Download Newton*s 1st Law

Newton’s 1st Law
Inertia
Force
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Anything capable of changing an object’s
state of motion
Any push or pull
Causes object to speed up, slow down, or
change direction (acceleration)
Galileo’s Experiment
Inertia
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Galileo reasoned without friction ball
would keep going
Also reasoned that ball initially at rest
would stay that way unless something
caused it to move
Inertia - what Galileo called the tendency
of an object to maintain its initial state
Law of Inertia
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Object at rest will remain at rest or object
in motion will remain in uniform motion in a
straight line
Newton
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Related mass to inertia
More massive objects resist change more
Newton’s 1st Law – in the absence of an
unbalanced force, a body at rest remains
at rest and a body in motion with a
constant velocity
Force(s) acting must be external and must
not total zero
Summary
Newton’s 2nd Law
Free Body Diagram
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Used to show all net forces acting on an object
What can an object with a net force of zero be
doing?
Draw the free body diagram for a box being
acted on by all the following forces: 5N (W),
3N(S), 8N (N), and 7N(E)
Can it be at rest?
Can it be at a constant speed?
What is it doing?
Force on Object
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Objects acted on by a net unbalanced
force will accelerate in the direction of the
force
This means they will speed up, slow down,
or change direction
Newton’s 2nd Law
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The acceleration of an object is directly
proportional to the net force acting on it
and inversely proportional to the mass.
The direction of acceleration is the same
as the direction of the force
a = ∑F / m
∑F = ma
Force Redefined
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Force can be redefined as an action
capable of accelerating an object
Unit of force is the Newton (N)
One Newton is the force needed to
accelerate a 1 kg mass at 1 m/s2
1 N = 1 kg m/s2
Examples
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What force is needed to accelerate a
1000kg car at ½ g?
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What is the net force required to bring a
1500 kg car to rest from 100 km/hr within
a distance of 55 m?
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A 50 kg go-cart starts from rest and is
accelerated by a 550 N force. If the cart
experiences a 280 N friction force and 35
N of air resistance, find
a) it’s acceleration
b) it’s velocity after 3.0 s
c) its displacement after 3.0 s
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What force must be exerted on a 500 kg
crate in order to move it from rest to 3.0
m/s in 1.0 s?