Transcript ENV PHY 1 lesson 01

Mechanics
Unit 5: Motion and Forces
5.7 Newton’s Laws of Motion
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Fundamentals of physics - Mechanics
Newton’s First Law of Motion
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Every body continues in its state of rest, or a uniform
speed in a straight line, unless acted on by a net force.
Tablecloth trick
Sudden braking...
In plain English:
An object at rest will
stay at rest.
 A moving object will
continue to move a
straight line
Fundamentals of physics - Mechanics
Newton’s First Law of Motion
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Every body continues in its state of
rest, or a uniform speed in a straight
line, unless acted on by a net force.
Implications: We can say that there is
a net force acting on an object if
 From rest it begins to move
 It is already moving it slows down or
stop moving, speeds up or changes
direction of motion
 From rest it begins to rotate
 It is already rotating it rotates
slower or stop rotating, rotates faster
or changes direction of rotation
In plain English:
An object at
rest will stay at
rest.
 A moving
object will
continue to move
a straight line
Fundamentals of physics - Mechanics
Newton’s Second Law of Motion
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The acceleration produced by a net force on an object is
directly proportional to the net force, is in the same
direction as the net force, and is inversely proportional to
the mass of the object.
In plain English:
Greater force greater acceleration.
 Acceleration is in the direction of the force
 Greater mass, lesser acceleration...
Fundamentals of physics - Mechanics
Newton’s Second Law of Motion
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In plain English:
Greater force
greater acceleration.
 Acceleration is in
the direction of the
force
 Greater mass,
lesser acceleration...
Fundamentals of physics - Mechanics
Newton’s Second Law of Motion
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When you accelerate in the direction of your velocity, you
speed up; when you accelerate against your velocity, you
slow down; when you accelerate at an angle to your velocity,
your direction changes.
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Fundamentals of physics - Mechanics
Newton’s Second Law of Motion
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Two ways of defining acceleration:
Previously, we defined acceleration as the time rate of
change of velocity a = [(change in v)/time]
Now saying that acceleration it is the ratio of force to
mass [a = F/ma)]
Question (1) A jumbo jet cruises at a constant velocity of
50 ms-1 when the thrusting force of its engines is a
constant 100,000 N.
(a) What is the
acceleration of the
jet?
(b) What is the
force of air
resistance on the
jet?
Fundamentals of physics - Mechanics
Newton’s Second Law of Motion
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Question (1) A jumbo jet cruises at a constant velocity of
50 ms-1 when the thrusting force of its engines is a
constant 100,000 N.
Answer
F = 0 (constant velocity – Newton’s First Law)
No net force – Acceleration = 0
(F = 0  Far + Ft = 0
Taking the right to
be positive
Far + 100000N = 0N
Far = -100000
-ve sign indicates
force is to the left
Fundamentals of physics - Mechanics
Newton’s Second Law of Motion
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Doing the math.
Newton’s Second Law of Motion can be written as F = ma
or a = F/m
Question (2) The engines of a jumbo jet at rest on the
runway at produces a thrusting force of 100,000 N. If its
mass is 10000kg, what is its acceleration when the brakes
are released. Ignore any air resistance.
Answer:
a = F/m
a = 100000N / 10000kg
a = 10ms-2
Fundamentals of physics - Mechanics
Newton’s Third Law of Motion
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Whenever one object exerts a force on a second object,
the second object exerts an equal and opposite force on
the first object.
We call one force the action force, and we can call the
other the reaction force.
The important thing is that they are coequal parts of a
single interaction and that neither force exists without the
other.
Action and reaction forces are equal in strength and
opposite in direction.
They occur in pairs, and they make up a single interaction
between two things
Fundamentals of physics - Mechanics
Newton’s Third Law of Motion
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Whenever one object exerts a force on a second object,
the second object exerts an equal and opposite force on
the first object.
Fundamentals of physics - Mechanics
Newton’s Third Law of Motion
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Action and Reaction on Different
Masses.
Quite interestingly, a falling object
pulls upward on Earth with as much
force as the Earth pulls downward on
it. The resulting acceleration of the
falling object is evident, while the
upward acceleration of Earth is too
small to detect.
Which falls toward the other, A or B?
Do the accelerations of each relate to
their relative masses?
Fundamentals of physics - Mechanics
Newton’s Third Law of Motion
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When a cannon is fired, there is an interaction between the
cannon and the cannonball. The sudden force that the
cannon exerts on the cannonball is exactly equal and
opposite to the force the cannonball exerts on the cannon.
This is why the cannon recoils (kicks). But the effects of
these equal forces are very different.
This is because the forces act on different masses.
The different accelerations are evident via Newton’s
second law, a = F/m
cannonball: A = F/M
Cannon a = F/m
Thus we see why the change
in velocity of the cannonball is
so large compared with the change in velocity of the
cannon.
Fundamentals of physics - Mechanics
Newton’s Third Law of Motion
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This is why the cannon recoils (kicks). But the effects of
these equal forces
are very different. This is because the forces act on
different masses. The different accelerations are evident
via Newton’s second law,
a = F/m
This is why the cannon recoils
(kicks). But the effects of
these equal forces
are very different. This is
because the forces act on
different masses. The
different accelerations are
evident via Newton’s second
law,