Transcript 6.1 Newton`s First Law

6.1 Newton’s First Law
pp. 138-140
NEWTON’S THREE LAWS OF
MOTION
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In his book, Principia,
Newton described the
three laws of motion.
He did not discover all
three laws, he
explained them in a
way that many people
could understand
NEWTON’S FIRST LAW OF MOTION
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This is also known as the Law of Inertia
It says : An object in motion will remain
in motion at a constant speed and in the
same direction unless acted on by an
unbalanced force.
An object at rest will remain at rest unless
acted upon by an unbalanced force.
6.1 Law of inertia
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Newton’s first law says that objects
continue the motion they already have
unless they are acted on by a net force.
If the net force is zero, an object at rest
will stay at rest.
If an object is acted upon by unbalanced
forces, its motion will change.
NEWTON’S FIRST LAW OF MOTION
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In other words…
Unless you apply a force, things tend to
keep on doing what they were doing in
the first place.
Objects at rest (not moving) will not begin
moving unless a push or pull is exerted on
them.
6.1 Net force
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Newton’s first law is
often written in terms
of the net force:
“An object at rest will
stay at rest and an
object in motion will
continue in motion at
constant velocity
UNLESS there is a net
force.”
According to these vectors, in
what direction is the net
force?
6.1 Force changes motion
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Forces can be used to increase or
decrease the speed of an object, or to
change the direction an object is moving.
NEWTON’S FIRST LAW OF MOTION
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An object moving at a certain velocity will
continue to move forever at that same
speed and in the same direction unless an
unbalanced force acts on it.
Inertia is related to mass. Mass is a
measure of inertia.
NEWTON’S FIRST LAW OF MOTION
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An object with a small
mass has less inertia
than an object with a
large mass.
For example…
 A tractor trailer has
more mass than a
motorcycle
6.1 Law of inertia
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Inertia is the
property of an object
that resists changes
in motion.
Objects with more
mass have more
inertia and are more
resistant to changes
in their motion.
Which ball has more
inertia?
NEWTON’S FIRST LAW OF MOTION
NEWTON’S FIRST LAW OF MOTION
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How do ABS
(automatic brake
systems) work in
cars?
The system is
controlled by a
computer that
prevents skidding by
sensing when the
wheels are about to
lock.
ABS BRAKES EXPLAINED
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The computer
releases and reapplies
the brakes up to 25
times a second.
Instead of skidding
out of control, the car
slows down and stops
safely.
Solving Problems
A car drives along the highway at constant velocity.
Find the car’s weight and the friction force if
the engine produces a force of 2,000 newtons
between the tires and the road and the normal
force on the car is 12,000 N.
Solving Problems
Looking for:
1.
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…weight of car in newtons, force due to
friction
Given:
2.
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…ForceN = 12,000N (up);
…ForceE = 2,000N (forward)
Relationships:
3.
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Newton’s 1st Law:
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net force = zero at constant velocity; so ForceN =
ForceW and ForceE = ForceF
Solution
4.
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FN = 12,000N
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Draw a free body diagram.
There is no net force
upward, so the weight of
FF = 200 N
the car is an equal
downward force of 12,000
N.
The forward engine force
balances the friction force
so the friction force is 2,000
N.
FW = 12,000N
Solving Problems
FE = 200 N