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Newton’s Laws of Motion
Sir Issac Newton, 1642-1727, English Scientist
Basic Terms
• Position = Distance from a Reference
• Motion = Change in Position
• Speed = Distance / Time
– How fast you are moving
– Example:
• A car travels 100 miles in 2 hours
• Speed = 100 / 2 = 50 mph
Force
Push
• A force is described by 2 parts
– Strength
– Direction
• Balanced Forces
– Same Strength
– Opposite Directions
• Unbalanced Forces
– Sum of all Forces acting on an object is Not Zero,
– resulting in a Net Force
Pull
Newton’s
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Law of Motion
An object at rest stays at rest.
An object moving at a constant speed in a straight
line will continue moving at a constant speed in a
straight line.
As long as no net (unbalanced) force acts on it.
• Inertia is tendency to resist change in motion
• 1st Law is also called Law of Inertia
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Law Examples
With no wind and no atmosphere
on the moon, assuming no moonquakes,
this footprint on Moon will not change
After being launched from earth and given
an initial push, this probe traveled about
48 billion miles to Mars, at a constant speed
with no engines running.
Another Basic Term
• Acceleration = (change in speed) / Time
– How fast your speed is changing
Lamborghini
0 to 60 mph in 3 sec
Acceleration = 60 / 3
= 20 mph per sec
Zero Acceleration
• An object can have No Acceleration 2 ways
(1) Speed is a constant, not changing
Example
– A car moving at a steady speed of 50 mph
(2) Speed is zero, not moving
Example
– A car parked car
Newton’s 2nd Law of Motion
Net Force = Mass x Acceleration
Acceleration = (Net Force) / Mass
Force on an object is directly related to its Acceleration
Increase Force then Acceleration will increase
Decrease Force then Acceleration will decrease
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2
Law Examples
The stronger the pitcher’s arm, the faster
the ball will accelerate toward the batter
A strong force is applied to this tennis ball,
it will change direction and accelerate to
the right
Free Fall
• Without Air Resistance everything falls at
the same rate.
• With Air Resistance heavier objects fall
faster.
• Coffee Filters
Newton’s
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Law of Motion
Forces are not “one-sided”
Whenever one object exerts a force on a 2nd object, the
2nd exerts an equal and opposite force on the 1st
This is often stated as:
“to every action there is an equal and opposite reaction”
Remember that the “action” force and “reaction” force are
acting on different objects
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Law Examples
Reaction
force of
table
Reaction
Action
weight of
book
Action
Book on a Table
Rocket engine pushes out gases,
the gases exert an equal and opposite
force back on the rocket