Chapter 11
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Force – A push or pull that one body exerts on
another
Force is measured in a Newton (N) or (Kg m/s2)
Can be measured with a spring scale
 What forces are being exerted on the Fkick
football?
▪ Force of kick
▪ Force of gravity
▪ Maybe force of air resistance
Fgravity
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There are two types of forces:
 Balanced Forces – no movement occurs
▪ Equal forces on both sides
▪ Ex) tug of war game with neither side moving
 Unbalanced Forces – movement occurs
▪ Unequal forces on both sides
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Normal (@ a right angle)
Friction/Drag (slows it down)
Contact (touching)
Gravitational (a pull)
Electric (movement of electrons)
Magnetic
Air Resistance (slows it down)
Compression (pushing)
Tension (pulling)
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Exists between two solid objects when their
surfaces are pressed together due to other
forces acting on one or both objects
 Ex) a solid sitting on or sliding across a table
 Ex) a magnet attached to a refrigerator
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Always directed at a right angle
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Force that opposes sliding between two surfaces
The force on an object always points in a
direction opposite to the relative motion of the
object
Friction causes thermal energy between objects
Which direction is the friction?
Motion
Pushing Force
Friction
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Friction is greater:
 Between rough surfaces
 When there’s a greater force between the
surfaces (for example – more weight)
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The diagram below shows two people having a
tug-of-war. Determine the unknown tension
force.
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T = 900 N
 Tension is constant throughout the entire rope.
Tension is always opposite and equal.
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When an object falls, it is pulled down by
gravity
Also pushed up by air resistance
The force of air resistance is in the opposite
direction to the force of motion
Amount of air resistance depends on the
speed, size, and shape of the object
If no air resistance is present, then an apple
and a feather fall at the same rate
Example of Feather and Hammer
Falling at the Same Rate because
there is no air on the moon to slow it
down from Apollo 15 Mission.
They fell at
the same
rate because
they have a
similar
shape, not
because of
their mass.
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Acceleration towards the center
Requires an unbalanced force
Ex) On a roller coaster, the coaster cars zoom
around the loop, the track exerts a centripetal
force toward the center of the loop
Ex) Bucket of water spun around
 Water stays in bucket
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Gravitational Force is proportional to its mass
Weight can be found by:
Fg = m x g
Fg = force of gravity
m = mass
g = 9.8 m/s2 on Earth
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A robot rover used by NASA has a
mass of 90 kg.
 What is its mass on Earth?
▪ Fg = mass x gravity
▪ Fg = (90 kg) x (9.8 m/s2)
▪ Fg = 882 kg m/s2
 On the Moon (gravity is 1/6 less than on
Earth)?
▪ Fg = mass x gravity
▪ Fg = (90 kg) x (1.6 m/s2)
▪ Fg = 147 kg m/s2
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If the same astronaut stands on the Earth and
the Moon their weight is different.
 Why?
▪ The mass of an object
stays the same
wherever it is, but its
weight can change.
▪ The Moon has less mass
than the Earth, so its gravity is less than the Earth's
gravity. This means that objects weigh less on the Moon
than they do on the Earth.