of iron at 18 deg. C to 42
deg. C?
Example
The specific heat for iron 0.11 calories/g deg.
C
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Apply these concepts to a heat mixture problem
If we are given 50 g of water at 15 deg. C and
mix it with 80 g of water at 28 deg. C, what is
the final equilibrium temperature?
Example
Hot water initial temperature 28 deg. C and
final temperature T.
Temperature difference
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T
Cold water initial temperature 15 deg. C and
final temperature T.
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Temperature difference
Specific Heat of water 1 calorie/g deg. C
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The heat lost by the hot water the heat gained
by the cold water.
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Newtons 3rd Law
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Newtons 3rd Law
Demo 5
Jet Propelled Soda Can
A soda can with a horizontal glass tube inserted
at the bottom (like a straw), is placed in a open
box of Styrofoam which is floating in water. If
the soda can is filled with water, which escapes
through the tube horizontally.
By the 3rd law, the can exerts a force on the
water, forcing it out of the tube, thus the tube
exerts an equal force on the can in the opposite
direction.
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Newtons 3rd Law

• Action/ Reaction
• When two objects interact the force that object
  A exerts on B is equal and opposite to the force
  that B exerts on A.

This says that for interacting objects, forces
always exist in pairs and are equal and opposite
to one another.
An ice skater facing south, throws a heavy rock.
The skater will be pushed north.
implication
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The 3rd law must be understood in terms of the
2nd law
Suppose that the ice skater has mass 100 kg and
she throws a 10 kg rock south with acceleration
of 1.4 m/s2
Example
The 2nd law says that the force she exerts on
the rock is
south
The 3rd law says that the rock exerts 14 N on the
skater, but in a north direction.
Thus the skater accelerates north while the rock
accelerates south.
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How much is the acceleration of the skater?,
the rock?
Use the 2nd law to find out.
Skater
north
rock
south
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Events explained by Newtons 3rd law
Cannon exerts a force F on a cannonball
Cannonball exerts same force - F (opposite
direction)
Because the cannonball has small mass compared to
the cannon, its acceleration is much larger.
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Events explained by Newtons 3rd law
Pogo Stick
www.physicsforums.com
The boy exerts a force F on the pogo stick The
pogo stick exerts a force -F on the boy.
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Events explained by Newtons 3rd law
Baseball Batter
www.firstcontactllc.com/ news.htm
Baseball comes in contact with the bat, exerts
force F on the bat Bat exerts force - F on the
ball
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The Horse Cart and the 3rd Law
Suppose that a horse exerts 100 lbs on a cart.
Then by Newtons 3rd Law, the cart exerts -100
lbs on the horse. Do these forces cancel each
other with the result that neither the horse nor
the cart move?
We know that they both move, so we must examine
the 3rd law more closely.
Newtons 3rd law is for two objects that
interact, which clearly applies to the horse and
cart. It says that the force that the horse
exerts on the cart is equal and opposite to the
force that the cart exerts on the horse.
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Thus it is correct to say that if the horse
exerts 100 lbs on the cart that the cart exerts
100 lbs in the opposite direction on the horse.
These forces do not cancel because they apply to
different objects. The force on an object affects
only that object.
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We must examine each object in isolation
(separately)
The 100 lbs the horse exerts is on the cart
applies to the motion of the cart only.
100 lbs
horse exerts on the cart
Also the cart exerts a backward force of 100 lbs
on the horse .The horse does not go backward
because the hooves exert 100 lbs (backwards) on
the road which exert 100 lbs (forward) on the
horse. So the 100 lbs (forward) apply to the
horse, thus it moves forward.
100 lbs
cart exerts on horse
100 lbs
100 lbs
The horse exerts 100 lbs on the road.
The road exerts -100 lbs on the horse
The road does not move
The horse moves
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The Archer on Ice (the 2nd and 3rd laws)
An archer standing on ice shoots an arrow. We can
use the 2nd and 3rd laws to predict what happens.
The bow exerts a force F on the arrow which by
the 2nd law results in its acceleration forward
(F ma).
The arrow, by the 3rd law, exerts an equal and
opposite force on the bow. Since the bow is being
held by the archer, this force is transmitted to
the archer, which causes him to slide backward.