Transcript Newton’s Laws of Motion

Newton’s Laws of Motion
• Newton’s Laws
• Forces
• Mass and Weight
Serway and Jewett 5.1 to 5.6
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Newton’s First Law (Law of Inertia)
An isolated object, free from external forces, will
continue moving at constant velocity, or remain at rest.
(This law was actually due to Galileo.)
Earlier, Aristotle said objects were “naturally” at rest, and needed a
continuing push to keep moving.
Galileo realised that motion at constant velocity is “natural”, and
only changes in velocity require external causes.
Objects in equilibrium (no net external force) also move
at constant velocity.
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Forces
• A force is a push or pull that tends to cause motion
(more exactly, changes in motion)
• From the Second Law, force should have units of
1 kg  m/s 2  1 newton (N)
• Force is a vector.
• In Newton’s dynamics, all influences on a particle
from its surroundings are expressed as forces
exerted on that particle.
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Newton’s Second Law


Fnet  ma
Fnet (or Ftotal) is the vector sum of all forces acting on the
particle of mass m:



Fnet   Fi   mai
i
i
The acceleration a is parallel to the total force, and proportional
to it. The proportionality constant is the particle’s mass.
Newton defines mass as a measure of an object’s inertia.
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Contact Forces : direct contact is required
examples - normal forces, friction, air resistance,
buoyancy, ...
Non-Contact Forces :
gravity, electromagnetic, weak and strong forces
The gravitational force is also called weight and is
measured in newtons.
Weight is proportional to mass : Fw = mg, where g is the
gravitational field (and is also the acceleration of an object
in free fall).
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Weight and Mass
Weight is a force; it can be measured using a spring scale
On Earth, a baseball
weighs 2.40 N
On the moon, a
baseball weighs 0.40 N
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•
Mass is a measure of inertia : on the earth or on the moon, a
24.5 N force applied to the baseball will give it an acceleration of
100 m/s2 (its mass is m = F/a = 0.245 kg)
•
We can compare masses with a balance, because of the
remarkable property :
weight  mass
Weights are equal
when masses are
equal

Fg ,1

Fg,2
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Newton’s Law of Gravitation
Gravity is a universal attractive
force between masses.
The force is proportional to each
mass, and inversely proportional to
the the square of the distance
between them (from center of
mass):
Mm
F G 2
r
M
r
Units of G ?
What is g ???
m
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Quiz
Astronauts in orbit are seen floating motionless inside
the space shuttle. They can do this because:
a) They have no mass in space.
b) There is no gravity in space.
c) The shuttle is not an inertial reference frame.
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Newton’s Third Law (action and reaction)
If object A exerts a force on object B, object B
exerts an equal, opposite force back on A.
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Newton’s Third Law : examples
1.)
What is the “reaction” to the following forces?

Fg

 Fg
Gravity (of block)
pulls earth up
Gravity pulls block down
2.)
Block pushes down
on table (contact)
Table pushes up on block
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3.)
Balloon pushes air backwards
Pressure forces on balloon
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Quiz
A 140-kg wrestler and a 90-kg wrestler try to push each other
backwards out of the ring. At first they are motionless as they push;
then the large wrestler moves the other one backwards. Compare the
forces they exert on each other. Which statement is correct?
a) The forces are always equal.
b) The larger wrestler always exerts a larger force.
c) When they are motionless, the forces are equal; they start to
move when the large wrestler exerts a larger force on his
opponent than his opponent exerts back on him.
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Contact Forces
Examples : A heavy block on a table
• The table must push up on the
block to prevent it from falling
Forces on Block

Fg
• The type of contact force is
called a normal force if it is
perpendicular (normal) to the
surfaces in contact.
• The normal force will be as large
as necessary to hold the block
(until the table breaks)

Ftable
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If we look closely, the normal force
arises from the table being bent :
as the table tries to straighten, it
pushes back.
This is really an elastic force; the table behaves like a
spring.
At the atomic level, all contact forces are due to
electromagnetic forces.
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
Fg
• We divide the contact
force into two
components”

FA

f

n

- normal force : n is perpendicular to the surfaces
in contact

- friction : f is parallel to the surface
friction has a more complex behaviour than
the normal force (next lecture)
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Summary
•
Newton’s Three Laws
1. Objects naturally move at constant velocity in a straight line
2. Accelerations are caused by unbalanced forces
3. Forces come in action-reaction pairs, with the ‘action’ and
‘reaction’ always on different objects.
•
•
•


Fg  mg
Gravity :
Weight is a force, mass is a measure of inertia.
All familiar forces require direct contact, except for
gravity and electromagnetism (and nuclear forces).
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Practice: Chapter 5
Questions 5, 7, 10
Problems 1, 3, 11
(5th ed) Questions 1, 9, 18
Problems 1, 3, 15
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