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Acceleration and Force
Graph of Speed (distance/time)
Speeding up and Slowing down
Which car is accelerating?
Why it’s m/s2
• a = change in velocity (m/s)
• divided by time
• _m/s__ =
m/s X 1/s = m/s2
Balanced and Unbalanced
• A balanced force is present when equal
forces are applied to each side of an object.
• An unbalanced force is present when
unequal forces are applied to each side of an
• In the picture below, the forces are balanced
when the horizontal bar is level.
• The forces are unbalanced when the
horizontal bar is unleveled.
Direction of Forces is Important
Total Force
Force #2
Force #1
Force #1
Total Force = 0
Force #2
Friction is a Force
Force on box
by person
Force on floor by box
Force on person
by box
Force on box
by floor
It’s the sum of all the forces that determines the acceleration.
Every force has an equal & opposite partner.
Friction Mechanism
Corrugations in the surfaces grind when things slide.
Lubricants fill in the gaps and let things slide more easily.
Why Doesn’t Gravity Make the
Force of Floor acting on Box
Force from floor on box
cancels gravity.
If the floor vanished, the
box would begin to fall.
Force of Earth acting on Box (weight)
What’s missing in this picture?
Force on box
by person
Force on floor by box
Force on person
by box
Force on box
by floor
A pair of forces acting between person and floor.
Air Resistance –
air creates friction
What is gravity?
Gravity (g)
Gravity is a force of
attraction between objects.
The more massive the object, the
greater the pull. However, the object
has to be really massive, like Earth, for
the pull to
be obvious to the casual
What are some EFFECTS of gravity?
• Weight
• Hot air rises,
cool air sinks.
• Liquids
conform to
the bottom
of a
• Objects fall.
• Liquids of
density settle
in layers.
Which falls faster?
Gravitational Acceleration in Free
Fall is 9.8 m/s2