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Unit 3
Lesson 7
Force and Laws of
Motion
S.8.C.3.1.1,2
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Force
•
•
•
•
1 N= 0.225lbs
A push or pull on an object.
Can change the motion of an object
Measured in Newtons (N) (kg*m/s2)
Friction and gravity are examples.
Friction
• A force that acts against motion.
• Between objects that are touching
•Between the legs of a desk and the
floor when you push a desk
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Gravity
• A force that pulls matter together
• Between objects that are not touching
• Isaac Newton
– “the force of gravity acts between all
objects in the universe”
• Gravity exists between all matter
– Gravity exists between you and your pencil
however it is very weak
• The more mass an object has, the
stronger its gravity
• Keeps objects in orbit around a star,
planet or moon.
• Makes objects “fall”
3
Combining Forces
Force Diagrams:
• We use arrows to show the different forces acting on
objects
• Direction of arrow shows the direction the force is acting
• Length of the arrow shows the strength (size or magnitude)
of the force
• Net force: the combination of all forces acting on
the object
5N
5N
= 10N
5N
10N
=
5N
5N
= 0 N
5N
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Balanced Forces
• When two forces are the same
strength (magnitude or size) but
acting in opposite directions on a
the same object.
•Cannot change an objects
motion (speed and direction)
•An object at rest to start with
balanced forces will continue at
rest
•An object moving at 30 mph to
the left with balanced forces
acting on it will continue to move at
30 mph to the left
•Forces are equal
•Motion remains the same
•If not moving, it continues
to not move
•If moving , it continues to
move exactly the same
5
Balanced Forces
10 N
10 N
Forces on the
person are
balanced:
Gravity pulls
down on person,
chair pushes up
on person
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Unbalanced Forces
•Will change an objects motion
(speed and direction)
• A net force results.
•Net force is calculated by adding
forces in the same direction or
subtracting forces in opposite
directions
•Net force of 40 N to the Right
• 40 N
• New motion depends on the
starting motion
• If an object is starting at rest when a
net force of 40 N  is applied, the
object will move to the right
•If an object is already moving to the
right at a constant speed when a net
force of 40 N  is applied, the object
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will speed up.
Newton’s 1st Law of Motion
Law of Inertia
•The tendency of an object to resist a change in
motion.
• The higher the mass of an object , the higher its
inertia is.
– Object at rest will remain at rest
– Object in motion will continue in
motion in a straight line
Unless an
unbalanced force
acts upon it
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Inertia
9
Newton’s 2nd Law of Motion
• F = MA
• The larger the net force on an object , the more the
objects motion will change (greater acceleration).
• Objects with larger masses, need more force to
change their motion.
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Newton’s
nd
2
Law of Motion
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Newton’s 3rd Law of Motion
• When one object exerts a force on another object,
the second object exerts an equal but opposite force
on the first object
•They are not balanced forces because the forces act
on different objects
100 N
100 N
12
Newton’s 3rd Law
• Action-Reaction
•For every action (force), there is an equal but
opposite reaction (force)
Newton’s 3 Laws
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